Volcanic and Magmatic Study Group (2013) Bristol

Volcanic and Magmatic Studies Group Annual Meeting 2013

Supported by The Mineralogical Society of Great Britain and Ireland; registered charity number 233706.

School of Earth SciencesUniversity of Bristol7th-9th January 2013

VMSG 2013 – Welcome

Welcome

Bristol Volcanology welcomes you to VMSG 2013! Volcanology research at Bristol spans a truly wide range, encompassing everything from fluid dynamics and volcano physics to magmatic petrology, geochemistry, and risk assessment. We are pleased to be welcoming over 230 delegates from the U.K. and abroad to this year’s conference. Following the success of VMSG 2012, we have maintained a two-and-a-half day format and have scheduled dedicated poster sessions throughout the day on Tuesday. Oral sessions were vastly oversubscribed, and over 70% of the received abstracts are presented as posters, so please use poster sessions as an opportunity to take in the majority of the presented research.

Bristol Earth Sciences The University College Bristol was founded in 1876 and was the first university in the country to admit women on an equal basis to their male counterparts. King Edward VII approved the Charter creating the University of Bristol in 1909. Today the university comprises six faculties, with over 13,000 undergraduates and over 4000 postgraduates. The School of Earth Sciences is located in the neo-Gothic Wills Memorial Building. Geology has been offered as a subject since 1876, when topics such as solid crust, minerals, volcanoes and the history of the Earth were taught. For two lectures a week for two terms, this would cost you a sum of £3 and 3 shillings. Today, the School has six highly active research groups studying such diverse topics as climate, seismology, palaeobiology, petrology and geochemistry, and geologic hazards.

Acknowledgments

The Organising Committee gratefully acknowledges the Mineralogical Society for running the online registration and financial aspects of the conference and the VMSG Committee for their support throughout the organisation of this meeting. Hazel Bunting of the Bristol University Conference Office assisted with the venue hire and was endlessly helpful in accommodating the large number of poster presentations. We also thank our supporters (as of early December), Springer, AWE, SciMed, Thermo Fisher, Oxford Instruments, and FEI, as well as the Clifton Hotels Group and the The Grand Hotel by Thistle for offering discounted rates to delegates. Lorraine Field of the British Geological Survey provided many of the images used in publicity materials. Thanks again to all members of the Bristol Volcanology Research Group who have contributed to the organisation of this meeting and are not named below. Finally, cheers to those of you whose abstracts required no reformatting – you are the true heroes of VMSG 2013.

The Organising Committee

Alison Rust, Kate Saunders, Rose Burden, Jonathan Hanson, Emma Johnson, Elena Melekhova, Jenny Riker, Anne Schöpa, and Susanne Skora.

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We would like to thank the following organsiations for their kind support of VMSG 2013

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Monday 7th January Tuesday 8th January Wednesday 9th January

Welcome

Registration opens 11 amEast Foyer, Chemistry

Magma Storage andDegassing (Session 1)

Lecture Theatre 1,Chemistry Building

CoffeeEast Foyer, Chemistry




Conference Dinnerand Ceilidh

Wessex SuiteThe Grand Hotel by Thistle

Research in Progressand General Presentations

(Session 1)

Auditorium, Victoria Rooms

VMSG AGMCoffee

Victoria Rooms

CoffeeVictoria Rooms

LunchVictoria Rooms Lunch

East Foyer, Chemistry

Japan IODP Project

Poster Session AVictoria Rooms

Poster Session BVictoria Rooms

Poster Session Cand Drinks Reception

Victoria Rooms

VMSG Award Talk:Mike Branney

Behaviour of Giant Pyroclastic Density Currents: Deductions from Deposits


Physical Volcanology


GeophysicsLecture Theatre 1,Chemistry Building

Student Forum

Closing remarks

John Guest (Session 1)Lecture Theatre 1,Chemistry Building


Magma Storage andDegassing (Session 2)


(Session 2)

John Guest (Session 2)

Programme Overview

Public Lecture: Ellen Stofan

@Bristol Science Centre

Mapping of Volcanic Terrains acrossthe Solar System

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VMSG 2013 – Conference Logistics

Venues and Registration Conference activities will be split between two main venues – the Chemistry Building and the Victoria Rooms (please see campus map). The entrance to Chemistry can be accessed from Cantocks Close, off of Woodland Road at the rear of Wills Memorial Building. Directions to Lecture Theatre 1 will be clearly posted within the building. The entrance to the Victoria Rooms is at the intersection of Queens Road and Whiteladies Road, and a detailed map of the venue is included in this volume. Registration will take place on Monday, January 7th, from 11:00 to 13:45 in the East Foyer of the Chemistry Building. For those arriving on Tuesday, we will also host a registration table at the entrance to the Victoria Rooms.

Information for Presenters

Oral presentations. All oral presentations (excepting the VMSG award talk) will be allotted 12 minutes, plus an additional 3 minutes for questions. Acceptable formats are Microsoft PowerPoint or Adobe PDF. Please bring your presentation on a USB memory stick to be uploaded at the end of the preceding session. Presenters in the first session on Monday, January 7th, please upload your talk on arrival. Presenters in the first morning sessions on Tuesday and Wednesday, please upload your talk after the final session of the previous day. Poster presentations. Poster boards are portrait orientation and the maximum poster size is A0 (W × H = 84 cm × 119 cm). All posters should be on display throughout Tuesday. Presenters are requested to stand by their poster during their allocated poster session. Please note your poster number, as this refers to your poster board, and the letter of your poster session (A, B, C), which indicates the session time. This information can be found in the meeting programme in this volume.

Internet Access Fast eduroam wireless internet is available for delegates from organisations participating in the eduroam federation. Internet access will also be available in all conference venues via the VisitorNet Wi-Fi service. VisitorNet passwords and connection instructions can be collected by delegates at registration. Please only request a VisitorNet password if you are from a non-eduroam institution.

Conference Dinner and Ceilidh

The conference dinner will take place at The Grand Hotel on Monday, January 7th. The Grand is roughly 10-15 minutes walk from the Chemistry Building (see campus map). The cash bar in the Wessex Suite will be open from 6:30 p.m. for drinks, and dinner will begin at 7:15 p.m., with a Ceilidh following the meal.

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orialBuilding

Chemistry

January 7th and 9th(enter via Cantocks Close)

@Bristol Science

Centre

Washington

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ouse

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otel(conference

dinner)

The RodneyH

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Berkeley Square Hotel

Victoria Rooms

January 8th

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Main Entrance

Victoria Rooms Floor Plan (Tuesday 8th January)

PORTERSInformation

andRegistration

Desk

Exhibitors

AUDITORIUMOral Presentations

RECITAL ROOM

Geophysics Posters


Posters

Refreshment location(please use both locations)

VICTORIA’S ROOM

Physical VolcanologyPosters

LECTURE ROOM G12

Magma Storage andDegassing Posters

John Guest Posters

STAGE

THEATRE BAR(seating area)

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VMSG 2013 – Food and Drink

Food and Drink In addition to Monday night’s conference dinner, lunch will be provided for delegates on Tuesday and Wednesday, and we will host a drinks reception concurrent with Poster Session C on Tuesday evening. If you wish to venture out on your own, there are plenty of great places to eat and drink in Bristol. A thorough guide to a wide range of pubs, bars, and restuarants, including maps, is provided below. All are within 20 minutes walk of the conference venues. Clifton Village Restaurants 1. Clifton Sausage (7 Portland Street, BS8 4JA) – Good quality, locally sourced British dishes with sausages an obvious speciality. Party Menu; 2 courses for £15 or 3 for £19. 2. Pizza Provencale (29 Regent Street, BS8 4HR) – The best pizzas in Bristol (often shared). £8-13 for 12”, £12-18 for 16”. 3. The Thali Café (1 Regents Street, BS8 4HW) – Award-winning curries made with locally sourced ingredients and a modern British kitsch twist. Main course < £10. Pubs 4. The White Lion (Sion Hill, BS8 4LD) – Part of the Avon Gorge Hotel, including a large heated beer terrace with spectacular views of the famous Clifton Suspension Bridge. 5. The Mall (The Mall, BS8 4JG) – Cask ales, a secluded secret garden and great pub food. 6. The Cori Tap (8 Sion Place, BS8 4AX) – Famous throughout Bristol for serving the exclusive and infamous Exhibition Cider. Intrigued? You should be, it can only be served in half-pints… 7. The Lansdown (8 Clifton Road, BS8 1AF) – Traditional Bristish pub with a large heated outdoor area.

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Park Street, Park Row, and The Harbourside Restaurants 8. Jamie’s Italian (87-89 Park Street) – Housed in a stunning listed building Jamie Oliver’s signature restauarant serves rustic, Italian food in a relaxed, friendly environment. Mains £10-20. 9. ZaZa Bazaar (Canon’s Road, BS1 5UH) – A huge all-you-can-eat world banquet catering for up to 1000 guests. Definitely a sight to behold! Dinner £15. 10. Bordeaux Quay (Canon’s Road, BS1 5UH) – Award-winning restaurant serving European food in a stunning harbourside setting. Brasserie Menu ~£10. 11. La Tomatina (2 Park Street, BS1 5HS) – A newly opened Spanish tapas restaurant drawing inspiration from the festival of the same name. £3-5 per tapas dish. Pubs 12. Bristol Ram (32 Park Street, BS1 5JA) – Good size local pub with a range of ales and bitters. Also serves food. 13. The Woods (1 Park Street Avenue, BS1 5LQ) – A modern pub serving a wide range of drinks and cocktails in a vibrant environment with a buzzing atmosphere. 14. The Ship (8 Lower Park Row, BS1 5BJ) – A comfy, classic pub experience with a large variety of drinks on tap. 15. Zero Degrees (53 Colston Street, BS1 5BA) – Very good beers produced in the onsite microbrewery. They also do nice pizza’s. 16. White Harte (54-58 Park Row, BS1 5LH) – A ‘Scream’ pub with cheap food and drinks and offers-a-plenty. Popular with students. Bars 17. Goldbrick House (69 Park Street, BS1 5PB) – A classy champagne and cocktail bar with highly knowledgable bar tenders catering to every taste. 18. The Apple (Welsh Back, BS1 4SB) – Situated on a boat, this cider bar has won many awards and serves up a range of approximately 40 different ciders and cider-related drinks. Clubs Java (9 Park Street) – A cocktail bar and club with a reputation as a premium destination for the city’s trendy young professionals. Agora (55 Park Street) – Good for dancing after 2 a.m.

The Centre Restaurants 19. Guiseppes (59 Baldwin Street, BS1 1QZ) – A delicious yet affordable, traditonal, family-owned Italian restaurant. Highly reccomended. Before 7PM: main + a drink £6.50, or 2 courses + a drink £11.95. 20. Pizza Express (35 Corn Street, BS1 1HT) – Standard chain pizza restaurant. Pizzas from £8.

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Pubs 21. The Bank (8 John Street, BS1 2HR) – A fantastic pub with friendly staff hidden away in the city centre, yet conveniently close to the Conference Dinner venue! 22. Commercial Rooms (45 Corn Street, BS1 1HT) – A Wetherspoons pub, we need say no more. 23. Horts (49 Broad Street, BS1 2EP) – A few doors down from the Conference Dinner venue. Bars 24. The Rummer (All Saints Lane, BS1 1JH) – Independent cocktail bar and restaurant within a listed building in Bristol’s Old City Quarter, holding over 400 premium spirits from around the globe. Ideal for a post-dinner whiskey… 25. Start the Bus (7-9 Baldwin Street, BS1 1RU) – A lively bar that often plays host to a variety of live music. Open late. 26. The Big Chill (15 Small Street, BS1 1DE) – Artistically creative, bright and cheery. This laid-back place is open late and caters for a variety of tastes.

The Triangle (…and around) Restaurants 27. Wagamama (63 Queens Road, BS8 1QL) – Japanese restaurant and Noodle Bar down the road from the Victoria Rooms. Mains ~£10. 28. Browns (38 Queens Road, BS8 1RE) – A stylish blend of modern and clasic dishes as well as post-work cocktail happy hours. Mains £10-20. 29. Racks (St. Pauls Road, BS8 1LX) – A friendly and cosmopolitan bar & kitchen literally behing the Victoria Rooms. Highly reccomended. Mains from £8.

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30. Cosmo (30 Triangle West, BS8 1ET) – A large all-you-can eat Pan-Asian dinery. Dinner £13. Pubs 31. The Eldon (6 Lower Clifton Hill, BS8 1BT) – Good beer and decent pub grub. 32. Quinton House (Park Place, BS8 1JW) – Old man pub with good ales and pork scratchings. Limited seating space. Not recommended for large groups. 33. Brass Pig (1 Triangle West, BS8 1EJ) – The traditional feel of a public house in a more modern (and large) setting. 34. The Hope & Anchor (38 Jacob's Wells Road, BS8 1DR) – An equal rival to The Eldon. 35. The Berkeley (15-19 Queens Road, BS8 1QE) – Another Wetherspoons, opposite the Earth Sciences Department. Bars 36. Mbargo’s (38-40 Triangle West, BS8 1ER) – Consistently busy with students and locals alike, happy-hour prices and dodgy DJ’s till late. What more could you ask for? 37. Illusions (2 Byron Place, BS8 1JT) – A multi award winning magic bar with free live shows. 38. Hermanos (55 Queens Road, BS8 1QQ) – An independent, neighbourhood café-bar serving coffee and cake by day and beer, cocktails and fine wines by night.

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VMSG 2013 – Meeting Programme

Programme Monday 7th January

11:00 – 13:45 Registration in the East foyer, Chemistry Building Afternoon session: Lecture Theatre 1: Chemistry Building Please ensure that oral presentations are uploaded prior to the start of your session. 13:45 Welcoming remarks (S) denotes a student presentation Session: Magma, storage & degassing 1 Chaired by: Catherine Annen, University of Bristol 13:50 Keynote: Changing the metaphor – from magma chamber to magma reservoir

Kathy Cashman & Guido Giordano 14:05 (S) Experimental constraints on coupled degassing and crystallisation at Mount St. Helens

Jenny Riker, Jon Blundy, Alison Rust 14:20 Degassing regime of Hekla volcano in 2012

Evgenia Ilyinskaya, Alessandro Aiuppa, Bergur Bergsson, Thrainn Fridriksson, Audur Agla Oladottir, Finnbogi Oskarsson, Katharina Lechner, Richard Yeo, Gaetano Giudice

14:35 (S) Triple magma batches and a complex history of a monogenetic volcano: geochemical analysis of Mt. Rouse, Newer Volcanics Province, Australia. Julie Boyce, Ian Nicholls, Reid Keays, Patrick Hayman

14:50 (S) Assembling a super-eruption: linking magma accumulation and eruption timescales at Toba. David Budd, Val Troll, Ester Jolis, Frances Deegan, Vicki Smith, Martin Whitehouse, Chris Harris, Carmela Freda, David Hilton, Sæmundur Halldorsson

15:05 (S) Shallow storage of dacites beneath Uturuncu volcano, SW Bolivia Duncan Muir, Jon Blundy, Alison Rust

15:20 The behaviour of a volcanic system with two linked magma chambers Stephen Blake

15:35-16:05 Coffee: East Foyer, Chemistry Building Session: Magma, storage & degassing 2 Chaired by: Evgenia Iiyinskaya, British Geological Survey 16:05 Petrological Cannibalism – the chemical and textural consequences of pulsatory

growth of magma bodies. Jon Blundy & Kathy Cashman

16:20 (S) Plagioclase as a recorder of magma-crust interaction beneath the Faroe Islands Börje Dahren, Val Troll, Abigail Barker, Fiona Meade, Carmela Freda, Paul Holm, Nina Søager

16:35 (S) A new model for granitic emplacement: the Newry Igneous Complex, Northern Ireland Paul Anderson, Carl Stevenson, Mark Cooper, Rob Ellam, Ian Meighan, Colm Hurley, John Reavy, James Inman, Dan Condon, Quentin Crowley.

16:50 (S) Reconstructing the emplacement of the Lago della Vacca complex, Adamello Batholith, Italy, through field observations, image analysis and AMS. Anne Schöpa, Catherine Annen, Jon Blundy, Michel de Saint-Blanquat, Patrick Launeau

17:05 (S) Slurry remobilisation in a layered mafic sill (Franklin LIP, Victoria Island, Artic Canada Ben Hayes, Jean Bédard, Johan Lissenberg, C.D. Beard.

17:20 (S) Crystal-melt relationships and the record of deep mixing and crystallisation in the AD 1783 Laki eruption, Iceland. David Neave, Emma Passmore, John Maclennan, Godfrey Fitton, Thor Thordarson

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17:35 Glacial loading probes mantle heterogeneity beneath Iceland John Maclennan, Kenneth W.W. Sims, Janne Blichert-Toft, Evelyn Mervine, Jurek Bluzstajn, Karl Grönvold

Please vacate the Chemistry Building promptly at the end of the last session. The cash bar at the Wessex Suite, The Grand by Thistle will be open from 6:30 pm for drinks. It is a 10-15 minute from the Chemistry Building and there are several pubs/bars along the way for a pre-dinner drink. 19:00 Conference Dinner and Ceilidh: Wessex Suite

The Grand by Thistle, Broad Street, Bristol BS1 2EL (Please note the cost of the dinner is included in your conference registration)

Tuesday 8th January

Morning session: Auditorium: Victoria Rooms Please ensure that oral presentations are uploaded prior to the start of your session. Session: Research in Progress and general presentations 1 Chaired by: Susanne Skora, University of Bristol 9:00 Keynote: Mission Immiscible for two subduction components; evidence from Pagan

Volcano, Mariana arc Yoshihiko Tamura, Osamu Ishizuka, Robert J. Stern, Akiko Nunokawa, Hiroshi Shukuno, Hiroshi Kawabata, Yuka Hirahara, Qing Chang, Takashi Miyazaki, Jun-Ichi Kimura, Robert W. Embley, Sherman Bloomer, Yoshiyuki Tatsumi

9:15 (S) Melting beneath the Izu volcanic arc: constraints from Uranium series isotopes Heye Freymuth, Tim Elliott, Yoshihiko Tamura

9:30 (S) Diatreme volcanism facilitating Pb-Zn mineralisation in the Irish Orefield? Holly Elliott, Tom Gernon, Stephen Roberts, Patrick Redmond

9:45 (S) The Comsos Greenstone terrane; Insights into a mineralised Archean Arc from U-Pb dating, volcanic stratigraphy and geochemistry Alexandra de Joux & Thor Thordarson

10:00 (S) Carbonatite genesis: An experimental approach in the CMASK-CO2 system Sorcha McMahon, Mike Walter, Ken Bailey

10:15 Rheology of Three-Phase magmas Mattia Pistone, Luca Caricchi, Peter Ulmer, Eric Reusser, Federica Marone, Luigi Burlini

10:30 (S) Disturbed 40Ar/39Ar ages in basalt lavas: Chemical and X-ray computed tomographic (CT) evidence for fluid/basalt chemical interaction Elizabeth Cramer, Sarah Sherlock, Kate Dobson, Alison Halton, Stephen Blake, Tiffany Barry, Peter Lee, Simon Kelley, David Jolley

10:45 Taking geology to the IMAX: 3D and 4D insight into geological processes using micro-CT Kate Dobson, Peter Lee, David Brown, Tim Tomkinson, Elizabeth Cramer, Sarah Sherlock, Chedtha Puncreobutr, Kristina Kareh

11:00 VMSG AGM Andy Saunders

11:15-11:45 Coffee: Recital Room and Lecture Room G12 Poster sessions: Recital Room, Lecture Room G12 and Victoria’s Room 11:45- 12:45 Poster Session A

Magma, Storage and Degassing A (Lecture room, G12)) Geophysics A (Recital Room) Research in Progress and General presentations A (Recital Room)

12:45-13:45 Lunch: Recital Room and Lecture Room G12

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Japan IODP project 13:45 Ultra-deep drilling into arc crust

Yoshihiko Tamura, Yoshiyuki Tatsumi, Osamu Ishizuka, Robert J. Stern, James B. Gill, Julien A. Pearce, Richard Arculus and others

14:00- 15:00 Poster Session B

Physical Volcanology B (Victoria’s Room) Research in Progress and General presentations B (Recital Room) Magma, Storage and Degassing B (Lecture Room, G12)

15:00-15:30 Coffee: Recital Room and Lecture Room G12. VMSG Award Talk: Auditorium: Victoria Rooms 15:30 Behaviour of giant pyroclastic density currents: deductions from deposits

Mike Branney

Supported by 16:30- 17:45 Poster Session C

Geophysics C (Recital Room) Physical Volcanology C (Victoria’s Room) John Guest (Lecture room, G12)

A wine reception will be served during this poster session form the Recital Room and Lecture Room G12. Public Lecture: @Bristol, Harbourside, BS1 5DB Mapping of Volcanic Terrains across the Solar System

Ellen Stofan Please note 100 places have been reserved for VMSG delegates. Tickets will be allocated on a first come, first served basis from the Information Desk.

Wednesday 9th January

Morning session: Lecture Theatre 1: Chemistry Building Session: Physical Volcanology Chaired by: Mike Branney, University of Leicester 9:00 Buoyancy of plume-sourced ash clouds: implications for ash transport modelling

Steve Sparks, R. Baines, Rose Burden, Sam Engwell, Andrew Hogg, Herbert Huppert, Chris Johnson, Jessica Kandlbauer, Jeremy Phillips, Mark Woodhouse

9:15 (S) The duration of volcanic eruptions: controls and forecasts Leanne Gunn, Stephen Blake, Chris Jones, Hazel Rymer

9:30 (S) Climate and carbon cycle response to the 1815 Tambora eruption: pre-industrial vs. future Earth system simulations. Jessica Kandlbauer, Peter Hopcroft, Steve Sparks, Paul Valdes

9:45 (S) Newly discovered components of magmatism from Santorini are revealed during cryptotephra studies of marine cores. Christopher Satow, Emma Tomlinson, Paul Albert, Sarah Collins, Katharine Grant, Sabine Wulf, Luisa Ottolini, Eelco Rohling, Martin Menzies, Simon Blocklet, Vicki Smith, Christina Manning, John Lowe

10:00 (S) The nature and scale of lava-water-sediment interaction: an example from the Fife-Midlothian Basin, Kinghorn, eastern Scotland. Heather Rawcliffe, David Brown, Brian Bell

! ! ! ! ! ! ! !

Scientific Instruments Stafford House Hemel Hempstead +44 (0)1442 233555 www.thermofisher.com Boundary Way HP2 7GE +44 (0)1442 233667 fax United Kingdom Thermo Fisher Scientific is the trading name of Thermo Electron Manufacturing Limited. Registered Office:19 Mercers Row, Cambridge, CB5 8BZ, United Kingdom Registered No.441506 VAT No.827900029

Hermel Hempstead, December 10 2012

OBJECT: Supporting the VMSG 2013 Dear Organizer, We would like to support the VMSG 2013 award and meeting with a contribution of £ 700. Regards Thermo Scientific Team

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10:15 Inclined Vulcanian explosions at Soufriere Hills Volcano: causes and consequences Paul Cole, Adam Stinton, Roderick Stewart, Henry Odbert

10:30 Assigning a volcano alert level: negotiating uncertainty, risk, and complexity in decision-making processes Carina Fearnley

10:45-11:15 Coffee: East Foyer, Chemistry Building Session: Research in progress and general session 2 Chaired by: Lucy Porritt and Anthony Burnham, University of Bristol 11:15 Pyroclastic granulation in explosive volcanic eruptions

Tom Gernon, Martin Wood, Richard Brown, Chris Medlin, Mark Tait, Thea Hincks 11:30 (S) Continental crust formation in the Southern Central Andes: new insights from O and

Hf isotopes in zircon Rosie Jones, Linda Kirstein, Simone Kasemann, Bruno Dhuime, Tim Elliott, Vanesa Litvak

11:45 (S) Melting rocks with magma: Sediment pyrometamorphism as a tracer of magma flow localisation in sills and dykes Clayton Grove, Dougal Jerram, Richard Brown, Jon Gluyas

12:00 (S) MORB-like halogens in basalts of the Azores archipelago Lisa Jepson, Ray Burgess, Vera Fernandes, Chris Ballentine

12:15 (S) Entablature Anne Forbes, Steve Blake, Dave McGarvie, Hugh Tuffen

12:30 Dihedral angles as a proxy for crystallisation times in dolerites Marian Holness & Chris Richardson

12:45 Student Forum Chaired by the VMSG student representative

12:45-13:45 Lunch: East Foyer, Chemistry Building Afternoon session: Lecture Theatre 1: Chemistry Building Session: Geophysics Chaired by: Andy Nowacki and Mark Woodhouse, University of Bristol 13:45 The onset and evolution of pit crater collapse: insights from events at Pu’u O’o,

Hawaii in 2011 Eoghan Holohan, Thomas Walter, Martin Schöpfer, John Walsh, Tim Orr, Mike Poland

14:00 (S) Structures of the rift zone in northern Iceland Sion Hughes & Hazel Rymer

14:15 (S) Volcanic unrest in Kenya: a satellite perspective Elspeth Robertson, Juliet Biggs, Marie Edmonds, Charlotte Vye-Brown

14:30 Magmatic systems within sedimentary basins Nick Schofield, Simon Holford, David Jolley

Session: John Guest Chaired by: Chris Kilburn, UCL 14:45 A tribute to John Guest

Chris Kilburn 15:00 Contrasting lava flow dynamics and morphologies on planetary bodies

Lionel Wilson 15:15 (S) The evolution of volcanism in Syrtis Major Planum (Mars): drawing insight from

terrestrial analogues Peter Fawdon, Matt Balm, Charlotte Vye-Brown, Dave Rothery, Colm Jordan

15:30 – 16:00 Coffee: East Foyer, Chemistry Building

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Session: John Guest Chaired by: Chris Kilburn, UCL 16:00 Mapping of volcanic terrains across the Solar System

Ellen Stofan 16:15 Pulsatory andesite lava flow at Bagana Volcano

Geoff Wadge, Steve Saunders, Ima Itikarai 16:45 John Guest’s legacy on Mt. Etna: a spreading volcano with no chamber

John Murray 17:00 Closing remarks

Andy Saunders, VMSG Chair

Poster Contributions Posters should be displayed throughout the day on Tuesday. Posters have been arranged alphabetically per session. Please note the number of your poster as this refers to your poster board and the letter of your Poster Session (A, B, C). Poster sessions for particular themes have been spilt across Poster Sessions to enable easy access to all posters and to facilitate interaction between researchers. Please be present at your poster during the appropriate session. Magma, storage & degassing: Posters 1-40, Lecture Room G12 Research in progress and general presentations: Posters 41-64, Recital Room Geophysics: Posters 65-86, Recital Room Physical Volcanology: Posters 87-113, Victoria’s Room John Guest: Posters 114-119, Lecture Room G12

Magma, storage & degassing (Lecture Room G12) 1_A (S) Geochemistry of Quaternary magmatism in the Greater Caucasus

Samuel Bewick, Nigel Harris, Ian Parkinson, S. Adamia 2_B Eruption chemistry recorded by accretionary lapilli within pyroclastic density

current deposits, Kilchrist, Isle of Skye, NW Scotland. David Brown, Simon Drake, Andrew Beard

3_A Long-term deformation at Uturuncu volcano in Bolivia and igneous diapirism in the crust Rodrigo del Potro, Mikel Díez, Cyril Muller, Jo Gottsman

4_B Geochemical and petrological analysis of products from the 2011 eruption of Nabro volcano, Eritrea Amy Donovan, Iris Buisman, Clive Oppenheimer

5_A Magma plumbing and degassing during the 2008-present summit eruption of Kilauea Volcano, Hawai’i Marie Edmonds, Tamsin Mather, Rob Martin, Isobel Sides, Richard Herd, Don Swanson

6_B Re-evaluating ‘super’ volcanoes: the case of Yellowstone Ben Ellis, Darren Mark, Michael Rowe, Olivier Bachmann

7_A (S) Hydrothermal cells and the Thermal Boundary Layer around the Cuillin Gabbro, Isle of Skye Fran Entwistle, Bruce Yardley, Adrian Boyce

8_B 2011-2012 eruption of Nyamuragira Marco Fazio, Adrian Jones, Andy Beard

9_A Petrological constraints on deep degassing prior to large basaltic fissure eruptions: CO2 in Laki melt inclusions Margaret Hartley, John Maclennan, Marie Edmonds, Thor Thordarsson, Dan Morgan

10_B (S) Silicic volcanism in the Main Ethiopian Rift: A case study of Alutu Volcano William Hutchison, Juliet Biggs, Tamsin Mather, David Pyle, Gezahegn Yirgu

11_A (S) Modelling of magmatic hydrothermal systems: Phlegrean Fields case study Alia Jasim, Fiona Whitaker, Alison Rust

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12_B Magma mixing, storage and degassing during the 1959 Kilauea Iki eruption, Hawai’i Isobel Sides, Marie Edmonds, John Maclennan, Bruce Houghton, Don Swanson

13_A (S) Post-caldera explosive activity at Furnas volcano, São Miguel, Azores Adam Jeffery, Ralf Gertisser, Brian O’ Driscoll, A. Pimental, José Manuel Pacheco, Stephen Self

14_B (S) Compositional heterogeneity of the Earth’s convecting mantle: constraints from olivine-hosted melt inclusions from a continental flood basalt setting Eleanor Jennings, Sally Gibson, John Maclennan, Robert Thompson

15_A Tracking changes of magma transfer beneath Mt. Etna: Evidence from crystal zonation and real-time gas monitoring Maren Kahl, Sumit Chakraborty, Fidel Costa, Massimo Pompilio, Marco Liuzzo, Marco Viccaro

16_B (S) Timescales of magmatic processes at Mt. Ruapehu, New Zealand: linking mineral diffusion rates to monitoring data. Geoff Kilgour, Kate Saunders, Jon Blundy, Heidy Mader

17_A (S) Disequilibrium during volcanic eruption: effect of cooling rate on plagioclase-liquid element exchange Marthe Klöcking, Marie Edmonds, Madeleine Humphreys

18_B Melting events in the Rum layered intrusion Julien Leuthold, Jon Blundy, Marian Holness

19_A (S) The origin of the Igwisi Hills Kimberlite (Tanzania) constrained from He and O isotopes Freya Marks, Finlay Stuart, Richard Brown

20_B (S) An integrated study of SO2 degassing from Tungurahua volcano, Ecuador Brendan McCormick, Jian Yang, Marie Edmonds, Tamsin Mather, Simon Carn, Silvana Hidalgo, Baerbel Langmann, Michael Herzog

21_A (S) (Mis)understanding bubble growth in magma: Evidence from preserved volatile concentration gradients in glass Iona McIntosh, Ed Llewellin, Madeleine Humphreys, Alain Burgisser, Ian Schipper, Alex Nichols

22_B Granitoid magmatism during continental rifting: preliminary insights from the Oslo Rift, Norway Claire McLeod, Alan Brandon, Reidar Trønnes, Tom Lapen

23_A (S) Norwegian Larvikites: Colours & origin: An oxygen isotope study Ian Meighan, Tony Fallick, Rob Ellam

24_B Exploring volcanic-plutonic connections Vali Memeti & Jon Davidson

25_A (S) Magma storage and differentiation beneath Dabbahu Volcano, Afar, Ethiopia Hilary Milroy, Catherine Annen, Jon Blundy

26_B (S) Constraining the pre-eruptive storage conditions for Pollara eruptions of Salina, Italy Helena Moretti, Jo Gottsman, R. Sulpizio, Jon Blundy

27_A (S) Making granites in the BPIP: How did the evolved rocks in Centre 3, Ardnamurchan form? Joanne Murray, Fiona Meade, Rob Ellam, Val Troll

28_B (S) Degassing of sulphur from sediments in the Siberian Traps Large Igneous Province Svetlana Novikova, Christine Yallup, Marie Edmonds, Alexandra Turchyn, John Maclennan, Henrik Svensen

29_A The post-Minoan plumbing system behaviour at Santorini Volcanic Field: implications for the current unrest phase Chiara Maria Petrone, Lorella Francalanci, George Vougioukalakis

30_B (S) Geochemical evidence for relict degassing pathways in andesite Melissa Plail, Marie Edmonds, Madeleine Humphreys, Jenni Barclay, Richard Herd

31_A (S) Textural variations of groundmass microlites in the 2006 and 2010 eruptive products of Merapi volcano, Indonesia: evidence for magma ascent and shallow conduit processes. Katie Preece, Jenni Barclay, Ralf Gertisser, Richard Herd

16


32_B (S) Reconciling sulphur dioxide emissions from satellite data with petrological volatile data for explosive eruptions of Mount Etna, Italy Lois Salem, Marie Edmonds, Brendan McCormick, Simon Carn

33_A (S) Effects of shear strain on deformation and degassing of three-phase magmas Jessie Shields, Luca Caricchi, David Floess, Heidy Mader, Mattia Pistone

34_B Effect of fO2 on phase relations and sulphur mobility during magmatic differentiation of a basaltic andesite Susanne Skora & Jon Blundy

35_A (S) Timescales of upper crustal residence at Campi Flegrei, Italy Vicki Smith, Kate Saunders, Roberto Isaia

36_B Triggering of major volcanic eruptions recorded by actively forming cumulates on Tenerife Michael Stock, Rex Taylor, Thomas Gernon

37_A The Alpehué Eruption, Sollipulli Caldera, Southern Chile Karen Strehlow, Armin Freundt, Steffen Kutterolf, Julie Christin Schindlbeck

38_B Evidence for an open magma system feeding the compositionally diverse Laacher See (Germany) eruption Emma Tomlinson, Vicki Smith, Martin Menzies

39_A (S) What lies beneath? A Sr and Pb isotope study of intrusive rocks on the Isle of Mull Marie Turnbull, Fiona Meade, G.R. Nicoll, Rob Ellam, Val Troll

40_B (S) Degassing of sulphur from shale adjacent to a dolerite sill in Skye: implications for the volatile budget of large igneous provinces Christine Yallup, Marie Edmonds, Alexandra Turchyn

Research in Progress and general presentations (Recital Room) 41_A (S) MORB like noble gas signatures within Western Antarctic Rift Zone

Michael Broadley, Chris Ballentine, Ray Burgess 42_B The Snap Lake Kimberlite; A true composite intrusion

Richard Brooker, Rachael Ogilvie-Harris, Tom Gernon, Steve Sparks, Matthew Field

43_A (S) Vesiculation of a rhyolitic melt: new insights from hot-stage microscopy experiments John Browning, Hugh Tuffen, Mike James

44_B Trace element systematics of zircon from I- and S-type granites Anthony Burnham, Andrew Berry, Ian Williams, Ryan Ickert

45_A (S) A tale of two magmas: Petrological insights into mafic and intermediate explosive volcanism at Volcán de Colima, Mexico Julia Crummy, Ivan Savov, Daniel Morgan, Marjorie Wilson, Carlos Navarro-Ochoa, Sue Loughlin

46_B Dynamics of deforming partially molten regions and the nucleation of dykes Mikel Diez, Jon Blundy, Andrew Hogg

47_A (S) The Red Hills intrusive system: Easternmost porphyry copper deposit in southwestern North America Amy Gilmer & Richard Kyle

48_B (S) The structure and evolution of shallow magmatic systems emplaces in fold-and-thrust belts – a case study of Cerro Negro, Neuquén Province, Argentina Derya Gürer, Fernando Corfu, Olivier Galland

49_A (S) Water/Rock interaction and volcanic behaviour Brioch Hemmings, Alia Jasim, Fiona Whitaker, Ben Buse, Jo Gottsmann

50_B (S) Experimental insights into the formation of amphibole reaction rims: Texture, mineralogy, and processes of formation Sarah Henton De Angelis, Jessica Larsen, Michelle Coombs, A. Dunn

51_A (S) Changes in heavy metal distribution and deposition at Poás Volcano, Costa Rica Melanie Hinrichs, Hazel Rymer, Steve Blake, Mike Gillman

52_B (S) Platitnum group element geochemistry of the Scourie Dykes: Insights ito the Lewisian subcontinental lithospheric mantle Hannah S.R. Hughes, Iain McDonald, Andrew Kerr

17


53_B Fe3+/ΣFe in hydrous glass Madeleine Humphreys, Richard Brooker, Don Fraser, Vicki Smith

54_A Depositional and textural characteristics of “dry” maar volcanoes in northern Tanzania Hannes Mattsson

55_A (S) Temporal geochemical changes in the Miocene Ignimbrite succession on Gran Canaria: crustal contamination or mantle heterogeneity? Peter Nicholls, Val Troll, Ben Ellis, Abigail Barker, IIya Bindeman

56_B The source of A-type magmas in two contrasting settings: constraints on processes and tectonics from U-Pb, Lu-Hf and Re-Os isotopes. Matthew Pankhurst, Bruce Schaefer, Simon Turner

57_A Sm-Nd and U-Pb isotope geochemistry of the Sweetwater Wash and North Piute plutons, Mojave Desert, California. Stacy Phillips, John Hanchar, Calvin Miller

58_B The lattice strain model applied to coexisting garnet and cpx. Joe Pickles, Jon Blundy, Chris Smith

59_A (S) The role of ice cavities in lave lobe formation Hannah Reynolds, Duncan Woodcock, Jennie Gilbert, Steve Lane

60_B (S) Phreatomagmatic edifices produced by lava-sediment interaction Peter Reynolds, Rich Brown, Ed Llewellin, Thor Thordarsson, Kevin Fielding

61_A (S) Concentrations of critical metals in the Carnmenellis biotite granite, Cornwall, UK Bethany Simons, Jens Andersen, Robin Shail

62_B The nature of deep mantle from Afar plume picrites Finlay Stuart, Nick Rogers, Ian Parkinson, Heather Davies

63_A (S) Recognising mush disaggregation in basaltic systems: The distribution of olivine compositions in Icelandic basalts and picrites Andrew Thomson & John Maclennan

Geophysics (Recital Room) 65_A (S) Loading, compaction and injection: investigating ground deformation on Mt.

Etna’s northeast crater flowfield Alistair Davies

66_C (S) Detection and categorization of geyser eruption dynamics: insights from infrasound monitoring at Yellowstone National Park Philippa Demonte, Jeffery Johnson, Aida Quezada-Reyes

67_A On the lack of InSAR measurements of deformation at Central American Volcanoes Susanna Ebmeier, Juliet Biggs, Tamsin Mather

68_C Hydroacoustic, infrasonic and seismic monitoring of the submarine eruptive activity and subaerial plume generation at South Sarigan May 2010 David Green, Läslo Evers, David Fee, Robin Matoza, Mirjam Snellen, Pieter Smets, Dick Simons

69_A (S) Crustal deformation between volcanic segments of the Askja and Kverkjöll central volcanoes, Northern Iceland Robert Green, Robert White, Tim Greenfield, Jon Tarasewicz, Heidi Soosalu, Janet Key

70_C (S) Local earthquake tomographic imaging of a magma chamber beneath Askja volcano, Iceland Tim Greenfield, Robert Green, Janet Key, Hilary Martens, Michael Mitchell, Robert White

71_C (S) Large-scale ground deformation at Uturuncu volcano: evidence for magma rise from the Altiplano-Puna magma body James Hickey, Jo Gottsmann, Rodrigo del Potro

72_A (S) A comparison of seismically imaged hydrothermal vents with field and laboratory analogues Murray Hoggett, Nick Schofield, Stephen Jones

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73_C (S) Anatomy of the onset of the current repose period at Volcán de Colima during July 2011 Oliver Lamb, Nick Varley, Tamsin Mather, David Pyle

74_A Modes of volcano growth and linkages to sub-volcanic intrusions determined using seismic reflection data from the Ceduna Sub-basin (offshore S Australia) Craig Magee, Esther Hunt-Stewart, Christopher Jackson

75_C (S) Sill geometries in 3D seismic data: Implications for sill emplacement Ben Manton & J. Cartwright

76_A (S) Causes of continuous activity at Arenal volcano, Costa Rica: preliminary results from a volcano-tectonic study Cyril Muller, Rodrigo del Potro, Jo Gottsmann, Juliet Biggs, Mikel Diez, Marino Protti, Gerardo Soto, Waldo Taylor

77_C ARGOS: Geophysical study of Alutu Andy Nowacki, Mike Kendall, Ian Bastow, Matthew Wilks, Juliet Biggs, Atalay Ayele, Shimeles Fisseha, Elias Lewi, Will Hutchison, David Pyle, Friedemann Samrock, Alexei Kuvshinov, Andy Jackson

78_A (S) Seismic and acoustics indices using the registered energies on Tungurahua volcano, Ecuador Pablo Palacios, Heidy Mather, Mike Kendall

79_C (S) Monitoring Cascade volcanoes using InSAR Amy Parker, Juliet Biggs, Tim Wright, Zhong Lu

80_A (S) A photogrammetric feasibility study for DEMs of gulleys in Ecuador Jacqueline Ratner, David Pyle, Tamsin Mather

81_C (S) Decreases in LP seismicity before the May 2011 eruption of the persistently restless Telica Volcano, Nicaragua Mel Rodgers, Halldor Geirsson, Molly Witter, Diana Roman, Peter LaFemina, Angelica Muñoz, Virginia Tenorio

82_A Geodetic data shed light on on-going caldera subsidence at Askja, Iceland Hazel Rymer & Elske de Zeeuw-van Dalfsen

83_C (S) Locating the source of volcanic noise, a picture tells a thousand hertz Elizabeth Swanson, Scase Mathew, David Green

84_A (S) Examining seismic precursors to eruptions at volcanoes in extensional stress fields using an experimental approach Richard Wall, Chris Kilburn, Philip Meredith

85_C (S) Digital mapping of accommodation structures associated with emplacement of the Maiden Creek intrusion, Henry Mountains, Utah Penelope Wilson, Ken McCaffrey, Robert Holdsworth, Jon Davidson, Pamela Murphy

86_A Comparing predictions of an integral model with observations of the Eyjafjallajökull 2010 plume Mark Woodhouse, L. Dowson, Jeremy Phillips

Physical Volcanology (Victoria’s Room) 87_B Dispersal and timing of major eruptive events at Ischia (Italy), insights from

distal tephra records Paul Albert, Emma Tomlinson, Lucia Civetta, Sabine Wulf, Richard Brown, Vicki Smith, Giovanni Orsi, Christine Lane, Martin Menzies

88_C (S) Does volcanology work? Evidence from volcanic fatalities record Mel Auker, Steve Sparks, Lee Siebert, Sian Crosweller, J. Ewert

89_B (S) A glimpse into the future – Earth Science on trial! Richard Bretton, Jo Gottsmann, R. Christie

90_C (S) The structure and emplacement of the Rocche Rosse obsidian lava flow, Aeolian Islands, Italy Liam Bullock, Ralf Gertisser, Brian O’ Driscoll

91_B (S) A statistical method for determining the volume of volcanic fall deposits Rose Burden, Li Chen, Jeremy Phillips

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92_C (S) Ecological impacts of degassing and deposition from recent activity at Volcán Turrialba, Costa Rica Bethan Burson, Rob Martin

93_B Advances in the construction of volcanic records from marine sediment cores: A review and case study (Montserrat, West Indies) Michael Cassidy, Jess Trofimovs, Martin Palmer, W. Symons

94_C (S) Landslide processes at Montserrat, Lesser Antilles, and their implications for tsunami generation Maya Coussens, Peter Talling, Sebastian Watt, Mike Cassidy, Marin Palmer

95_B Interactions between mitigation strategies: implications for the scientific bases of mitigation policy Simon Day & Carina Fearnley

96_C (S) Eruption and depositional facies of the Stob na Doire Ignimbrite Member, Glencoe, NW Scotland: fault-bounded rheomorphic/lava-like and eutaxitic ignimbrite Jonathan Dietz, David Brown, Ross Dymock

97_B (S) Bentonised silicic pyroclastic fall deposits at the base of the Palaeogene Skye Lava Field which possess welded ignimbrite like fabrics Simon Drake & Andrew Beard

98_C (S) The importance of conduit erosion Jonathan Hanson, Alison Rust, M. Pavier, Jeremy Phillips

99_B Modelling disaster risk scenarios at La Soufrière, Guadeloupe Susanna Jenkins, Robin Spence, Peter Baxter, Jean-Christophe Komorowski, Sara Barsotti, Tomaso Esposti-Ongaro, Augusto Neri

100_C (S) Revised estimates for the volume of the Minoan eruption Emma Johnston, Steve Sparks, Jeremy Phillips

101_B (S) Stratigraphy and eruption history of peralkaline welded ignimbrites, Island of Pantelleria, Italy Nina Jordan, Rebecca Williams, Mike Branney, Mike Norry

102_C (S) Drilling into a super-eruption caldera? Initial report of the proximal rhyolites revealed by the Snake River deep drill hole, Idaho Tom Knott, Mike Branney, Marc Reichow, Mike McCurry & the HOTSPOT team

103_B (S) Glacial modulation of eruptive activity at Volcán Sollipulli, Chile Stefan Lachowycz, David Pyle, Tamsin Mather, Katy Mee, J. Naranjo

104_C Development of a database of volcanic ash layers from ocean drilling cores as a record of global explosive volcanism Sue Mahony, Steve Sparks, N. Barnard

105_B (S) Lithofacies architecture of the Stallachan Dubba Ignimbrite Member, Ardnamurchan, NW Scotland: valley-filling ignimbrites and the incursion of pyroclastic density currents into a lake Charlotte McLean, John Buchanan, Peter Reynolds, Peter Nicholls, Ross Dymock, Caroline Patmore, David Brown

106_C (S) The respiratory health hazard of volcanic ash: factors affecting the formation and toxicity of cristobalite Claire Nattrass & Claire Horwell

107_B (S) Exploring the mechanisms of basaltic fragmentation: insights from textural analysis Emma Nicholson, Kathy Cashman, Alison Rust

108_C Deforming a volcano by surface deposit loading: how loading may mislead classic deformation analyses Henry Odbert, Benoit Taisne, Steve Tait

109_B Pele’s tears and spheres – insights into the fragmentation of low viscosity magmas Lucy Porritt, James Russell, Steve Quane

110_C (S) The atmospheric habit of fine volcanic ash Gemma Prata, Benjamin Reed, Tamsin Mather, David Pyle, Dan Peters

111_B (S) The Sky’s the limit: mapping volcano deposits using kites Jonathan Stone, Jenni Barclay, Paul Cole, Sue Loughlin, Peter Simmons

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112_B Causes and effects of variations in groundwater inflows in a Stombolian to phreatomagmatic explosive eruption: the Cova de Paúl Crater eruption on Santo Antão, Cape Verde Islands. Bob Tarff & Simon Day

113_C Towards quantifying the arc-scale and global magmatic response to deglaciation Sebastian Watt, David Pyle, Tamsin Mather

John Guest (Lecture Room, G12)

114_C (S) Constraints on the physical characteristics of volcanic activity on Venus

Martin Airey, Tamsin Mather, David Pyle 115_C Effusive activity at Somma-Vesuvius: lava flow-field characteristics from 1631

to 1944 Sarah Brown, Carmen Solana, Chris Kilburn

116_C (S) Lava channel networks Hannah Dietterich & Kathy Cashman

117_C (S) Kalkarindji – The forgotten volcanic province Peter Marshall & Mike Widdowson

118_C Levee control on the evolution of lava flow fields Marie Nolan, Carmen Solana, Chris Kilburn

119_C Inflation, drainage and lava-water interaction during the emplacement of the Nesjahraun, Iceland John Stevenson, Neil Mitchell, Mike Cassidy, Harry Pinkerton

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22

VMSG 2013 Presentation Abstracts

Oral and poster presentation abstracts are presented in the same order as in the

meeting programme. A complete delegate list follows the abstracts.

23

VMSG 2013 – Oral Presentation Abstracts

Changing the metaphor – from magma chamber to magma

reservoir K.V. CASHMAN*1, G. GIORDANO2

1 School of Earth Sciences, University of Bristol, UK. (*[email protected])

2 Dipartimento di Scienze Geologiche, Univ. Roma Tre, IT.

A long-lived conceptual model for pre-eruptive magma storage envisions long-lived “magma chambers”, or large coherent bodies of magma (with or without included crystals). However, this model is difficult to reconcile with thermal models that suggest that large coherent volumes of eruptible magma should represent transients within the crust, and with new data showing that erupted magma batches may be assembled shortly prior to eruption. Mafic explosive caldera-forming eruptions are particularly difficult to explain by conventional models, because storage of large melt volumes is thermally implausible and explosive ejection of large volumes of low vesicular magma cannot be viewed as solely gas-driven. To explain these eruptions, we suggest instead melt storage within, and eruptions directly from, magma “reservoirs”, where we use the term reservoir in the sense used for water-, oil- and gas-bearing systems, that is, as pockets of liquid contained within a rigid framework, where here the framework is a largely solidified magma body. This model extends the concept of eruptible melt assembled from a rigid sponge to the idea that the sponge itself may feed eruptions. Tapping an over-pressured network of melt pockets within a rigid crystal framework provides an attractive model for several reasons: (1) it does not require a large (thermally and physically unstable) body of molten magma to be assembled prior to an eruption, but instead allows erupted magma to be stored within a thermodynamically stable crystal mush; (2) it allows syn-eruption tapping of large melt volumes from within the reservoir through permeable networks established both prior to and during eruption; (3) decompression of an over-pressured reservoir provides a physical mechanism for magma ascent and eruption that does not rely on ascent-related gas exsolution as the only driving force; and (4) a reservoir model can link the timing of caldera collapse directly to the strength of the reservoir framework. We then speculate on the extent to which a magma reservoir model may explain more general aspects of caldera formation.

Experimental constraints on coupled degassing and

crystallisation at Mount St. Helens J.M. RIKER*1, J.D. BLUNDY1, A.C. RUST1


Degassing and concomitant crystallisation play a major

role in modulating the chemical and physical properties of erupting magmas. Although the compositions and textures of natural volcanic rocks give evidence of varied and complex degassing histories, a limited experimental framework exists within which to interpret such observations. To address this problem, we present results of a series of isothermal experiments on a synthetic Mount St. Helens rhyodacite saturated with H2O and H2O-CO2 fluids. Our runs simulate equilibrium crystallisation driven by volatile exsolution at depths between a mid-crustal magma storage region and the near-surface (400–25 MPa). Fluid compositions range from XH2Ovapour 0.4 to 1.0, as estimated from measured glass H2O and CO2 contents using a compositionally-dependent solubility model. Within this parameter space, the stability of all phases (plagioclase, amphibole, orthopyroxene, Fe-Ti oxides, and rarer apatite and silica) varies as a function of both pressure and fluid composition, such that phase assemblages and proportions depend strongly on melt H2O content. Similarly, glass compositions (69–78 wt% SiO2), plagioclase anorthite content (An52–An33), and total crystallinity (0–44 wt%) can be parameterised as simple functions of pH2O using data derived from experimental run products. Such parameterisations provide a context for comparing the compositional and textural evolution of magmas subject to different gas transport scenarios, including closed-system degassing, isobaric vapour fluxing, and vapour-buffered ascent. For each scenario examined, we evaluate the extent to which magmas evolve along crystallisation trajectories that are distinct from one another and from the simpler case of water-saturated ascent. A preliminary suite of dynamic decompression experiments provides a counterpoint to the equilibrium case. Our results demonstrate the potential of erupted products to reveal information about the composition, quantity, and transport of gases in magmatic systems. They also underscore the importance of coupled degassing and crystallisation in determining petrologic indicators of volcanic processes.

24


Degassing regime of Hekla volcano in 2012

E. ILYINSKAYA*1,2, A. AIUPPA3,4, B. BERGSSON1, T. FRIDRIKSSON5, A.A. OLADOTTIR5, F. OSKARSSON5, K.

LECHNER1, R. YEO1, G. GIUDICE4 1 Icelandic Meteorological Office, Reykjavik, Iceland.

(*[email protected]) 2 British Geological Survey, Edinburgh, EH9 3 LA.

3 University of Palermo, Palermo, Italy. 4 INGV, Palermo, Italy. 5 Iceland GeoSurvey, Reykjavik, Iceland.

Hekla is one of the most frequently active, yet also one of the most unpredictable volcanoes in Europe. It is unknown whether the frequent activity of Hekla during the past century is a beginning of a new eruptive trend. There is also still a significant uncertainty about the depth of Hekla’s magma source, and the intrusion pathways.

In the summer of 2012 we collected an original data set in order to quantify Hekla’s gas emissions during a quiescent interval. The data set includes 1) near-continuous MultiGAS measurements over a 2 months period, 2) quantification of CO2 flux (emitted through diffuse degassing) and 3) direct sampling of gases for compositional and stable isotope analysis.

Our findings show that Hekla’s gas emissions are dominated by CO2, and restricted to the top crater which erupted last in 1981. This is an interesting result as more recently active craters (1991 and 2000) have higher ground temperatures, but no detectable degassing. The carbon isotope signature in the gas is believed to be close to the that of the magma source. This presentation will discuss our results, and the obtained insights into Hekla’s degassing behaviour.

Triple magma batches and a complex eruption history of a

monogenetic volcano: Geochemical analysis of Mt. Rouse, Newer Volcanics Province, Australia

J. BOYCE*1, I. NICHOLLS1, R. KEAYS1, P. HAYMAN1 1 School of Geosciences, Monash University, Clayton, VIC

3800, Australia. (*[email protected])

Mt. Rouse is the largest eruption centre in the Newer Volcanics Province (NVP), a continental intraplate basaltic volcanic field. With triple the magma volume of that erupted from other volcanic centres in the province, Mt. Rouse is a composite volcano of lava and pyroclastic deposits, featuring at least eight eruption points and a lava field extending at least 60 km to the coast.

Monogenetic volcanoes are the most abundant volcano type on Earth and are usually thought to involve single magma batches and simple evolutionary histories. However, recent research reveals that this may not be the case for every volcanic centre. Detailed stratigraphic and geochemical analysis of Mt. Rouse reveals three distinct magma batches and a complex eruption sequence.. There is evidence of both sequential and simultanous eruption of the three magma batches, which differ in chemical composition, ranging from basanites to trachybasalts through to alkali basalts. The southern cone features a basal sequence of batch C then B scoriae and batch A lava before becoming more complex. An unusual proximal surge deposit containing distinct products of two magma batches in the form of Pele’s tears and hair from batch A and scoriae from batch B is found between the basal lava and overlying scoriae of batch B. Most surprisingly, this sequence shows evidence that the three magma batches erupted either from the same vent, or within close proximity to one another, with no evidence of mixing. Through preliminary petrogenetic modelling, it is suggested that the magma batches were sourced at different depths from an enriched mantle source.

The fact that Mt. Rouse has undergone such a complex evolution, along with other recent research into supposedly monogenetic volcanic centres may have implications for petrogenetic modelling in the NVP and the study of other monogenetic fields.

25


Assembling a super-eruption: Linking magma accumulation and

eruption timescales at Toba D.A. BUDD1*, V.R. TROLL1,5, E.M. JOLIS1, F.M.

DEEGAN1,3, V.C. SMITH2, M.J. WHITEHOUSE3, C. HARRIS4, C. FREDA5, D.R. HILTON6, S.A.

HALLDORSSON6 1 CEMPEG, Uppsala University, Sweden. (*[email protected]) 2 Research Lab. for Archaeology, University of Oxford, UK.

3 Swedish Museum of Natural History, Sweden. 4 Dept. Geo. Sciences, University of Cape Town, South

Africa. 5 Istituto Nazionale di Geofisica e Vulcanologia, Italy. 6 Scripps Institution of Oceanography, UCSD, USA.

The Toba caldera located in Sumatra (Indonesia) is the result of the four successive eruptions at 1.2, 0.84, 0.5 and 0.074 Ma [1]. This study presents oxygen isotope data for a suite of whole rocks and quartz crystals erupted as part of the Young Toba Tuff (YTT), an eruption event producing 2,800 km3 of material some 74 ka ago [1, 2]. Oxygen isotope data have been obtained from whole rock (conventional fluorination), single mineral grains (laser fluorination-LF) and in-situ (SIMS) in combination with cathodoluminescence (CL) imaging in order to establish the relative roles of magmatic fractionation, magma-crust interaction and crystal recycling occurring in the Toba magmatic system. The CL images of quartz crystals exhibit defined patterns of zoning that often coincide with fluctuations in δ18O values, allowing correlation of textural and compositional information. Measured δ 18Oquartz values from SIMS and LF range from 6.7 to 9.4 ‰, independent of their position on the crystal. Whole rock values, in turn, range from 8.2 to 9.9 ‰. The δ 18Omagma values calculated from quartz (assuming δ 18Oquartz-magma = 0.7 ‰), suggest a minimum value of 6.0 ‰, similar to that expected from a mantle derived magma [3], and a maximum value of 8.7 ‰. Several quartz crystals, however, have rims with lower δ18O values, suggesting a late, low-δ18O contaminant. This indicates multiple sources to the Toba system, including at least two crustal components, one with high- and one with low-δ18O. Helium isotope data obtained from pyroxenes from the oldest Toba eruption (R/RA = 0.7 and 1.8) are consistent with a significant crustal contribution.

Barometry calculations from feldspar and amphibole suggest the magma chamber system resided at similar depth (~ 10 km) for all four Toba eruptions. The system probably persisted as a crystal mush, which was repeatedly re-mobilised by fresh magma injections. Crystal recycling, consistent with compositional and textural features in most of the YTT quartz crystals, seems an integral part of how super-eruptions are assembled. Therefore, large volumes of isotopically heterogeneous sources were mixed to make the final YTT cocktail, including a late low-δ18O contaminant, substantial high-δ18O crustal contributions, and considerable amounts of recycled antecrysts from the three previous eruptive episodes of the Toba system.

[1] Rose & Chesner (1987) Geology 15, 913-917. [2] Aldiss & Ghazali (1984) J Geol. Soc. 141, 487-500. [3] Taylor & Sheppard (1986) Rev. Min. 16, 227-271.

Shallow storage of dacites beneath Uturuncu volcano, SW Bolivia

D.D. MUIR1*, J.D BLUNDY1, A.C. RUST1 1 School of Earth Sciences, University of Bristol, Wills

Memorial Building, Queens Road, Bristol, BS8 1RJ. (*[email protected])

Volcanic unrest detected with InSAR recently drew

attention to Uturuncu, a dormant dacitic stratovolcano in the Bolivian altiplano that last erupted 271 ka1. Magma intrusion in the mid to upper crust is thought to be responsible for the ~80 km diameter deformation footprint calculated as 1-2 cm/yr central uplift rate between 1992 and 20062.

Over a ~1 million year period dacite lavas and domes have been erupted effusively at Uturuncu with no evidence of explosive activity. Mineral assemblages almost exclusively consist of plagioclase, orthopyroxene, biotite, ilmenite, magnetite and apatite set in a rhyolite glass. Rims and cores of plagioclase and orthopyroxene phenocrysts span a large compositional range from An45-90 and Mg#30-70, respectively. Mean magmatic temperatures calculated from coexisting oxides from 18 samples are 854±50°C. H2O contents of 3.2±0.7 wt% have been measured in plagioclase-hosted melt inclusions using SIMS and CO2 is generally less than 100 ppm. Assuming magmas were volatile-saturated, H2O and CO2 compositions provide trapping pressures of 45 to 107 MPa with xH2O from 0.9 to 1.0 in the fluid phase.

UTDM41B, a microlite-poor dacite with total crystallinity of 37 vol%, a calculated magmatic temperature of 873±13°C and ƒO2 around NNO was chosen as a starting composition for experimental study. Crystallisation phase equilibria experiments were run with the intention of reaching equilibrium conditions from two end-member synthetic starting compositions based on UTDM41B bulk-rock and groundmass glass. Runs were conducted between 50 and 250 MPa, and 750 and 900°C under H2O-saturated and mixed volatile-saturated conditions in cold-seal hydrothermal apparata and all requisite phases were reproduced. At most P-T conditions, either orthopyroxene or biotite are present. However, only at 870°C and pressures shallower than 100 MPa are both phases stable together. Crystallinities of experimental runs are consistently lower than observed in natural rocks which contain a significant proportion of ante- and xenocrysts.

Calculated melt inclusion trapping pressures and experimental findings indicate Uturuncu dacite magmas are stored ≤3 km below surface – much too shallow to produce the broad deformation observed at the surface. If the deformation has a magmatic source it is more likely intrusion of magma into, or movement of magmas out of the Altiplano-Puna Magma Body at ~17 km below surface without concurrent inflation of shallow reservoirs where dacites are stored prior to eruption. [1] Sparks et al. (2008) American Journal of Science 308,

727—769. [2] Pritchard, M, E and Simons, M. (2002) Nature 418,

167—171.

26


The behaviour of a volcanic system with two linked magma chambers

S. BLAKE*1 1 Department of Environment, Earth & Ecosystems, The

Open University, Milton Keynes, MK7 6AA. (*[email protected])

A typical volcanic plumbing system contains a deep

magma storage reservoir which supplies magma to higher crustal levels and the surface. The parameters which control the rates of magma flow, chamber pressurization, and eruption in such a system are studied using a mathematical model of two chambers linked by a conduit. The lower reservoir is hosted in (hot) viscoelastic rock and contains buoyant magma whereas the shallow chamber is hosted in (cold) elastic low-density country rock. The model describes the time evolution of pressure in both chambers, the rates of magma flow into and, during eruption, out of the shallow chamber, and the volumes of magma transferred.

During inflation of the shallow chamber, pressures and flow rates can respond on two timescales controlled by either the elastic properties of the two chambers or the viscosity of the deep country rock. At short (elastic) timescales, the maximum achievable overpressure in the shallow chamber is determined by magma buoyancy in the deep part of the system and the elastic properties and volumes of the two chambers. If this overpressure cannot break open the chamber, then slow viscous relaxation of the deep reservoir’s surroundings drives more magma to the shallow chamber, increasing the overpressure to a level now limited solely by magma buoyancy. The time required to trigger an eruption is influenced by the elasticity of the system, strength of the shallow country rock, buoyancy, and the ratio of the magma and country rock viscosities in the deep part of the system.

During deflation of the shallow chamber, eruption rate is moderated by decompression of shallow magma and influx of deep magma. For large country rock viscosities and small deep reservoirs, the deep supply system behaves elastically and eruption rate falls to zero over time. Lower country rock viscosities around large deep reservoirs allow the deep reservoir to continuously leak magma, prolonging the eruption. Although many basaltic systems appear to operate in a solely elastic regime (e.g., Hawaii), the model suggests that large volume basaltic systems (flood basalts) and some viscous magmatic systems may operate in a regime controlled by viscous deformation of deep country rocks.

Petrological Cannibalism – the chemical and textural consequences

of pulsatory growth of magma bodies

J.D. BLUNDY*1, K.V. CASHMAN1 1 School of Earth Sciences, University of Bristol, Bristol

BS8 1RJ, UK. (*[email protected])

Few magma bodies are assembled in a single magmatic pulse. The growing consensus, based largely on field evidence and geochronology, is that assembly of magma bodies large and small involved emplacement of multiple pulses into an expanding reservoir. Where successive pulses differ significantly in composition there may be clear evidence of mingling and mixing of magmas with contrasted physical properties, including synplutonic dykes and mafic enclaves. Where successive pulses have very similar composition, evidence for interaction between pulses may be cryptic, often limited to subtle textural variations that can be mapped out in the field, but are not evident chemically. Pulsed emplacement leads, inevitably, to fluctuations in temperature as successive pulses re-heat their cooler, partially solidified ancestors. Depending on the size of pulses and the time lapse between them the amount of re-heating may vary from a few degrees to a few hundreds and may involve many cycles of heating and cooling, although these become damped with time as the body grows. These cycles of heating and cooling can generate textures of crystal growth and resorption that cannot be explained by any conventional process of magma differentiation. These textures are especially well-preserved in plagioclase because of slow diffusive re-equilibration of major (but not trace) components. Thermal modeling shows that the size and frequency of pulses is reflected in the zoning styles of crystals, which may provide a powerful tool to retrieve such information. This approach is especially useful when chemical zoning in major and trace elements can be used to reconstruct the chemistry of the various magma pulses to which an individual crystal was exposed and the timing of these pulses prior to eruption. We term the process of physical and chemical interaction between magma pulses and their partially consolidated ancestors “petrological cannibalism” and propose that it can account for a wide range of textural and chemical features seen in magmatic rocks. We illustrate our proposal with field, textural and chemical examples from Mount St. Helens volcano, USA.

27


Plagioclase as recorder of magma-crust interaction beneath the Faroe

Islands B. DAHREN*1, V.R. TROLL2, A. BARKER2, F.C.

MEADE2 C.M. FREDA3 P.M. HOLM4 N. SØAGER4 1 CEMPEG, Department of Earth Sciences, Uppsala

University. Villavägen 16, Uppsala 752 36, Sweden. (*[email protected])

2 Istituto Nazionale de Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy.

3 Geological Institute, University of Copenhagen, Geocenter Copenhagen, DK-1350, Copenhagen, Denmark.

The opening of the North Atlantic in the early

Paleogene resulted in extensive volcanism as evident in now extinct volcanic centres and large basalt piles in e.g. Scotland, Ireland, Greenland and the Faroe Islands. This volcanic region, commonly referred to as the North Atlantic Igneous Province (NAIP) is still highly active on e.g. Iceland and Jan Mayen. The Faroe Islands Basalt Group (FIBG) is itself linked to the early Icelandic hot spot (55 Ma), and decompressional melting, resulting from rifting and the eventual breakup from east Greenland.

The FIBG is up to 6 km thick and is underlain by up to 40 km continental crust intruded by mafic sills, as suggested by geophysical surveys. The exact nature of these continental rocks is unknown, though previous studies have presumed a Pre-Cambrian basement, probably overlain by sediments related to the pre-volcanic rifting. Potential onshore equivalents of the basement rocks may be found in NW Scotland and East Greenland.

Here, we employ multiple geobarometric models coupled with Sr, Pb and Nd isotope signatures in plagioclase crystals to decipher crustal influences in the Faroe basalts. Isotope analyses was performed In-situ (microdrilling) as well as on whole rock and plagioclase separates. The 87Sr/86Sr signatures range between 0.703 - 0.705. Correlation of calculated magma storage depths with geochemical contamination signatures allows us to construct “virtual geochemical boreholes” through the basalts into the underlying crustal basement.

The wide range of the isotopic signatures indicate a complex plumbing system with variable degrees and depth levels of crustal contamination. Using the available data, we attempt to construct the general lithostratigraphy of the sub-basaltic basement from the combined geobarometry and contamination patterns.

A new model for granitic emplacement: The Newry Igneous

Complex, Northern Ireland P. ANDERSON1, C. STEVENSON1, M. COOPER2 , R. ELLAM3, I. MEIGHAN2, C. HURLEY4, J. REAVY5, J.

INMAN1, D. CONDON6, Q. CROWLEY7 1 The University of Birmingham, UK.

(*[email protected]) 2 GSNI, Belfast

3 SUERC, East Kilbride 4 Soil Mechanics, Lisburn, Northern Ireland

5 University College Cork, Ireland 6 NIGL, BGS

7 Trinity College Dublin

The Newry Igneous Complex (NIC) is comprised of three largely granodioritic plutons, together with an intermediate-ultramafic body at its NE end. The recent Tellus survey of Northern Ireland (GSNI, 2007) has highlighted several geophysical anomalies within the complex, including two previously unrecognised concentric aeromagnetic structures. U-Pb zircon ages and a geochemical study suggest that these features represent magmas intruded at different times, and that each pluton was emplaced through a series of inward-younging, concentric pulses.

A combination of anisotropy of magnetic susceptibility (AMS) and field relations were used to investigate the emplacement of these pulses, with a view to linking this with one of the classic emplacement mechanism models (i.e., cauldron subsidence, laccolithic inflation). AMS reveals strong, dominantly oblate, concentric fabrics; whilst field mapping highlights an aereole of deformed host rock surrounding the complex. Such features imply forceful emplacement and are characteristic of laccolithic inflation (but not cauldron subsidence). However, analysis of field relationships suggests that the complex was intruded as steep, concentric, sheet-like pulses, which preclude laccolithic inflation and are consistent instead with cauldron subsidence. Therefore, the evidence fits neither of these classical emplacement mechanism models.

Further investigation of the host rocks has provided what seems to be an appropriate solution. The structure of these suggest that the NIC was intruded into a tension-releasing bend on a strike-slip fault. The local extensional regime resulting from this feature would have created the tectonic conditions and some of the space for intrusion to take place. This occurred as steeply orientated sheets within a SW-migrating weakened zone. However, within sheets themselves magma pressure exceeded local lithostatic pressure, creating further space and causing the surrounding rocks to be deformed. Therefore, the NIC shows that granitic intrusions can show complex emplacement histories, with elements of more than one of the classical models.

Beamish, D., Kimbell, G. S., Stone, P. and Anderson, T. B.,

(2010). J. Geol. Soc. Vol 167, 4, p. 649-657. Geological Survey of Northern Ireland, 2007. The Tellus

project: Proceedings of the end-of-project conference, Belfast,http://www.bgs.ac.uk/gsni/tellus/conference/index.html.

28


Reconstructing the emplacement of the Lago della Vacca complex,

Adamello Batholith, Italy, through field observations, image analysis

and AMS A. SCHÖPA*1, C. ANNEN1, J. BLUNDY1, M. DE SAINT-

BLANQUAT2 AND P. LAUNEAU3 1 School of Earth Sciences, University of Bristol, Wills

Memorial Building, BS8 1RJ Bristol, UK. (*[email protected])

2 Géosciences Environnement Toulouse / Observatoire Midi-Pyrénées, 31400 Toulouse, France

3 Laboratoire de Planétologie et Géodynamique de Nantes, CNRS/Université de Nantes, 44322 Nantes, France

To shed light into the emplacement style and sequence

of the Eocene Lago della Vacca complex (LVC) in the southern Adamello Batholith, we used three different techniques to analyse fabrics in this granitoid intrusion.

First, we measured magmatic silicate fabrics defined by the orientation of hornblende and biotite directly in the field. Second, we used an image analysis software [1] to obtain crystal distribution anisotropies in photographs of rock surfaces taken in the field and of cut rock specimen. Third, we took samples for an anisotropy of magnetic susceptibility (AMS) study to gain insight into the magnetic fabrics of the LVC.

In general, direct field measurements and image analysis results agree well with each other. Planar fabrics prevail indicative of forceful emplacement of the LVC as earlier injected magma pulses would have been deformed by later magma injections. Foliations commonly strike parallel to the eastern and southern borders of the intrusion and dip steeply towards a common midpoint in the northwest.

Although there are discrepancies between the macroscopic silicate fabrics and the magnetic fabrics some similarities are obvious. For instance, macroscopic and magnetic foliations are deflected around the Blumone Complex in line with an emplacement model of lateral magma movement and inflation [2]. This is supported by a higher mean magnetic susceptibility in the vicinity of the gabbroic Blumone Complex, reflecting changes to more mafic granite compositions.

As revealed by AMS and image analysis likewise, linear fabrics dominate in the northwest of the LVC with moderate plunges of lineations to the west. This could mimic magma flow at higher levels of the intrusion not significantly influenced by later injected magma pulses and agrees with the concept that this part of the LVC is close to the source region of the magma.

To sum up, we combined different approaches to obtain fabrics in a silicic intrusion to link them to emplacement modes and magma intrusion sequences. [1] Launeau et al. (2010) Two- and three-dimensional shape

fabric analysis by the intercept method in grey levels Tectonophysics 492, 230-239.

[2] John and Blundy (1993) Emplacement-related deformation of granitoid magmas, southern Adamello Massif, Italy GSA Bull. 105, 1517-1541.

Slurry remobilisation in a layered mafic sill (Franklin LIP, Victoria

Island, Arctic Canada) B. HAYES*1, J.H. BÉDARD2, C.J. LISSENBERG1 , C.D.

BEARD3 1 School of Earth & Ocean Sciences, Cardiff University,

Wales, UK. (*[email protected]) 2 Geological Survey of Canada, Québec City, Canada. 3 School of Earth Sciences, University of Bristol, Bristol,

UK.

The >2500 km Neoproterozoic (~723-716 Ma) Franklin large igneous province (LIP) is extremely well exposed and preserved within the NE-SW trending Minto Inlier on Victoria Island. The magmatic plumbing system is dominated by sills (m’s-100 m’s in thickness), which intruded into the sediments of the Shaler supergroup. These sills were fed by localised feeder dyke systems, and there is unequivocal evidence of syn-magmatic faulting, with the migration of melts and crystal slurries up-section along faults1. Coeval ~1.1 km thick Natkusiak flood basalts overlie the Shaler stratigraphy. Recent trace element and isotopic work demonstrates the linkages between the sills and lavas, as well as revealing internally heterogeneous sills. Thus, we have been able to reconstruct the plumbing system of a LIP.

The sills have been divided into geochemical groups. Group A, the basal sills (below the Minto Inlet formation) are LREE-enriched and are characterized by olivine-cumulate bases, with diabasic roofs. Sills higher up in the stratigraphy (Groups B and C) are typically unenriched and are characterized by porphyritic or sub-ophitic diabasic textures. The Lower Pyramid Sill (LPS), which we have studied in detail here, is an example of one of the LREE-enriched sills. We believe, based on stratigraphic relationships, that this sill extends from its proposed feeder site, the Uhuk massif some ~40 km to the east. The LPS is its distal equivalent, and its gradual thinning from ~40 m at Uhuk to ~21 m at the LPS indicates a propagation direction towards the west.

The LPS shows remarkable layering for a thin magmatic body, being comprised of: (i) a lower chill; (ii) lower border zone (LBZ; olivine gabbro); (iii) olivine-cumulate (OZ); (iv) sub-ophitic diabasic roof (DZ); (v) upper border zone (UBZ; diabase), capped by a chill. The OZ is defined by a jump in olivine Fo and NiO contents above the LBZ, as well as reversely zoned olivine. A thin layer of clinopyroxene-cumulates above the OZ possibly formed from the interaction between evolved melts (sourced from the olivine-slurry?), and the diabasic mush. Based on these relationships, we propose that an initial intrusion of primitive melt (average 12% MgO based on chilled margin analyses) resulted in the formation of a diabasic mush. Replenishment led to the emplacement of an olivine-slurry which underplated the crystallising diabasic mush, forming the OZ. Radiogenic isotopes (207/204Pb/206/204Pb) support this differentiation model, as they show that the LPS is internally heterogeneous and that the OZ and DZ formed from geochemically discrete magmas.

[1] Bédard, J, H., et al (2012) GSA Bulletin, Vol 124, page

723-736.

29


Crystal-melt relationships and the record of deep mixing and

crystallisation in the AD 1783 Laki eruption, Iceland

D.A. NEAVE*1, E. PASSMORE2, J. MACLENNAN1, G. FITTON3, T. THORDARSON3

1 Dept. Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK. (*[email protected])

2 Dept. Earth Science & Engineering, Imperial College, London, SW7 2AZ, UK.

3 School of GeoSciences, University of Edinburgh, Edinburgh, EH9 3JW, UK.

Recent studies in the north and southwest of Iceland

have indicated that the composition of erupted basaltic magmas is controlled by concurrent mixing and crystallisation of variable mantle melts en route to the surface. Careful thermobarometric calculations have constrained the depth of the crystallisation interval to between the mid crust and uppermost mantle. In order to address the question of whether similar processes also take place in the plumbing systems that feed the large fissure eruptions characteristic of the Eastern Volcanic Zone (EVZ) we present the results of an investigation into crystal-melt relationships and deep magmatic processes in the large and environmentally damaging AD 1783 Laki eruption (Skaftár Fires). The collapse of olivine-hosted melt inclusion trace element variablity with progressive magmatic evolution indicates that concurrent mixing and crystallisation has occurred in the deep plumbing system. Plagioclase macrocrysts contain three distinct zones that also preserve a record of melt evolution and variability: high anorthite cores, oscillatory zoned mantles and low anorthite rims. Mineral-melt equilibrium partition coefficients indicate that the rims are in equilibrium with the erupted carrier liquid. High-anorthite cores are more primitive than any other crystal or melt inclusion composition in the magma and cannot be related to the carrier liquid by fractional crystallisation models, which assume a single liquid line of descent. High-anorthite cores may have grown from high Ca/Na melts of the shallow mantle with depleted compositions that have been mixed into the eventual carrier liquid early in the course of magmatic evolution. Furthermore the crystal size distribution (CSD) of plagioclase macrocrysts suggests plagioclase cores represent an accumulated or assimilated population, acting as nucleii for the growth of oscillatory zoned plagioclase mantles. Plagioclase mantle compositions may be related to the carrier liquid by fractional crystallisation models involving eutectic co-crystallisation with clinopyroxene and olivine. Melt barometry indicates that the carrier liquid last equilibrated with plagioclase, clinopyroxene and olivine in the shallow crust at 1-2 kbar. However, clinopyroxene-melt barometry suggests that the bulk of macrocryst growth occurred in the mid crust at 2.5-6 kbar. Much of the macrocryst content of the magma occurs as glomerocrysts, of which only the rims in contact with the groundmass are in equilibrium with carrier liquid. Glomerocrysts therefore formed before rim growth, during the deposition of crystal mushes in the mid crust, which dissagregated on transport to the shallow crust prior to eruption.

Glacial loading probes mantle heterogeneity beneath Iceland

J. MACLENNAN*1, K.W.W. SIMS2, J. BLICHERT-TOFT3, E. MERVINE4, J. BLUZSTAJN4, K. GRÖNVOLD5

1 Earth Sciences, Cambridge (*[email protected]) 2 Geology & Geophysics, University of Wyoming 3 Laboratoire de Géologie de Lyon, ENS Lyon 4 Department of Geology and Geophysics, Woods Hole 5 Institute of Earth Sciences, University of Iceland

Glacial modulation of melting beneath Iceland provides

a unique opportunity to better understand both the nature and length scale of mantle heterogeneity. At the end of the last glacial period, ~13,000 yr BP, eruption rates were ~20-100 times greater than in glacial or late postglacial times and geophysical modeling indicates that rapid melting of the large ice sheet covering Iceland caused a transient increase in decompression mantle melting rates. Here we present the first time-series of Sr-Nd-Hf-Pb isotopic data for a full glacial cycle from a spatially confined region of basaltic volcanism in Northern Iceland. Basalts and picrites erupted during the early postglacial burst in volcanic activity are systematically offset to more depleted isotopic compositions than those erupted during glacial or recent (<7 kyr) times. These new isotopic data, coupled with major and trace element data, show that the mantle underneath northern Iceland is heterogeneous on small (<100 km) scales. The temporal response of the isotopic compositions of the basalts to glacial unloading indicates that the isotopic composition of mantle heterogeneities can be linked to their melting behavior. The geochemical data can be accounted for by a melting model where a lithologically heterogeneous mantle source contains an enriched component which is more fusible than the depleted component.

While spatial variations in the compositions of basalts from mid-ocean ridges, seamounts and ocean islands have previously been understood in terms of such short-lengthscale variations, prior studies have not been able to deconvolve the effects of long wavelength variations in melting process and bulk source composition. This unusual sampling of mantle heterogeneity by glacial unloading provides strong evidence of the role of short- lengthscale mantle lithological variation in controlling the composition of erupted basalts. This finding has important implications for the compositional relationship between basalts and the upwelling mantle: preferential sampling of enriched fusible streaks will bias the isotopic composition of the melt away from the average of the rising mantle. The occurrence of such sampling bias may provide a mechanism for producing long wavelength isotopic features in basalts from long-wavelength variations in the melting process and outflow rather than underlying long-wavelength mantle heterogeneity.

30


Mission Immiscible for two subduction components; evidence from Pagan Volcano, Mariana arc

Y. TAMURA*1, O. ISHIZUKA2, R.J. STERN3, A. NUNOKAWA1, H. SHUKUNO1, H. KAWABATA1, Y.

HIRAHARA 1, Q. CHANG1, T. MIYAZAKI1, J. KIMURA1, R.W. EMBLEY4, S. BLOOMER5, Y. TATSUMI1

1 IFREE, JAMSTEC, Yokosuka, Japan. (*[email protected])

2 GSJ/AIST, Tsukuba, Japan.

3 U. Texas at Dallas, Richardson, USA. 4 NOAA, Newport, USA. 5 OSU, Corvallis, USA.

Pagan is one of the largest (2,160 km3; Bloomer et al., 1989) volcanoes along the Mariana arc magmatic front, but most of the volcano is submarine and unexplored. Bathymetric mapping and ROV Hyper-Dolphin (HPD1147) dive on the NE submarine flank of Pagan were carried out during NT10-12 (R/V Natsushima) in July 2010. There are no systematic differences between subaerial and submarine lavas with > 52 wt % SiO2, suggesting derivation from the same magmatic system. Twenty least-fractionated basalts (48.5-50 wt % SiO2) extend to higher MgO (10-11 wt %) and Mg# (66-70) than subaerial lavas. Compositions of olivine (up to Fo94) and spinels (Cr# up to 0.8) suggest that Pagan primitive magmas formed from high degrees of mantle melting.

Two geochemical groups of clinopyroxene olivine basalts (COB1 and COB2) can be distinguished at similar 10-11 wt % MgO; these erupted about the same time, 500 m apart. Lower TiO2, FeO, Na2O, K2O, incompatible trace element abundances, and Nb/Yb suggest that COB1 formed from higher degrees of mantle melting. In addition, LREE-enrichment and higher Th/Nb in COB2 contrast with LREE-depletion and lower Th/Nb in COB1. Higher Ba/Th and Ba/Nb and lower Th/Nb indicate that main subduction addition in COB1 was dominated by hydrous fluid, whereas that in COB2 was dominated by sediment melt. Sr-Nd-Pb-Hf isotopes are also consistent with this scenario.

Hydrous fluid could not be miscible with silicate (sediment) melt in the depth of the subducting slab below the volcanic front (Kawamoto et al., 2012; Mibe et al., 2011), which might have resulted in two neighboring, but completely different primary magmas. In contrast to Pagan, we observed two primary magmas (COB and POB) in the NW Rota-1 volcano (NWR1), ~40 km behind the volcanic front. NWR1 COB has a greater subduction component, both hydrous fluid and sediment melt, than POB, perhaps reflecting that the subducting slab below NWR1 is > 100 km deeper than that beneath Pagan. At such higher pressures, hydrous fluid and sediment melt could mix into a uniform supercritical fluid (Kawamoto et al., 2012; Mibe et al., 2011), with different proportions yield distinct NWR1 COB and POB (Tamura et al., 2011). Bloomer, S. H. et al. (1989) Bull Volc 51, 210-224. Kawamoto, T. et al. (2012) PNAS in press. Mibe, K. et al. (2011) PNAS 108, 8177-8182. Tamura, Y. et al. (2011) J Pet 52, 1143-1183.

Melting beneath the Izu volcanic arc: Constraints from uranium-

series isotopes H. FREYMUTH*1, T. ELLIOTT1, Y. TAMURA2

1 Bristol Isotope Group, University of Bristol, UK (*[email protected])

2 Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan

Uranium-series isotopes can provide information about

the time-scales of processes in subduction zones such as fluid transfer from the subducted slab to the mantle wedge, melt generation within the mantle wedge and ascent of the melts to the surface. Yet, critical aspects of these isotopic systems are not well understood. Traditionally, 238U excesses over 230Th in arc magmas has been interpreted to reflect the relative affinity of U for an aqueous fluids that transports it into the subarc mantle while Th remains immobile. Recent experimental work (e.g. Klimm et al., 2008) suggest that U-Series disequilibria are primarily controlled by properties of the accessory phases present in the subducting slab during dehydration rather than the fluid. Moreover, recent U-Series models for the Mariana arc (Avanzinelli et al., 2012) infer that some Th is transferred to the arc in the ‘fluid’. These considerations potentially affect conclusions on the time-scales of subduction zone processes. Another uncertainty is the origin of trace element enriched end-members in arc magmas, some of which have Th excess over U. These have so far been interpreted as dominated by sediment melts.

Both of these aspects are addressed by our study of U-Th disequilibria in samples from the Izu volcanic arc. A comparison to other arcs worldwide and in particular to the more extensively studied Mariana arc in the southern part of the Izu-Bonin-Mariana (IBM) arc system shows that the Izu arc is highly depleted in most incompatible elements which allows components derived from the subducting slab to be more clearly identified. Samples from several islands of the Izu arc have large 238U excesses over 230Th, showing that the compositions of magmas from this part of the IBM arc are dominated by fluids derived from the subducting slab. Sediments subducted at the Izu arc segment are fundamentally different to those of the Mariana arc and should lead to lower (230Th/232Th) in the Izu samples. However, the opposite is observed, suggesting that not sediments alone constitute the enriched end-member but that at least for some arc magmas an additional source is needed. This could be upper parts of the oceanic crust that melt together with the sediments at subarc depths.

Klimm, K., Blundy, J.D., Green, T.H.(2008) Journal of

Petrology 49, 523-553 Avanzinelli, R., Prytulak, J., Skora, S., Heumann, A.,

Koetsier, G., Elliott, T. (2012) Geochimica et Cosmochimica Acta 92, 308-328

31


Diatreme volcanism facilitating Pb-Zn mineralisation in the Irish

Orefield? H.A. ELLIOTT1*, T.M. GERNON1, S. ROBERTS1, P.B.

REDMOND2 1 Ocean and Earth Science, National Oceanography Centre,

University of Southampton, Southampton SO14 3ZH (*[email protected])

2 Teck Ireland Ltd., 5 Wentworth Place, Wicklow, Ireland

The Limerick Basin in southwest Ireland is an important element of the Pb-Zn Orefield within the Irish Midlands. Until the recent discovery of a cluster of basaltic diatremes within the Stonepark area, the carbonate hosted Pb-Zn deposits were not thought to be related to Lower Carboniferous volcanics within the area. Thus, the genetic relationship between mineralisation and magmatism is poorly understood and highly controversial. The diatremes emplaced into the Lower Carboniferous carbonate stratigraphy, are attributed to increased heat flow and magmatism during extensional tectonic activity in the Tournasian. The preferred NE-SW trend of diatremes suggests that they might have been fault controlled. In close proximity to the diatremes a series of extra-crater basaltic lava flows and pyroclastics (Knockroe Formation) is commonly interbedded with argillaceous and crinoidal limestones formed in a shallow marine environment (Lough Gur Formation) [1]. The Knockroe sequence was likely sourced from the diatreme eruptions, given the apparent thickening toward the diatremes, and similarities in both composition and texture between the deposits. The large-scale geological relationships, low vesicularity of juvenile lapilli [2] and other deposit characteristics suggest that eruptions were largely phreatomagmatic, shedding volcaniclastic material into a shallow marine setting. Base metal mineralisation occurs as replacement of Black Matrix Breccias (BMB), formed by the passage of hydrothermal fluids through the limestone country rock. The BMB signature mineral phases are calcite, quartz and dolomite. The occurrence of diatreme clasts within mineralised BMB suggests that mineralisation post-dates or is contemporaneous with magmatic activity. The presence of dolomite within the lowest part of the diatremes can be explained by the passing of BMB hydrothermal fluids through the lower sections of the diatremes before entering the adjacent country rock.

[1] Somerville, I.D., Strogen, P., and Jones, G. (1992). Geological Journal, 27, 201-222

[2] Ross, P., and White, J. (2012), Journal of Volcanology and Geothermal Research, 245-246, 55-67

The Comsos Greenstone terrane; insights into a mineralised Archean

arc from U-Pb dating, volcanic stratigraphy and geochemistry

A. DE JOUX*1, T. THORDARSON1 1 School of GeoSciences, University of Edinburgh, UK

(*[email protected])

The Archean Kalgoorlie Terrane, within the Yilgarn Craton, contains several world-class nickel-sulphide ore bodies. The origin of these ultramafic-hosted nickel sulphide deposits remains contentious, particularly in the Agnew-Wiluna Greenstone belt where recent reviews have reiterated a dominantly intrusive origin for thick, nickel-sulphide hosting, ultramafic adcumulate bodies associated with felsic volcanics (1). The Cosmos mine site lies on the western edge of the Agnew-Wiluna Greenstone belt and this previously unstudied mineralised volcanic succession contrasts markedly in age, geochemistry, emplacement mechanisms and probable tectonic setting to that of the majority of the belt (1,2).

The underlying succession to the Cosmos mineralised ultramafic body consists of a complex succession of both fragmental and coherent extrusive lithologies, ranging from basaltic-andesites through to rhyolites, plus later-formed felsic intrusions. The occurrence of thick sequences of amygdaloidal intermediate lavas intercalated with extensive sequences of dacite tuff, coupled with the absence of marine sediments or hydrovolcanic products, indicates that the succession, including the mineralised komatiite lavas, were formed in a sub-aerial environment. Chemical composition of the non-ultramafic lithologies is dominated by a calc-alkaline signature, indicative of a volcanic arc setting. REE data shows that the compositional variability was not achieved via fractional crystalisation alone, and that crustal assimilation and/or different sources must be invoked to explain the observed basaltic-andesite to rhyolite magma suite.

Recent U-Pb dating, undertaken on several samples from various levels within the established stratigraphy, has indicated that the emplacement of the Cosmos volcanic succession took place between ~2739Ma and ~2653Ma, making it significantly older and longer-lived than other dated volcanic successions within the Kalgoorlie Terrane. Extrusive periodic volcanism spanned ~55Ma with three cycles of bimodal intermediate/felsic and ultramafic volcanism occuring between 2739Ma and 2685Ma. Periodic intrusive activity lasted for a further ~32Ma until ~2653Ma.

The age, composition and geochemistry, particularly a lack of TTD affinity, of the intermediate and felsic succession at Cosmos contrasts with the adjacent Agnew-Wiluna greenstone belt (1), indicating the Cosmos succession is a separate, older terrane with a distinct sub-aerial extrusive volcanic succession formed in a long-lived arc setting.

[1] Florentini, Beresford, Barley, Duuring, Bekker,

Rosengren, Cas & Hronsky (2012) Economic Geology, 107, 781-796.

[2] Rosengren, Cas, Beresford & Palich, (2008) Precambrian Research 161, 34-52.

32


Carbonatite genesis: An experimental approach in the

CMASK-CO2 system S. C. MCMAHON*1, M. J. WALTER1, D. K. BAILEY1

1 Department of Earth Sciences, University of Bristol, BS8 1RJ. (*[email protected])

Liquid immiscibility between carbonate and silicate

melts is a possible mechanism for the genesis of carbonatites and associated silica-undersaturated rocks [1]. Compelling textural evidence for liquid immiscibility is displayed by carbonate globules associated with silicate melts of melilititic compositions at numerous carbonatite provinces [2]. These textures include sharp curved menisci against a silicate melt, budding and coalescing of globules, and dumb-bell and amoeboid forms. One possibility is that unmixing occurs as magma evolves in the shallow mantle or crust [3].

However, carbonate globules are sometimes found in deeply sourced mantle xenoliths and as inclusions in xenocrystic olivines with primitive compositions [4]. One such example of these features is at the Calatrava Volcanic Province in central Spain; an alkaline ultramafic province comprising over 250 monogenetic cones and vents [5]. Here, the carbonate ‘globules’ are possible examples of near-primary carbonatitic melts derived at high pressure in the mantle.

Experiments show that immiscibility can occur in partial melts of carbonated eclogite at 3 GPa [6]. To investigate the possibility of a primary origin for carbonatite-silicate melts at the carbonated peridotite solidus, we have conducted piston cylinder experiments in the synthetic system CaO-MgO-Al2O3-SiO2-K2O-CO2 (CMASK-CO2) at 30 kbar (equivalent to ~100km depth). Potassium is a significant constituent at many alkaline volcanic provinces associated with carbonatites, with high contents in unusual rock types such as kamafugites [7]. Given the widespread evidence for the presence of CO2 in the upper mantle [8], an anhydrous system has been investigated to assess the role of CO2 in carbonatite generation. We also test the hypothesis of whether H2O is necessary for immiscibility in the mantle.

Our experiments will track isobaric invariant equilibria where liquid coexists with an assemblage of olivine, garnet, clinopyroxene, orthopyroxene and carbonate. These phase relations can be used to determine the solidus temperature for a variety of model mantle lherzolite compositions, and provide the composition of solidus and near-solidus melts in a carbonate-bearing mantle. Geochemical and textural observations of the experimental products, in comparison with natural rocks, will have wider implications for the genesis of carbonatites and their associated silicate rocks.

[1] Le Bas, (1987), Geol Soc, London, Special Publications 30. [2] Bailey & Kearns, (2012) Mineral Mag, 76(2), 271–284. [3] Lee & Wyllie, (1998) J. Pet. 39(11-12), 2005-2013. [4] Humphreys et al., (2010) Geology 38(10), 911-914 [5] Bailey, K. et al. (2005). Mineral Mag 69, 907-915. [6] Dasgupta et al., (2006) J. Pet. 47, 647-671 [7] Bailey & Collier, (2000) Mineral Mag, 64(4), 675–682. [8] Dalton & Presnall, (1998a). Contrib Mineral Pet, 131(2), 123–135.

Rheology of three-phase magmas M. PISTONE*1, L. CARICCHI2, P. ULMER1, E. REUSSER1,

F. MARONE3, L. BURLINI1, 1 Department of Earth Sciences, ETH-Zurich,

Clausiusstrasse 25, CH-8092, Zurich, Switzerland. (*[email protected])

2 Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, CH-1205, Geneva, Switzerland.

3 Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland.

We present experimental results from a study aiming to

constrain the dependence of rheology of three-phase magmas (ranging from dilute suspensions to crystal mushes) on the viscosity of the suspending silicate melt, on the relative contents of crystals and bubbles and on the interactions occurring between the three phases during deformation. Hydrous haplogranitic magmas containing variable amounts of quartz crystals (between 24 and 65 vol.%), and a fixed bubble volume (9-12 vol.% CO2-rich bubbles) were deformed in simple shear with a Paterson-type rock deformation apparatus at high temperatures (823-1023 K) and high pressure (200 MPa), in strain-rate stepping (5·10-5 s-1 - 4·10-3 s-1) from low to high deformation rate. The rheological results suggest that three-phase suspensions are characterized by strain rate-dependent rheology (non-Newtonian behavior). Two kinds of non-Newtonian behaviors were observed: shear thinning (decrease of viscosity with increasing strain rate) and shear thickening (increase of viscosity with increasing strain rate). Microstructural observations suggest that: shear thinning dominantly occurs in crystal-rich magmas (55-65 vol.% crystals) because of crystal size reduction and shear localization; shear thickening prevails in dilute suspensions (24-44 vol.% crystals) due to outgassing promoted by bubble coalescence. We also propose possible scenarios of shear thickening and shear thinning rheology of magmas ascending within volcanic conduits.

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Disturbed 40Ar/39Ar ages in basalt lavas: Chemical and X-ray

computed tomographic (CT) evidence for fluid/basalt chemical

interaction E.L. CRAMER*1, S.C. SHERLOCK1, K.J. DOBSON2,

A.M. HALTON1, S. BLAKE1, T.L. BARRY3, P.D. LEE2, S.P. KELLEY1, D.W. JOLLEY4.

1 Department of Environment, Earth and Ecosystems, The Open University. Milton Keynes. MK7 6AA. (*[email protected])

2 Manchester X-Ray Imaging Facility (MXIF), School of Materials. University of Manchester, RCaH, Didcot, OX11 0FA.

3 Department of Geology, University of Leicester, University Road, Leicester. LE1 7RH.

4 Geology and Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen. AB24 3UE.

Being able to accurately model fluid migration through

a basalt would be a fundamental step forward in our understanding of post-emplacement effects upon the potential disturbances to K and Ar, and ultimately its effects on the 40Ar/39Ar dating system. Here we explore whether such effects can explain observed intralava 40Ar/39Ar age variations in a suite of lavas from the Faroe Islands.

Vesicular Palaeogene basalts from the Malinstindur and Enni Formations, Faroe Islands were sampled for 40Ar/39Ar dating and textural analysis. Potential reservoirs and pathways for fluids through our selected basalts are interconnected vesicles, most of which appear to be partially or fully filled with various species of precipitated minerals. Using CT imaging, the size, shape and interconnectivity of the vesicles was measured.

Early results suggest a high level of vesicle connectivity, while initial electron microprobe chemical element maps indicate the presence of K within the vesicle fills.

These early results suggest that K-bearing fluids may have led to post-emplacement alteration and will be further examined for information on processes controlling the re-distribution of K and Ar in basaltic rocks.

Taking geology to the IMAX: 3D and 4D insight into geological

processes using micro-CT K. J. DOBSON*1, P.D. LEE1, D.J. BROWN2, T.

TOMKINSON2, E.L. CRAMER3, S.C.SHERLOCK3, C. PUNCREOBUTR4, K.M. KAREH4

1 Manchester X-ray Imaging Facility, School of Materials, University of Manchester, Oxford Rd., M13 9PL. (*[email protected])

2 School of Geographical & Earth Sciences, University of Glasgow, G12 8QQ.

3 Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, MK7 6AA.

4 Department of Earth Science and Engineering, Imperial College London, SW7 2AZ.

Geology is inherently dynamic. Full understanding of

the geological system can therefore only be achieved by considering the processes by which change occurs. Analytical limitations mean that our existing knowledge has been largely developed from ex situ analyses of the products of geological change, rather than of the processes themselves. “Snap shot” sampling using 2D sections taken through 4D systems has limited ability to capture 3D and 4D behaviour. Serial sectioning and experiments quenched at different times can give some insight into the third and fourth dimension, but the true scaling of the processes from generally 2D laboratory to the 4D crust is still poorly understood.

Micro computed tomography (uCT) can visualise the internal structures and spatial associations within geological samples non-destructively, at pixel resolutions from 200 microns down to 50 nanometres. As well as enabling detailed descriptive assessment in 3D, uCT data permits a range of quantitative analyses, and can provide the geometric data for FEM, CFD and other modelling techniques. 3D assessment allows much better understanding of the role of the complex geometries and associations within the samples; but the challenge of capturing the processes that generate these structures remains. To this end we have developed the experimental capability to perform in situ melt and deformation experiments using synchrotron based x-ray tomography to achieve a full 3D data set per second; taking quantification of the igneous system into 4D.

We will present examples from recent work showcasing these capabilities, visualising features such as compositional zonation in feldspars, vesicle bubble structures in low grade ignimbrites, and rarely reported crystalline precipitations in basalt vesicles networks in 3D. We will show results from quantitative mineral mapping, characterisation of crystal and vesicle size distributions, and determination of preferred crystal orientations. Finally, we will present results from in situ high temperature melting and deformation experiments, where we have mapped changing melt and crystal locations, geometries and crystal motions during extrusion. We will discuss how these imaging and quantitative techniques can be applied to the volcanic and magmatic system, and how uCT can begin to record extrusion, flow, crystallisation and deformation in real time.

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Ultra-deep drilling into arc crust Y. TAMURA*1, Y. TATSUMI1, 2, O. ISHIZUKA1, 3, R.J.

STERN4, J.B. GILL5, J.A. PEARCE6, R. ARCULUS7 AND OTHERS

1 IFREE, JAMSTEC, Yokosuka, Japan. (*[email protected])

2 Kobe University, Japan.

3 GSJ/AIST, Tsukuba, Japan.

4 U. Texas at Dallas, Richardson, USA. 5 UCSC, USA. 6 Cardiff University, UK. 7 ANU, Australia.

What is raw and juvenile continental crust? Furthermore, how does it form and evolve into mature continental crust? The continental crust we observe on the surface of the earth has been deformed, metamorphosed, and otherwise processed perhaps several times from its creation in subduction zones to the present. It’s impossible to imagine a wild tuna fish from opening a can of processed tuna; the same might be said about juvenile versus mature continental crust. Although there are many examples of accreted arc crust on the margins of continents, during- and/or post-collision geochemical changes are widespread, and we don’t have the ability to observe active crust-forming processes in modern arcs except by what we can infer from eruptions at the surface, and by remote sensing of arc interiors. “ULTRA-DEEP DRILLING INTO ARC CRUST” is the best way to sample unprocessed juvenile continental-type crust, to observe these active processes that produce the nuclei of new continental crust, and to examine the nature of juvenile continental crust as first generated at intra-oceanic arcs.

Key questions for comprehending arc crust formation are: (1) What is the nature of the crust and mantle in the region prior to the beginning of subduction? (2) How does subduction initiate and initial arc crust form? (3) What are the spatial changes of arc magma and crust composition of the entire arc? (4) How do the middle arc crust evolve? Possible strategies for answering these questions include drilling by IODP at the IBM arc system. IODP has proposals to drill at the IBM, including three non-riser holes (IBM-1, IBM-2 and IBM-3) and one riser, ultra-deep hole (IBM-4), which answer these questions, respectively, and the four drillings result in comprehensive understanding of the arc evolution and continental crust formation. JR drillings at three sites (IBM-1, IBM-2 and IBM-3) are scheduled in 2014.

We had a workshop in Hawaii from September 18 to September 21, 2012, which aimed to gather a wide range of geophysicists, geologists, geochemists and petrologists who are interested in the nature of arc crust and how this is modified in collision zones and preserved in continental crust (http://www.jamstec.go.jp/ud2012/). Our goal has been to discuss the merits, methods and implications of “ULTRA-DEEP DRILLING INTO ARC CRUST” from both thematic (formation of continental crust) and regional (Izu-Bonin-Mariana) scope.

I’d like to show the IODP ‘Project IBM’ and discussion in the Hawaii workshop. The IODP proposals and abstracts of attendees are uploaded in the website (http://www.jamstec.go.jp/ud2012/) and the workshop report will also appear on the web until the end of 2012.

Buoyancy of plume-sourced ash clouds: Implications for ash

transport modelling

R.S.J. SPARKS1, R. BAINES2, R. BURDEN1, S. ENGWELL1, A. HOGG3, H.E. HUPPERT1, C. JOHNSON3,

J. KANDLBAUER1, J.C. PHILLIPS1 AND M. WOODHOUSE3


2 Dept. Infrastructure Engineering, University of Melbourne, Australia.

3 School of Mathematics, University of Bristol, UK. Volcanic plumes ascend high into the atmosphere where they spread out at a level of neutral buoyancy to form intrusions. The structure of these intrusions depends on the relative strength of the intrusions, the ambient wind and the local atmospheric stratification. In a strong wind moderate to weak sized eruptions form bent over plumes. While typically in more powerful eruptions or in eruptions with a weak wind, they form umbrella clouds, which spread in all directions. Irrespective of the wind the plumes eventually reach dynamical equilibrium with the wind field further from the volcanic source. The motion of such plume-fed intrusions is governed by buoyancy. The spreading of intrusions is controlled by the volumetric flux of the feeding plume at the height of neutral buoyancy and the density stratification in the atmosphere, but not by the density of intrusion itself. For the case of a symmetrically spreading umbrella cloud the thickness decreases linearly with distance. Although more complex in detail, intrusions affected by the wind also thin quite rapidly with distance. Buoyancy thinning can explain why ash clouds are observed to become very thin quite close to source. Advection diffusion models are now widely used to forecast ash clouds dispersal and ash deposition. Such models typically assume ash is dispersed vertically above the source and assume ash particles act as heavy (sedimenting) tracers that are spread by atmospheric diffusion. Buoyancy effects are ignored. We contend that such models are not a correct description of the physics of ash clouds in regions where buoyancy effects are significant. For very powerful eruptions buoyancy effects are dominant to distances of hundreds of kilometres or more; it seems unlikely that an advection diffusion model could reproduce observed ash distributions since such models cannot have upwind or very extensive cross-wind spreading. For weaker plumes that are markedly affected by wind, advection-diffusion models are useful mathematical descriptions that can be calibrated to give good forecasts of ash transport. However, this does not mean they are good physical models of the process. Models of buoyancy spreading suggest that it can be the main cause of lateral spreading of wind blown clouds to significant distances (perhaps tens or hundreds of kilometres). Operational models of ash dispersal are likely to remain structured as advection diffusion models, so they may need some empirical adjustments to take account of buoyancy. For example it might be better to have source terms, which assume ash is concentrated in narrow height intervals above the source (at one level rather than distributed vertically). However, it seems unwise to continue using such models for very powerful eruptions.

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The duration of volcanic eruptions: Controls and forecasts

L.S. GUNN *1, S. BLAKE 1, C. JONES2, H. RYMER1 1 Environment, Earth and Ecosystems, The Open University,

Milton Keynes, MK7 6AA. (*[email protected]) 2 Mathematics, Computing and Technology, The Open

University, Milton Keynes, MK7 6AA.

During volcanic eruptions a pressing question is “How long will the eruption last?” A period of continuous magma discharge during basaltic eruptions can range from less than one day to several years. Such a range of possible durations can lead to considerably different consequences that require different responses. A greater understanding of the controlling factors on eruption durations, and the development of a tool to help constrain probable eruptions durations would be a great benefit to emergency response planning and provide insight on sub-volcanic processes.

We present historical datasets of basaltic volcanic eruptions from Iceland, Mt Etna (Sicily) and Piton de la Fournaise (Indian Ocean) and assess variations in eruption durations at and between different volcanoes. Statistical analysis of the data indicates a likely volcano specific control on eruption duration. For example on Iceland, 56 % of basaltic eruptions from volcanic systems situated within the active rift zone have durations of less than or equal to 10 days, whereas 92 % of those outside of this zone last longer than 10 days.

A probabilistic statistical model has been developed to forecast the duration of future volcanic eruptions at the studied volcanoes. The model also forecasts the eventual duration of an eruption which has already been on-going for a known amount of time and the potential use of the model in a real life situation is demonstrated.

Climate and carbon cycle response to the 1815 Tambora eruption: Pre-

industrial versus future Earth system simulations

J.KANDLBAUER*1, P.O. HOPCROFT2, R.S.J. SPARKS1, P.J. VALDES2

1 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, BS8 1RJ, Bristol, UK. (*[email protected])

2 School of Geographical Sciences, University of Bristol, University Road, BS8 1SS. Bristol, UK.

The sulphur released by the 1815 Tambora eruption

and converted to H2SO4 aerosols in the stratosphere resulted in a maximum global cooling of about 1°C in summer 1816. The cold climate was responsible for crop failures, leading to serious famine and high food prices in Europe and Northern America. 1816 became widely known as the ‘year without summer’.

We performed a series of climate simulations with the UK Met Office model HadGEM2-ES to assess if a Tambora eruption in a futuristic high CO2 climate in 2045 (RCP 6.0 radiative forcing scheme) would lead to a different outcome than in a ‘1815-like’ pre-industrial environment.

First results show that in both scenarios the temperature decreases by about 1°C the year after the eruption and global precipitation reduces by about 4% in summer 1816. Interestingly, the vegetation productivity, as well as the different plant fractions do not show any strong relative anomalies between the two scenarios, although the vegetation distribution provides a fairly different initial model situation.

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Newly discovered components of magmatism from Satorini are revealed during cryptotephra

studies of marine cores C. SATOW1, E. TOMLINSON2, P. ALBERT3, S. COLLINS4, K. GRANT5, S. WULF6, L. OTTOLINI7, E. ROHLING5, M. MENZIES3, S. BLOCKLEY1, V. SMITH 8, C. MANNING3,

J. LOWE1

1 Dept. of Geography, RHUL, UK.

(*[email protected]) 2 Dept. Geology, Trinity College Dublin, Dublin, Ireland 3 Dept. of Earth Sciences, RHUL, UK 4 Dept. Earth Sciences, University of Durham, Durham, UK 5 University of Southampton, National Oceanography

Centre, Southampton, UK. 6 Helmholz Centre Potsdam, GFZ German Research Centre

for Geosciences, D-14473, Potsdam, Germany. 7 Institute of Geosciences and Earth Resources, University

of Pavia, Italy. 8 RLAHA, University of Oxford, UK

Marine core tephra deposits have traditionally been used as isochrons to link together environmental records, particularly in the Mediterranean region (e.g. Bourne et al 2010, Albert et al. 2012). Recent advances in detection and extraction techniques have augmented both the number of known tephra layers, and the useful geochemical data which can be extracted from them. Now the tephra layers can contribute invaluably to the construction of volcanic histories. Visible and crypto-tephra layers were extracted from core LC21 in the Aegean Sea and geochemically analysed by EPMA (major elements) and LA-ICP-MS or SIMS (trace elements). Highly precise dating information is derived from the correlation of the core’s isotope stratigraphy to that of the Soreq Cave speleothem in Israel (Grant et al. 2012). The resulting volcanic chronology comprises 14 or 15 eruptions; from Santorini (9 eruptions), Kos/Yali/Nisyros (2 eruptions), Campanian (2 or 3 eruptions) and Pantellerian (1 eruption) volcanic systems, dating from ~166.1ka to the present day. This work demonstrates that:

• Tephra layers in distal settings preserve evidence of eruptions from Santorini that were not previously known from proximal deposits.

• Some of the Santorini magma compositions identified in LC21 are not known from proximal deposits, but are comparable to the compositions of magmas that were present in the upper crust prior to the Minoan eruption (Druitt et al. 2012).

• Tephra deposits in this marine core can date the eruptions with very high precision through a novel dating technique (Grant et al. 2012).

• Relative ages of eruptions from different volcanic sources can be established using tephrostratigraphy.

Albert.P.G. et al. (2012) JVGR 229-230 pp 74-94 Bourne.A. (2010) QSR 29 pp 3079-3094 Druitt.T. (2012) Nature 482 pp 77-82 Grant et al. (2012) Nature in press

The nature and scale of lava-water-sediment interaction: An example from the Fife-Midlothian Basin,

Kinghorn, eastern Scotland H. RAWCLIFFE*1, D. BROWN1, B. BELL1

1 School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ, UK. (*[email protected])

The Carboniferous-Permian Igneous Province of

northern Britain records the complex interplay of competing volcanic and sedimentary systems during rifting. The Early Carboniferous Fife-Midlothian Basin at Kinghorn, in Fife, eastern Scotland, comprises a 485m thick succession of basaltic lavas, interbedded with a variety of siliciclastic and carbonate sedimentary rocks, and volcaniclastic rocks. A range of depositional environments from sub-aerial through fluvio-deltaic to shallow marine/lagoonal have been recognised. The sequence broadly records a marine transgression; however, various uplift and subsidence events and fluctuating accommodation space have controlled the intrabasinal drainage system. In this study, we characterise the processes of lava-water-sediment interaction at both the basin and interface scale.

The lavas were typically emplaced in sub-aerial environments, but the presence of hyaloclastite, locally with pillow fragments, and peperite, indicates eruption into standing bodies of water and/or interaction with unconsolidated wet sediment. The presence of phreatomagmatic lapilli-tuffs and ash aggregates also records interaction of magma with water. Background sedimentation is dominated by siliciclastic input to fluvial and marginal marine environments, but locally, the basin is inundated by pulses of reworked volcanic material.

At the interface between volcanic and sedimentary units, three distinct types of lava-water-sediment interaction have been identified: i) loading/soft-sediment interaction (± pillows/hyaloclastite); ii) “passive” interaction and formation of peperitic margins (± pillows/hyaloclastite), and iii) “aggressive” interaction and formation of “invasive” disaggregated peperite. We record the nature and scale of these interactions and their local digenetic effects.

The accurate characterisation of these domains will aid our understanding of the processes that occur during lava-water-sediment interaction. These data are of particular importance for facies analysis and reservoir characterisation in volcanic rifted margins that are subject to hydrocarbon exploration.

37


Inclined Vulcanian explosions at Soufriere Hills Volcano: Causes and

consequences P. COLE*1,2, 3, A. STINTON2,3, R. STEWART2,3, H.

ODBERT2,4 1 Earth Sciences, Plymouth University, UK

(*[email protected]) 2 Montserrat Volcano Observatory, Montserrat, West Indies 3 Seismic Research Centre, University of the West Indies, 4 School of Earth Sciences, University of Bristol, UK

More than 100 Vulcanian explosions have taken place at Soufriere Hills Volcano between 1996 and 2010 and many of these have been well-observed and documented, with video, thermal video and photography.

Such a rich observational dataset, together with geophysical monitoring data and samples of the products allows detailed analysis of these explosions, including their initial moments. While many of the Vulcanian explosions were relatively vertically directed events, with a quite symmetrical radial distribution of products such as ballistics and pyroclastic density currents (PDCs), several others were associated with initially inclined eruption columns and at least some laterally directed components.

Clear examples of non-vertical explosions occurred in 1996, 2008 and 2010. Documentation of the products of some of these non-vertical events shows that they varied considerably in lithology. However they all have irregularly dispersed ballistic fields and /or PDCs of a range of types. Some explosions have occurred associated with an unloading event, such as dome collapse, where a significant portion of the lava dome was removed over several hours resulting in an asymmetric explosion crater morphology (e.g. 17 Sept 1996 and 11 February 2010).

In all cases crater asymmetry is a critical factor in contributing to the non-vertical nature of the explosion. Preferentially directed hazardous PDCs and /or ballistics are important consequences of these inclined explosions.

Assigning a volcano alert level: Negotiating uncertainty, risk, and

complexity in decision-making processes

C.J. FEARNLEY1 1 Institute of Geography and Earth Sciences, Aberystwyth

University, Llandinum Building, Penglais Campus, Aberystwyth, SY23 3DB, UK (*[email protected])

A volcano alert level system (VALS) is used to

communicate warning information from scientists to civil authorities managing volcanic hazards. This paper provides the first evaluation of how the decision-making process behind the assignation of an alert level, using forecasts of volcanic behaviour, operates in practice. Using ethnographic studies and interviews conducted from 2007-2009 at five USGS managed volcano observatories (Alaska, Cascades, Hawaii, Long Valley, and Yellowstone) two key findings are presented here. First, that observatory scientists encounter difficulties in interpreting scientific data and making decisions about what a volcano is doing when dealing with complex volcanic processes and high levels of scientific uncertainty. Second, the decision to move between alert levels is based upon more than volcanic activity and scientific information, with a complex negotiation of social and environmental risks playing a crucial role. This research establishes that warning systems are complex and non-linear, making decision-making processes problematic in the face of intrinsic uncertainties and risks. A consideration of different social science approaches to risk would, therefore, be beneficial in volcanic hazard management insofar as these suggest effective practices for communicating scientific uncertainty and risk.

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Pyroclastic granulation in explosive volcanic eruptions

T.M. GERNON*1, M. WOOD1, R.J. BROWN2, C. MEDLIN3, M.A. TAIT4 , T.K. HINCKS5

1 Ocean & Earth Science, University of Southampton, SO14 3ZH. (*[email protected])

2 Dept of Earth Sciences, Durham University, DH1 3LE 3 School of Geosciences, Monash University, Australia 3800 4 Rio Tinto Limited, Perth, Western Australia 6000.

5 School of Earth Sciences, University of Bristol, BS8 1RJ. Potassic and ultrapotassic volcanism (e.g. kimberlites,

lamproites) typically involves the formation of diverging pipes or diatremes, which are the locus of high-intensity explosive eruptions. A conspicuous and previously enigmatic feature of diatreme fills are ‘pelletal lapilli’ — well-rounded clasts that consist of an inner ‘seed’ particle with a complex rim, thought to represent quenched juvenile melt (Gernon et al., 2012). Such clasts are widely documented in a range of pyroclastic successions on Earth. New observations of pelletal lapilli in kimberlites show they coincide with a transition from magmatic to pyroclastic behaviour, thus offering fundamental insights into eruption dynamics and constraints on vent conditions. We provide strong evidence that pelletal lapilli form by fluidized spray granulation — a coating process used widely in industrial applications. We propose that pelletal lapilli are formed when fluid volatile-rich melts intrude into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which locally-scavenged particles are simultaneously fluidized and coated by a spray of low-viscosity melt. Most fine particles are either agglomerated to pelletal coatings or elutriated by powerful gas flows. The origin of pelletal lapilli is important for understanding how magmatic pyroclasts are transported to the surface during explosive eruptions. A similar origin may apply to pelletal lapilli in a range of alkaline volcanic rocks including carbonatites, kamafugites and melilitites.

Schematic showing the formation of pelletal lapilli in kimberlite diatremes (Venetia and Letseng, southern Africa). Gernon, T.M., Brown, R.J., Tait, M.A., Hincks, T.K. (2012)

Nature Communications 3, 832.

Continental crust formation in the Southern Central Andes: New

insights from O and Hf isotopes in zircon

R. JONES*1, L. KIRSTEIN1, S. KASEMANN2, B. DHUIME3, T. ELLIOTT3, V. LITVAK4

1 School of GeoSciences, University of Edinburgh, UK (*[email protected])

2 Department of Geosciences, University of Bremen, Germany

3 Department of Earth Sciences, University of Bristol, UK 4 Departamento de Ciencias Geológicas, Universidad de

Buenos Aires, Argentina

Subduction zones, such as the Andean convergent margin, are the main producers of new continental crust via arc magmatism. Arc magmas and hence new continental crust are composed of variable contributions from mantle, crustal and subducted reservoirs. This study aims to investigate how contamination of arc magmas in the southern Central Andes has varied during the Cenozoic. The study area lies within the currently volcanically inactive Pampean flat slab segment (~27º-33ºS) of the Central Andes. During the Miocene the angle of the subducting Nazca plate shallowed, leading to the eastward migration of the magmatic arc and eventual termination of arc magmatism in the Late Miocene [1]. Subduction erosion, tectonic erosion, and crustal contamination have all been highlighed as important processes in the region [2,3,4]. High resolution, in-situ oxygen and hafnium isotope analysis and U/Pb dating has been carried out on magmatic zircon obtained from Late Cretaceous to Late Miocene plutonics and volcanics collected from an east-west transect across the Andean Cordillera. The information obtained from this mineral scale investigation has also been combined with thermobarometry and whole rock major/trace element geochemistry. Mantle-like δ18O(zircon) and εHf(zircon) values suggest the Late Cretaceous to Mid Eocene arc magmatics were derived from mantle melts with little contribution from upper crustal material, either from the subduction of continental crust or from crustal assimilation. Oxygen and hafnium isotope values obtained for Oligocene to Miocene magmatics, combined with the presence of inherited zircon populations, reflects the assimilation of different basement terranes as arc magmatism migrated to the east. This demonstrates varying contamination of arc magmas with continental crust during the Cenozoic in relation to changing subduction zone geodynamics. [1] Pilger, R, H., (1984), Journal of the Geological Society

London, 141, 793 - 802. [2] Stern, C, R., (1991), Geology, 19, 78 - 81. [3] Hildreth, W., and Moorbath, S., (1988), Contributions to

Mineralogy and Petrology, 98, 455 - 489. [4] Stern, C, R., (2004), Rev. geol. Chile, 31, 161-206.

39


Melting rocks with magma: Sediment pyrometamorphism as a tracer of magma flow locallisation

in sills and dykes C. GROVE*1, D.A. JERRAM2, R.J. BROWN1, J. GLUYAS1

1 Durham University, Department of Earth Sciences, South Road, Durham. (*[email protected])

2 DougalEarth Ltd. (www.dougalearth.com).

Petrographic, mineralogical and geochemical data have been used to characterise sill and dyke contacts with aeolian sandstones(the Twyfelfontein Formation, Namibia, 132 Ma (Jerram et al., 1999)). These show a range of contact diagenesis and in extreme cases pyro-metamorphic reactions (pyrometamorphism – contact metamorphism where temperatures are hot enough to induce melting and pyrogenic mineral growth (Grapes, 2010)) Contact effects vary from minor compaction and carbonate cementation, inerpreted to be caused by hot fliuds either from and-or circulated by the intrusion, adjecent to most contacts, to partial melt of arkosic sediment components at sill-dyke junctions or multiple dyke intersections, where heat has been intesified to the point where pyrometamorhism occurs.

The most intense effects are where magma flow has been locallised, such as at a sill-dyke junction. Here, the feldspar component has melted and segregated from the quartz component of the sandstone. Near to the heat source (slower cooling) the melt has crystallised into a potassium feldspar, pyroxene and tridymite (quartz paramorph) matrix. Further away the melt matrix shows less seggregation and has vitrified into a clear glass containing floating detrital quartz rimmed with tridymite paramorphs (like hedgehogs). Further above is a carbonate pore filling cement and both feldspar and quarts granins are present, representing the boiling hydrothermal system.

At dyke convergences, where >3 dykes come together effects are also intensified. Melt and segregation is similar to the above example, but a fluid-fluid like contact exists with the igneous dolerite. A boundary zone is developed in the dolerite that is enriched with quartz and arkosic melt transferred from the sediments.

We infer magma flow locallisation within the magmatic plumbing system from pyrometamorphic intensity. In the case of the converging dykes, a feeder to a vent may have existed above. Grapes, R. (2010) Pyrometamorphism. Springer. Jerram, D., Mountney, N., Holzförster, F. and

Stollhofen, H. (1999) Internal stratigraphic relationships in the Etendeka Group in the Huab Basin, NW Namibia: understanding the onset of flood volcanism. Journal of Geodynamics, 28, 393-418.

MORB-like halogens in basalts of the Azores archipelago

L.D. JEPSON*1, R. BURGESS1, V.A. FERNANDES2, C. BALLENTINE1

1 School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, UK. M13 9PL. (*[email protected])

2 Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany.

The halogens (Cl, Br, I) are moderately volatile

elements that exhibit incompatible behaviour during melting, and are hydrophyllic; iodine is strongly fractionated by biological processes. Although the halogens share similar geochemical properties to the noble gases in many systems, the heavy halogens in particular have been underutilized as tracers, because of the analytical difficulties related to determining their low abundances in geological materials.

A suite of 85 ocean island basalts was collected during fieldwork to the Azores, Portugal in 2011. Halogen compositions have been determined in a sub-set (12) of these basalts, from the islands of: São Miguel, Terceira, Graciosa and Pico. Olivine and pyroxene separates were analysed using an extension of the Ar-Ar method – a neutron-activation technique that allows the halogens to be determined from noble gas isotopes formed during irradiation. The halogens are assumed to be mainly sited in melt inclusions observed within the mineral phases; noble gases were liberated by a combination of crushing and stepped heating. Only bulk halogen ratios are quoted here.

Initial results show that the Azores basalts have a similar range in I/Cl (9.92-253x10-6) as previously reported for MORB, with the Br/Cl values offset to slightly higher values (0.62-4.22x10-3). There appears to be some variation between islands, observed in the I/Cl values: the basalts from São Miguel < Terciera and Pico < Graciosa. This variation is consistent with a SE-NW oriented decrease in age along ~250km of the Terciera Rift.

40


Entablature A.E.S. FORBES*1, S. BLAKE1, D.W. MCGARVIE1, H.

TUFFEN2. 1 Department of Environment, Earth and Ecosystems, The

Open University, Walton Hall, Milton Keynes, MK7 6AA (*[email protected])

2 Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ

Entablature is the term used to describe zones or tiers of

irregular jointing in basaltic lava flows. It has previously been linked to lava-water interactions and forms when water from rivers dammed by the lava inundates the lava flow surface, and in lava-water interactions in subglacial settings. There has been no previous in-depth study of the different types of fractures present in entablature tiers and this is the focus of this work.

A number of different fracture types have been recognised in entablature outcrops of the Þjórsádalur valley, southwest Iceland. These are striae bearing column bounding fractures and pseudopillow fracture systems which themselves consist of two different fracture types: master fractures with dimpled surface textures and subsidiary fractures with curved striae. The interaction of pseudopillow fracture systems and columnar jointing causes chevron fracture patterns commonly observed in entablature. An entablature tier is generally bound by both an upper and lower colonnade of straight, regular, vertical columns.

Closely spaced striae and dendritic crystal textures in the entablature tier indicate rapid cooling. Master fractures show a thin band with an evolved composition at the fracture surface, and mineral textures show evidence of quenching of this material. We interpret this as late stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation, which is then quenched by an influx of water and/or steam when the master fracture fully opens. Master fractures appear to be the main conduit for coolant entering the lava flow during entablature formation.

Dihedral angles as a proxy for crystallisation times in dolerites

M. HOLNESS*1, C. RICHARDSON2 1 Dept. Earth Sciences, University of Cambridge, Downing

Street, Cambridge CB2 3EQ. (*[email protected]) 2 BP Institute, University of Cambridge, Cambridge.

The median of the population of augite-plagioclase-

plagioclase dihedral angles in mafic rocks (Θcpp) varies systematically within dolerite intrusions. Formation of augite-plag-plag three-grain junctions involves the growth of augite into the melt-filled space created by the juxtaposition of two plagioclase grains. Partially crystallised samples from the Kiluaea Iki lava lake shows that three-grain junctions form first where the two plagioclase grains meet at a high angle, while narrower melt pockets tend to cool through the glass transition before the melt crystallizes. Θcpp in the lava lake crust and in other rapidly cooled dolerites is ~78˚, higher than the 60˚ expected if augite perfectly pseudomorphed the melt geometry. In more slowly cooled bodies, augite-plag-plag junctions are not formed by the intersection of two planar augite-plag boundaries, but are curved towards higher angles: Θ cpp > 78˚. In the most slowly cooled dolerites, Θcpp may approach the equilibrium value of 109˚.

The change in junction geometry (and hence Θcpp) with decreased cooling rate is a consequence of a change in the relative growth rate of plagioclase and augite during the last stages of solidification at the three-grain junction.

We measured Θcpp from 10-15 samples across each of a family of dolerite sills ranging in thickness from 40m to 300m. Θ cpp varies smoothly and symmetrically across each sill. The smaller bodies have Θ cpp ~78˚ at the margins, rising to higher values in the centre. The widest bodies show the same pattern of dihedral angle variation but also have high values within a few tens of metres of their margins. There is no straightforward control of sill thickness on average plagioclase grain-size, but the coarsest horizons are stratigraphically above the sill centre in every case.

We constructed simple conductive cooling models, taking into account the latent heat of crystallisation. Θ cpp is a sensitive function of crystallisation time if that time exceeds >10 years. The high marginal angles in thickest sills are caused by localized sub-solidus modification of the grain boundary orientations, enhanced by the relatively fine grain-size of the chilled margins. Because grain-size information is dependent on (poorly known) crystal growth rates for constraining cooling times, Θ cpp is a more reliable proxy for dolerite cooling rates.

The new speedometer is applied to mafic rocks for which the cooling rate is unknown. In the eucritic meteorite Juvinas, Θ cpp = 93 ± 3˚, suggestive of a crystallisation time of order 100 years and a minimum crustal thickness ~70m for the supposed source of Juvinas, the asteroid 4 Vesta. Θcpp can also be used to determine relative intrusion age of Tertiary dykes in the Skaergaard Intrusion (E. Greenland).

41


The onset and evolution of pit crater collapse: Insights from events at

Pu'u O'o, Hawaii, in 2011 E.P. HOLOHAN*1,2, T.R. WALTER1, M.P.J. SCHÖPFER2,3,

J.J. WALSH2, T. ORR4, M. POLAND4 1 GFZ Potsdam, Sektion 2.1, Helmholtzstrasse 7, Potsdam

14467, Germany. (*[email protected]) 2 Fault Analysis Group, UCD School of Geological

Sciences, University College Dublin, Dublin 4, Ireland.

3 Department for Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria.

4 U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii, U.S.A. Deformation is often obscured at active vents. In such

very-near-field settings, the onset and evolution of high-strain deformation processes, such as crater collapse, are particularly ill-constrained. On March 2011, time-lapse cameras deployed at the highly-active Pu'u O'o crater, Hawaii, imaged a major collapse in unprecedented spatio-temporal detail. We analyzed these images with Digital Image Correlation (DIC) techniques to yield a semi-quantitative (pixel-unit) description of the collapse's structural development and associated surface displacements. We then ran numerical pit-crater collapse simulations based on the 2D Distinct Element Method (DEM) to examine how geometric and mechanical factors may influence such observations. The time-lapse images reveal initial stability during lava drain-back, followed by broad sagging of the crater floor, and then collapse controlled by two ring faults. The structural development and surface displacement patterns are best reproduced in DEM models with a shallow, vertically-elongated magma reservoir, and/or with a host-rock that is reasonably strong at depth. This highlights how new tools like DIC techniques and DEM models can help unravel physical processes at active volcanoes.

Structures of the rift zone in northern Iceland S. HUGHES*1, H. RYMER1

1 Ecosystems, & Earth Sciences, The Open University, Walton Hall, Milton Keynes, Bucks, MK7 6AA. (*[email protected])

There is speculation, based on historical ‘twinned’

eruptions and interconnected faults and eruptive fissures that activity at Eyjafjallajökull and neighbouring Katla are linked (Dahm and Brandsdottir, 2007) and there are other suggested ‘paired’ volcanoes further north (Gudmundsson and Hognado ́ttir, 2007). Pressure changes at one of the paired volcanoes may trigger activity at the other (Andrew and Gudmundsson, 2008; Einarsson, 2008), although geochemical evidence suggests the magma systems are not materially connected (Sturkell and Sigmundsson, 2000).

The focus of this project is on the northern part of the Icelandic rift zone - from Vatnajokull ice cap in the south, across Askja and Upptyppingar volcanoes, and north towards Krafla volcano - because of the recently identified magma movements there and the implications for future eruptive activity.

The aim of this work is to discover how magma is transported and stored at shallow depth beneath the NVZ in Iceland. Seismic studies indicate that there may be several magma sources even within a single volcanic system in Iceland’s NVZ (Soosalu et al., 2009). This work also directly addresses the question of whether separate volcanic systems are linked and, if so, at what depth.

The centre of the Askja caldera is sinking (at about 2-5 cm per year for the last 40 years) and recent modelling suggests that the long-term trend (>20 years) of magma drainage from beneath this caldera recently reversed, possibly indicating magma intrusion (Rymer et al., 2010). Over the same recent period, nearby dormant Upptyppingar volcano has become seismically active (Jakobsdottir et al., 2008). The question of where the magma is coming from and going to, and its ultimate fate naturally arises.

Results of recent microgravity and Bouguer gravity work will be presented here.

Andrew R. E. B. & Gudmundsson, A. (2008) J. Volcanol.

Geotherm. Res. 177: 1045-1054 Dahm, T. & Brandsdottir, B. (2007) Geophys J Int.: 130:

183-192. Einarsson, P. (2008) Jokull 58: 35-58 Rymer, H., Locke, C.A., Ófeigsson, B. G., Einarsson, P &

Sturkell, E. (2010) Terra Nova 22(4): 309-313. Jakobsdottir, S.S. et al., (2008) Stud. Geophys. Geod. 52: 513-528

Soosalu et al., S. (2009) Bull. Volcanol. doi:10.1007/s00445-009-0297-3

Sturkell, E. & Sigmundsson, F. (2000) J. Geophys. Res. 105: 25671-25684.

42


Volcanic unrest in Kenya: A satellite perspective

E. ROBERTSON*1, J. BIGGS1, M. EDMONDS2, C. VYE-BROWN3

1 School of Earth Sciences, Univeristy of Bristol, Bristol, BS8 1RJ, UK (*[email protected])

2 Department of Earth Sciences, Univeristy of Cambridge, Downing Street, Cambridge, CB5 3EQ, UK

3 British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK

The East African Rift (EAR) system is a 5,000 km long

series of fault bounded depressions that run from the Red Sea to Mozambique. The use of InSAR throughout the East African Rift has led to the discovery that a number of volcanoes, which haven’t erupted in human history, are actively deforming. In Kenya, the EAR hosts 14 Quaternary volcanoes that lie along its central rift axis. An initial InSAR study, covering the period 1997-2008, discovered that four volcanoes underwent geodetic activity during this time.

We present results from a long-term ENVISAT and ALOS PALSAR monitoring campaign of Kenyan Rift volcanoes. Using multi-temporal interferograms, we construct time series of ground deformation. We discover that two Kenyan volcanoes, Silali and Longonot, are undergoing slow subsidence, at mm rates which single interferograms are unable to detect. We also discover that a third volcano, Paka, has undergone a second period of uplift between 2009-2010 at a rate of 1.6 cm/year.

Satellite-based observations play an important role in monitoring and assessing volcanic hazard in this remote region. There is a limited historical record of volcanic activity in Kenya and detailed eruptive records are understudied. We present a remote-sensing study to investigate the temporal and spatial development of volcanic activity at Longonot volcano. Through high-resolution mapping using ASTER, SPOT5 and GDEM imagery, we identify boundaries of eruptive units and begin to establish Longonot’s eruptive history. Understanding the source of the geodetic signal, which may be magmatic, hydrothermal or a combination of both, using both geological and geophysical techniques will aid interpretation of InSAR signals observed elsewhere in the East African Rift and guide future hazard studies across Kenya.

Magmatic systems within sedimentary basins

N. SCHOFIELD*1, S. HOLFORD2, D. JOLLEY3 1 Earth Sciences, University of Birmingham

(*[email protected]) 2 Australain School of Petroleum, Adelaide.

3 Geology and Petroleum Geology, University of Aberdeen Discerning how magma moves around the subsurface,

and its connection too sub-aerial volcanic systems is integral to our understanding of how volcanic systems evolve. Our current understanding of magma transit through the crust is mainly based on petrological and geochemical methods. These methods have given us great insight, but are unable to give exact geometries of such systems; additionally they also, in general, cannot take into account movement of magma laterally through the crust.

The use of 3D oil industry seismic data has yielded incredible insight into understanding such systems and the ever increasing amount of seismic data available in frontier basins containing volcanics means that it is now possible to start to understand magma movement within the subsurface, and its connection to extrusive volcanism.

We present a series of examples from basins around the world, from both field and seismic data, illustrating key aspects of magma flow and eruption within a sedimentary basin.

43


Introduction to John Guest Tribute C.R.J. KILBURN*1

1 Aon Benfield UCL Hazard Centre, Dept of Earth Sciences, UCL, Gower St, London WC1E 6BT, U.K. (*[email protected]) This special session is a tribute to the legacy of John

Guest (1938-2012). John was a pioneer in volcanology and planetary science. He was among the generation of scientists who recognised the importance of transforming volcanology into a physical discipline and vigorously promoted advances in quantifying the effusive behaviour of volcanoes. In 1973, he led the UK contribution to a ten-year Anglo-Italian project to investigate the evolution of Mount Etna. The collaboration produced the modern geological map of the volcano and launched a new understanding of Etna’s magmatic feeding system and eruptive dynamics. He became one of the first non-US citizens on NASA’s programme for planetary exploration and joined the science teams for missions to Mercury (Mariner 10, launched in 1973), Mars (Viking, 1975) and Venus (Magellan, 1989). He also engaged with the Soviet Phobos mission to Mars (1988). In 1980, he founded at University College London the first NASA Regional Planetary Image Facility outside the USA. For his contributions to planetary geology, he received the 1991 G.K. Gilbert Award from the Geological Society of America; in the same year, asteroid 1982 HL was named Guest by the International Astronomical Union. An extended obituary has been published online by the Geological Society of London at: https://www.geolsoc.org.uk/en/About/History/Obituaries%202001%20onwards/Obituaries%202012/John%20Edward%20Guest.

Contrasting lava flow dynamics and morphologies on planetary bodies

L. WILSON*1 1 Lancaster Environment Centre, Lancaster University,

Lancaster LA1 4YQ. (*[email protected])

Lava flow morphology is dictated by magma composition (hence rheology), gas content (explosive or purely effusive at the vent), and source conditions (volume available for eruption; mass or volume discharge rate) and by the planetary environment into which an eruption occurs (atmospheric pressure; acceleration due to gravity). Even for the same broad composition (mafic to ultramafic) expected for the majority of magmas erupted on bodies in our Solar System (Earth's unique subduction zone magmas excepted), the environmental conditions can lead to strikingly different products. The Dawn mission to the asteroid 4 Vesta, now essentially confirmed as the source of the igneous HED meteorites, together with observations from MESSENGER in orbit around Mercury, have promped an ongoing reappraisal of what we expect to see and what we actually see as regards lava flow morphologies (Wilson and Head, 2012; Wilson and Keil, 2012) and the consequences of lava flows (Hurwitz et al., 2012 ) on planetary bodies.

Here I summarize some of the more important systematic results. Every object in the Solar System exhibiting mafic volcanism (now or in the past) has a smaller value for the acceleration due to gravity than Earth (granted only slightly so in the case of Venus) and lacks/lacked plate tectonics. Crustal stresses and magma buoyancy both scale with gravity but rock strength does not. Together these factors allow the pre-eruption accumulation of larger volumes of melt than on Earth and encourage greater eruption rates, thus explaining the presence of extensive flood lavas on the Moon, Mercury, Mars and Io. The greater thicknesses of flows on the surfaces of low-gravity bodies encourage heat retention which would itself imply greater flow lengths, other factors being equal. However, provided flows are not volume-limited, maximum flow lengths involve a balance between time for heat loss and time for travel, and flow speed itself depends on flow thicknesss and gravity. I will discuss how these inter-relationships play out in practice.

Hurwitz, D.M., Head, J.W., Wilson, L. and Hiesinger, H.

(2012) J Geophys Res - Planets 117 E00H14 Wilson, L. and Head, J.W. (2012) Volcanic eruption

processes on Mercury. Lunar Planet. Sci. XLIII, #1316. Wilson, L. and Keil, K. (2012) Volcanic activity on

differentiated asteroids: a review and analysis. Chemie der Erde (in press)

44


The evolution of volcanism in Syrtis Major Planum (Mars): Drawing

insight from terrestrial analogues P. FAWDON*1, M.R. BALME1, C.L. VYE-BROWN 2, D.A.

ROTHERY1, C.J. JORDAN3. 1 Department of Physical Sciences, The Open University,

UK (*[email protected]) 2 British Geological Survey, Edinburgh, UK 3 British Geological Survery, Keyworth, UK

The Syrtis Major Planum is an Hesperian (3.7 – 3.0 Ga) low-angle basaltic plains volcano on Mars. The edifice is 1500 km by 1100 km, covers 3.6% of the martian surface and has been poorly studied relative to other large martian volcanic terrains. This basaltic complex is dissected by extensional and compressional fault systems and has two distinct central calderas believed to contain evolved volcanic products. It is emplaced on, and fringed by, highland cratered terrain containing sites of astrobiological significance1.

We apply a remote sensing approach using modern NASA and ESA data sets of Mars and terrestrial analogue data sets in Afar, Ethiopia and Krafla, Iceland to analyse surface morphology and composition. We investigate the volcanic evolution of Syrtis Major Planum using the 3D visualisation software Geovisionary™ to build an architectural model of the evolution of the Syrtis Major volcanic complex.

We present initial results of our investigation into the rheological properties of the lava flows on [1] the flanks of Syrtis Major and [2] terrestrial lavas from Dabbahu (Afar) and Krafla (Iceland). These results are derived from morphometric measurements on high resolution (1 m/pix) DEMs, and a simple rheology model2. We use calculated yield strength, viscocity and effusion rate to investigate eruption conditions, melting and upper mantle conditions during the emplacement of Syrtis Major. We will also consider, with suitable environmental scaling, rheology derived using the same methrod of terrestial analogues to compare with the results from Syrtis Major.

Additionally we present an update on our high resolution mapping at ~15 m/pix, subdividing the previously-recognised unit Hs of Syrtis Major ridged plains3. Within this unit we identify: [1] lava flows 1-6 km wide, 5-30 m thick with flow field lengths up to 500 km that are distributed radially with respect to the central caldera complex.; [2] lava flows with distinct night/day thermal properties cross-cutting Hesperian ejecta containing contributory sinuous channels that might be fluvial in origin; and [3] ‘wrinkle-ridges’ – tectonic features concentric and radial to the central caldera complex; and [4] heavily eroded or buried ‘ghost’ craters. These structural features and DEM analysis1 indicate a total lava thickness of ~500m.

[1] Hiesinger, H., Head, J.W., et al. (2004) J Geophys Res

104 E01004. [2] Moore, H.J., Arthur D.W.G., et al. (1978) Proc Lunar

Planet Sci Conf, 9th 3 3351-3378. [3] Greeley, R., Guest, J.E. (1987). Geologic map of the

eastern eastern equatorial region of Mars. USGS.

Mapping of volcanic terrains across the solar system:

A tribute to John Guest E.R. STOFAN*1

1 Proxemy Research, PO Box 338 Rectortown VA 20140 USA (*[email protected])

The Mariner missions to the Moon, Mars, Venus and

Mercury were truly missions of discovery, with great debate in the scientific community over the impact vs. volcanic origin of much of the surfaces of these bodies. The initial group of geologists who interpreted these images, including Don Wilhelms, John Guest and Ron Greeley, pioneered the use of terrestrial geologic mapping techniques to interpret these alien surfaces. Recognizing that stratigraphic principles could be applied all over the solar system, John Guest produced maps of the surfaces of Mercury, the Moon and Mars that are still in use today (e.g., Guest and Greeley, 1983; Greeley and Guest, 1987).

Guest applied his knowledge of mapping of terrestrial volcanoes, from Chile to Italy to Hawaii, to provide in depth interpretations of volcanic processes across the solar system. His work on volcanism on Venus (e.g., Guest et al., 1992; Guest and Stofan, 1999) demonstrated that venusian volcano morphology can be used to discern the plumbing systems and evolution of these features, and what they imply for the interior evolution of a terrestrial planet. Guest focused on comparative planetology: how can we use what we learn from studying multiple planetary surfaces to better understand the physical process of volcanism.

Guest, J.E. and R. Greeley, Misc. Inv. Series I-1408, USGS,

1983. Guest, J.E. et al., J. Geophys. Res., 97, 15949-15996, 1992. Guest, J.E. and E.R. Stofan, Icarus, 139, 55-66, 1999. Greeley, R. and J.E. Guest, Misc. Inv. Series, I-1802-B,

USGS, 1987.

45


Pulsatory andesite lava flow at Bagana Volcano

G. WADGE*1, S. SAUNDERS2, I. ITIKARAI3 1 University of Reading, UK. (*[email protected]) 2 Rabaul Volano Observatory, Papua New Guinea.

3 Rabaul Volcano Observatory, Papua New Guinea. Using a time series of TerraSAR-X spaceborne radar

images we have measured the pulsatory motion of an andesite lava flow over a 14-month period at Bagana volcano, Papua New Guinea. Between October 2010 and December 2011, lava flowed continuously down the western flank of the volcano forming a 3 km-long blocky lava flow with a channel, levees, overflows and branches. We captured four successive pulses of lava advancing down the channel system, the first such behaviour of an andesite flow to be recorded using radar. Each pulse had a volume of the order of 107 m3 emplaced over many weeks. The average extrusion rate estimated from the radar data was 0.92 ± 0.35 m3 s-1

, and varied between 0.3 and 1.8 m3 s-1, with higher rates occurring earlier in each pulse. This, together with observations of sulphur dioxide emissions, explosions and incandescence suggest a variable supply rate of magma through Bagana’s conduit as the most likely source of the pulsatory behaviour.

John Guest's legacy on Mt Etna: a spreading volcano with no magma

chamber JOHN MURRAY*1,

1 Dept. Environment, Earth & Ecosystems, The Open University, Milton Keynes MK7 6AA. (*[email protected])

The present ground deformation network on Etna is

providing much of the information on how this volcano functions and why. Commissioned by John Guest and first established in 1975 as a precise levelling traverse 11 km long across the summit of the volcano, the present network covers the entire volcanic edifice and comprises 76 km of interconnected precise levelling lines, more than 100 dual-frequency GPS benchmarks, and 27 dry tilt stations.

Throughout the 38-year period of monitoring, major flank eruptions have consisted of dyke injection episodes, in which a subsiding trough radiates from the summit to the eruption site, with concomitant outward radial movement of the lower flanks.

Between flank eruptions, a similar but distinctly different type of movement takes place. The top 700 metres of the volcano moves towards the sea at rates of about 1 to 3 cm per year, and the flanks spread outward but not quite radially, those on the western upslope side having a tendency to spread away from a line between the summit and the town of Bronte to the east.

Attempts have been made by several workers to model these inter-eruptive movements as inflations and deflations of a subsurface magma chamber, but the modelled amount of volumetric expansion is much smaller than the volume of lava erupted. On Etna there is also little or no subsequent radial contraction that elastic models predict when a magma chamber drains during an eruption. Another peculiarity of deformation between eruptions is that the easterly outward movement increases with distance from the summit, contrary to syn-eruptive dyke injection. Furthermore, different eruptions and different methods give a huge spread of depths for a postulated magma chamber, from 2 km to 30 km in depth.

Analogue modelling has provided new insight into this unusual behaviour. Scaled models of brittle cones over an inclined ductile basement show that the pattern of gravitational spreading can be quite different from the radial movement of a level basement. For a basement slope of 1°, the direction and amount of movement closely resembles inter-eruptive deformation at Mt Etna, with the summit moving downhill, lateral spreading occurring on the upslope side, and movement greatest on the lowest downslope flanks.

This evidence therefore points to the fact that Etna has no magma chamber, but that magma rises in discrete pulses directly and rapidly from depth, resulting in dykes that reach the surface and erupt, but remain filled with magma that solidifies over a period of weeks and months. This conclusion is in agreement with recent data from seismic tomography and magma residence times.

46

VMSG 2013 – Poster Presentation Abstracts

Geochemistry of Quaternary magmatism in the Greater

Caucasus S. BEWICK*1, N. HARRIS1, I. PARKINSON1, S. ADAMIA2

1 Earth and Environmental Sciences, Open University, Milton Keynes, MK7 6AA. (*[email protected])

2 M. Nodia Institute of Geophysics, 1/1 M. Alexidze str., 0171, Tbilisi, Georgia

Collision of the Arabian-African and Eurasian plates in

the Early Miocene resulted in the final closure of the Tethys Ocean, crustal thickening, uplift, and magmatism which formed the present day Caucasus orogenic belt [1]. Throughout the Quaternary voluminous magmatism formed the Greater Caucasus, the highest peaks in Europe. Although there are several geochemical studies into related magmatism on the East Anatolian plateau (Turkey) and the Lesser Caucasus (Armenia and Georgia) [2], the geochemistry, and petrogenesis of Greater Caucasus volcanism has not been focus of a modern, systematic geochemical study.

Calc-alkaline to subalkaline magmatism, evolving from basaltic-andesites to dacites and minor rhyolites have erupted from the Keli-Kazbegi volcanic centre in northern Georgia throughout the upper Pliocene and Holocene [1, 3]. Despite the eruptions occurring on the thickest continental crust in the region [1], it has been suggested the source is subduction-enriched lithospheric mantle, with insignificant input from the continental crust [4].

Preliminary results are presented for the petrography and geochemistry from a small selection of samples from across the same region. Major and trace element bulk rock data are compared to the limited data from the covering the Lesser Caucasus and eastern Turkey [4]. Also presented are mineral analyses for Quaternary volcanics from the Greater Caucasus. Of particular interest is a pyroxene phase present in some basaltic-andesite samples that contains up to 17wt% Al2O3, as well as being rich in TiO2.

The underlying objective of this work is to compare petrogenic models for Quaternary magmatism in the Greater Caucasus with that of the Anatolian plateau, and to integrate these findings into a tectonic model for post-collision orogeny in the region. [1] Adamia et al. (2011) Turkish J. Earth Sci. 20, 1-57 [2] Keskin et al. (1998) J. Volcanol. Geotherm. Res. 85,

355-404 [3] Lebedev et al. (2008) Dokl. Earth Sci., 418, 169-173 [4] Tutberidze (2012) Turkish J. Earth Sci. 21, 799-815

Eruption chemistry recorded by accretionary lapilli within

pyroclastic density current deposits, Kilchrist, Isle of Skye, NW Scotland

D. BROWN*1, S. DRAKE2, A. BEARD2 1 School of Geographical and Earth Sciences, University of

Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ, UK (*[email protected])

2 Department of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, London, WC1E 7HX, UK Accretionary lapilli evolution remains controversial.

Based on field, modelling and rare geochemical studies most workers argue for formation and growth within an eruption plume, although recent studies suggest formation within a pyroclastic density current. Here we adopt an integrated field and geochemical approach to test existing models of formation. A sequence of massive lapilli tuffs and breccias from the Isle of Skye, NW Scotland, record deposition from the sustained passage of a pyroclastic density current. Accretionary lapilli in two vertically chemically zoned massive lapilli tuff units within this sequence display evolving chemical trends from core to rim. The accretionary lapilli formed initially as basaltic ash pellets in a co-ignimbrite plume before falling under gravity into the underlying density current, where they accreted progressively more evolved laminae. These laminae record reverse tapping of a magma chamber with sequential pulses of basaltic andesite, andesite and dacite magma being fed into the density current. Accretion of laminae was initially supported by available moisture and turbulence, but growth ceased with the input of dacitic magma and drier conditions. The accretionary lapilli were formed within a sustained pyroclastic density current, which demonstrates that accretionary lapilli are not necessarily formed by simple fallout from an ash plume. Our model demonstrates that accretionary lapilli may be used to trace physical and chemical characteristics of complex eruptions, and understand the magmas which fed those eruptions.

47


Long-term deformation at Uturuncu volcano in Bolivia and

igneous diapirism in the crust R. DEL POTRO*1, M. DÍEZ1, C. MULLER1, J.

GOTTSMANN1 1 University of Bristol, School of Earth Sciences, BS8 1RJ,

Bristol, UK. (*[email protected])

The presence of partial melt in the Earth’s crust causes a decrease in density, and hence a density contrast, that generates a potential field anomaly. Gravimetric techniques can quantify such an anomaly and invert its signature to produce a subsurface density distribution model. Here, we image 15 km-wide, low-density, vertically-elongated 3D structures in the mid-upper crust of the Central Volcanic Zone of the Andes in southern Bolivia. These bodies are massively rooted at the Altiplano-Puna Magma Body (APMB), and connect it with shallower crustal levels beneath the largest Neogene ignimbrite province, the Altiplano-Puna Volcanic Complex (APVC). Petrological constraints strongly suggest the presence of a melt fraction as the cause of the mass deficiency at depth. Moreover, the partially molten body beneath Uturuncu volcano has caused at least 1 meter of ground uplift in no more than 45 years. Following ground deformation, geological, petrological, tectonic and mechanical constraints, we provide evidence for the diapiric ascent of magma in a region of thickened continental crust.

Geochemical and petrological analysis of products from the 2011 eruption of Nabro volcano, Eritrea

A. DONOVAN*1, I. BUISMAN2, C. OPPENHEIMER1 1 Department of Geography, University of Cambridge

(*[email protected]) 2 Department of Earth Sciences University of Cambridge

The 2011 eruption of Nabro volcano in Eritrea displaced several communities and produced a lava flow over 15km in length along the border between Eritrea and Ethiopia. Field investigations took place several months after the onset of activity, and over a hundred samples were collected and analysed. Whole-rock geochemistry has been used to classify the products and compare them to other rocks from the Afar province. Nabro and Dubbi volcano to the north constitute a transverse alignment relative to the Erta Ale rift zone, and are considerably alkali-enriched relative to Erta Ale.

The 2011 eruption was trachybasaltic to trachy-basaltic andesite in composition, with phenocrysts of olivine, clinopyroxene, plagioclase, Fe-Ti oxides and minor apatite. It appears similar to older mafic eruptions from Nabro. Older rock samples were also analysed, ranging from primitive basalts to trachytes and rhyolites. Satellite images and field observations show the presence of large ignimbrite deposits on the flanks of Nabro, which forms a double caldera with Mallahale volcano in Ethiopia. Samples of these are currently being dated, and may represent the caldera-forming eruptions.

During the 2011 eruption, dynamic mixing occurred in the shallow conduit between the erupting basalt and an older rhyolite. Our analysis shows that the gas from the basalt was able to bubble through the molten rhyolite, and the two melts mingled at the vesicle rims. This allows for analysis of the mixing process between two melts of diverse compositions.

This poster will focus on the 2011 products, describing mineralogy, preliminary insights into eruptive conditions, and the superficial mixing of the magma with the rhyolitic pumice. It will also put this eruption in its local and regional context. Preliminary analysis from older rocks, particularly the ignimbrite, will also be presented.

48


Magma plumbing and degassing during the 2008-present summit

eruption of Kīlauea Volcano, Hawaiì

M. EDMONDS*1, T. MATHER2, R. MARTIN3, I. SIDES1, R. HERD4, D. SWANSON5

1 Earth Sciences Department, University of Cambridge, Downing Site, Cambridge CB2 3EQ, UK (*[email protected])

2 Department of Earth Sciences, University of Oxford, UK 3 Department of Geography, University of Cambridge, UK 4 School of Environmental Sciences, University of East

Anglia, UK 5 United States Geological Survey, Hawaiian Volcano

Observatory, Hawaiì 96718, USA

The 2008-current eruption of Kīlauea Volcano is the first explosive eruption to occur at the summit since 1924, and offers a unique opportunity to test models of degassing and magma plumbing. We measured sulphur dioxide (SO2) and carbon dioxide (CO2) concentrations in the gas plume from Halemau`maù using electrochemical and non-dispersive infrared sensors in April 2009. We analysed olivine-hosted melt inclusions from tephra erupted in 2008 and 2010 for major, trace and volatile elements. The gas and melt data are both consistent with a relatively evolved magma batch which equilibrated at depths of 1.2-2.0 km beneath Halemau`maù prior to the current eruptive activity. The differences in the volatile concentrations between the melt inclusions and matrix glasses are consistent with the observed gas composition, indicating that there is no need to invoke either addition of CO2 vapour, or scrubbing of SO2. A slight enrichment in SO2 in the gas over that calculated from the melt compositions is consistent with the decomposition of sulphides during magma decompression and degassing, thereby buffering melt sulphur concentrations. The degassing of sulphur and halogen gases from the melt requires low pressures and therefore we invoke convection to bring the magma close to the surface to degas, before sinking back into the conduit. The fluxes of gases (900 and 80 t/d SO2 and CO2) are used to estimate convective magma fluxes (1.2-3.4 m3/s). The observation of minimal loss of hydrogen from the melt inclusions implies a rapid rise rate (less than a few hours), which constrains the conduit radius to 1-2 metres (or a narrow dyke with a similar cross-sectional area). The inferred conduit radius is much narrower than the lava lake at the surface, implying a flared geometry. The melt inclusion data suggest that there is a progressive decrease in melt volatile concentrations with time during 2008-2010, consistent with convection, degassing and mixing in a closed, or semi-closed magma system. Our results are consistent with ground deformation (INSAR), thermal observations (of magma movement, gas pistoning and bubble bursting) and seismicity (source of long period earthquakes).

Re-evaluating ‘super’ volcanoes: The case of Yellowstone

B.S. ELLIS*1, D.F. MARK2, M.C. ROWE3, O. BACHMANN1

1 Institute for Geochemistry and Petrology, ETH Zurich (*[email protected])

2 NERC AIF, SUERC, East Kilbride, Scotland

3 School of the Environment, Washington State University, Pullman, WA, USA

Yellowstone is commonly held up as the archetypal

‘super-volcano’, having had three major eruptive episodes at ~ 2 Ma, 1.3 Ma, and 0.6 Ma. However, despite the importance of such large magnitude events, this idea has been held up to surprisingly little rigorous testing. Here we combine new high-precision Ar/Ar geochronology and mineral chemistry from multiple phases to shed new light on the explosive history of the Huckleberry Ridge (HRT) and Lava Creek (LCT) eruptions from the Yellowstone volcanic field.

Recent high precision 40Ar/39Ar geochronology has shown that member C of the HRT was erupted at least 6,000 years later than members A and B. This result is supported by differences in the compositions of fayalitic olivine, augite, sanidine and quartz between the members. Mafic minerals are compositionally homogeneous with augites and fayalites of member C less magnesian than those found in members A and B. Quartz grains show a variety of textures in CL imaging and have within-grain variations in titanium (determined via EMPA and LA-ICPMS) of up to a factor of 2. Again, member C of the HRT has distinct compositions of quartz (with higher Ti) than earlier erupted HRT. Quartz from LCT shows differences between members A and B with member A having generally lower Ti (avg. 55 ppm) than member B (avg. 102 ppm). Pb isotopes in sanidine further confirm the difference between members A and C of the HRT and between members A and B of the LCT.

Combining high-precision geochronology and detailed mineral-scale geochemistry from a number of different phases provides a robust method of distinguishing individual magma batches and clarifying the explosive history of a volcano. Our new data suggest that in some cases ‘super-eruptions’ might be better thought of as a series of large eruptions over a short timespan rather than a single gigantic event.

49


Hydrothermal cells and the thermal boundary layer around the

Cuillin Gabbro, Isle of Skye F. ENTWISTLE*1, B. YARDLEY1, A. BOYCE2

1 University of Leeds (*[email protected]) 2 SUREC

Fluid circulation and associated thermal regimes are crucial to understanding geothermal systems. Thermal boundary layers provide the connection between geothermal systems and the intrusive heat source and it is these thermal boundary layers that are now the focus for the exploration of Enhanced Geothermal Systems (EGS). EGS focus on high enthalpy systems with the fluids being supercritical.

Heat removed by water circulation around the intrusion is transferred by conduction from the magma through a thermal boundary layer, so that the more active the geothermal convection the thinner this layer must be. Thermal boundary layers are preserved as high grade metamorphic aureoles, in this case categorised by the presence of 2 pyroxene hornfels in the inner aureole, moving outwards into an amygdaloidal rich zone dominated by chlorite, epidote and actinolite with the most distal parts of the aureole containing amygdales containing zeolites. We propose that the inner hornfelses of the Skye aureole represent a thermal boundary layer while the rocks outside this zone have experienced hydrothermal alteration in a geothermal field. Areas of the contact where the hornfelses are thickest correspond to limited geothermal circulation, while where the hornfelses are thinnest geothermal activity was at a maximum.

Two distinct zones have been identified around the Cuillin gabbro; the widespread geothermal field dominated by convective heat transfer with large fluid fluxes and categorised by greenschist facies metamorphism. This zone is separated from the intrusive heat source by a (highly variable) high temperature zone of recrystalisation; representing a thermal boundary layer which is dominated by conductive heat transfer, minimal amounts of fluid flow and categorised by the presence of 2 pyroxene hornfels. This thermal boundary layer varies enormously around the aureole with minimum thicknesses ranging from 2m up to 150m at different locations around the aureole.

The thermal structure implied by the mineral zonation appears to be at odds with current interpretation of oxygen stable isotope analysis. The classic work of Taylor and Forester (1971) and Forester and Taylor (1977) reported depleted δ 18O values from the Cuillin gabbro and the surrounding aureole, with the lightest values in both cases close to the contact, and interpreted the depletion as the result of circulation of meteoric water. Current stable isotope analysis indicates that the lightest values are in fact those that have the highest degree of hydrothermal alteration, but with light values for samples from the thermal boundary layer. The challenge is to evaluate the relationship between oxygen isotope depletion and temperature and thereby develop a more sophisticated model for the palaeogeothermal activity, and second to understand the mechanisms by which high temperature assemblages have experienced extensive modification of their oxygen isotope composition without displaying any macroscopic features of related hydrothermal activity..

2011-2012 eruption of Nyamuragira M. FAZIO*1, A. JONES1, A. BEARD2

1 Earth Sciences, University College London (*[email protected])

2 Birkbeck College London

Between 6/11/2011 and 25/1/2012 a new fissure

eruption occurred at Nyamuragira Volcano (Democratic Republic of Congo)1. In this study we show new rock samples (provided by Abigail Church, former UCL PhD) coming from this event (that are now being studied) along with new specimens from the neighbouring volcano, Nyiragongo, which last erupted in 2002. Though they are just 15 km far from each other, their current bulk compositions are quite different ranging from nephelinite – melilitite - leucitite on Nyiragongo2 to tephrite – basanite - phonolite on Nyamuragira3.

We will mainly focus on the volatile contents, analyzing the melt inclusions in dry minerals, to characterize the magma source. On Nyamuragira this type of study has been conducted recently by Head et al.4, but it does not account for the last eruption, while on Nyiragongo the last melt inclusions analysis was run more than twenty years old5, well before the 2002 eruption, the last one after the famous 1977 event.

Due to tens of years of internal crisis, the North Kivu region, where both volcanoes lie, is one of the most dangerous area in the world. This explains the paucity of research. In an attempt to overcome the difficulty to carry on a fieldtrip, we found a clear resemblance between the rocks of both DRC volcanoes6-4and some of those at Vulture volcano (Italy)7. For example, they share similar TAS diagrams and K2O/NaO ratios.

The possibility to compare rock samples with similar chemical compostions allows us to run experiments to understand the rheology of lavas emitted by both volcanoes, and its dependance on chemical compositions and volatile contents. Note that the rheological properties of these lava were investigated just once by Giordano et al.8 who took account only of the highly fluid lavas of Nyiragongo.

[1] Volcano Discovery [homepage on the internet]. (cited

2012 Jan 27). [about 1 screen]. Available from: http://www.volcanodiscovery.com/view_news/4622/Nyamuragira-volcano-DR-Congo-update-eruption-continues-with-active-lava-lake-and-lava-flow-from-new-.html

[2] Platz, T., Foley, S.F., Andre, L., (2004) J. Volcanol. Geotherm. Res. 136, 269–295.

[3] Aoki, K., Yoshida, T., Yusa, K., Nakamura, Y., (1985) J. Volcanol. Geotherm. Res. 25, 1–28.

[4] Head, E. M., Shaw, A. M., Wallace, P. J., Sims, K. W. W., Carn, S. A., (2011) Geochem. Geophys. Geosyst. 12, Q0AB11, doi:10.1029/2011GC003699.

[5] Bailey, D.K., Hampton, C.M. (1990) Lithos 26, 157-165.

[6] Chakrabarti, R., Basu, A. R., Santo, A. P., Tedesco, D., Vaselli, O., (2009) Chem. Geol. 259, 273–289.

[7] Beccaluva, L., Coltorti, M., Di Girolamo, P., Melluso, L., Dilani, L., Morra, V., Siena, E., (2002) Miner. and Petr. 74, 277–297.

[8] Giordano, D., Polacci, M., Longo, A., Papale, P., Dingwell, D. B., Boschi, E., Kasereka, M., (2007) Geophys. Res. Lett. 34, L06301.

50


Petrological constraints on deep degassing prior to large basaltic

fissure eruptions: CO2 in Laki melt inclusions

M.E. HARTLEY*1, J. MACLENNAN1, M. EDMONDS1, T. THORDARSON2, D.J. MORGAN3

1 Department of Earth Sciences, University of Cambridge (*[email protected])

2 School of GeoSciences, University of Edinburgh

3 School of Earth and Environment, University of Leeds

The Laki eruption of AD 1783 produced over 15 km3 of basaltic lava and tephra, and led to three years of extreme climatic variability in Europe and North America. We have used a novel combination of micro-analytical measurements to constrain the history of deep-degassing CO2 loss from the Laki magma prior to eruption. The data provide important constraints on magma storage depths and the overall CO2 budget of the eruption, and have implications for the interpretation of seismic, geodetic and gas monitoring data from regions where large fissure eruptions occur.

We have analysed volatile, trace and rare earth elements by SIMS in over 100 naturally-quenched, olivine-hosted melt inclusions from Laki. Raman spectroscopy was then used to determine the fluid density, and hence the CO2 content, of bubbles in the melt inclusions. Bubbles are common in the Laki melt inclusions, and formed when post-entrapment crystallisation within an inclusion causes CO2 to exsolve from the melt. It has previously been assumed that the amount of CO2 in shrinkage bubbles is negligible. We have added the CO2 contained in the bubble to the glass CO2 content, to obtain the total CO2 of the Laki melt inclusions, which range from volatile-rich (4000-8000 ppm total CO2) to almost completely degassed (<50 ppm total CO2). Bubbles typically comprise <5 vol.% of the melt inclusion, but contain ~90% of the total CO2. The combination of SIMS and Raman analyses can thus be used to significantly improve estimates of melt inclusion entrapment pressures and quantification of the total mass flux of volatiles associated with Laki.

Solubility models for CO2 in basaltic melt can be used to predict the degassing path of the Laki magma for different crystallisation models. Carbon is expected to behave as an incompatible trace element similar to La or Nb in a CO2-undersaturated melt. CO2/Nb ratios can thus be used to deconvolve the effects of crystallisation and degassing. Laki melt inclusions hosted in Fo83-86 olivines have the highest CO2 concentrations and CO2/Nb ratios, indicative of entrapment during the deep crystallisation of a CO2-undersaturated magma. By contrast, melt inclusions hosted in Fo71-76 olivines have low CO2 and CO2/Nb, indicative of entrapment during crystallisation at a shallow level in the crust. The melt inclusions thus appear to record concurrent crystallisation and degassing in the Laki magma. We use solubility models to constrain the melt inclusion entrapment depth, and estimate the total CO2 loss from the Laki magma.

Further work will investigate the timescales of degassing in the Laki magma system, and used to estimate the potential flux of CO2 to the surface in the premonitory phases of the Laki eruption.

Silicic volcanism in the Main Ethiopian Rift: A case study of

Alutu Volcano W. HUTCHISON*1, J. BIGGS2, T. MATHER1, D. PYLE1, G.

YIRGU3 1 Department of Earth Sciences, University of Oxford, UK

(*[email protected]) 2 Department of Earth Sciences, University of Bristol, UK

3 Department of Earth Sciences, University of Addis Ababa, Ethiopia The Main Ethiopian Rift is lined by a series of regularly

spaced silicic volcanic complexes, typically 10-20km in diameter. While most of these complexes show signs of large caldera forming eruptions in the recent past, little is known of how active these volcanoes are currently and what the frequency and magnitude of past eruptions has been.

Our work will address these issues via a detailed multi-diciplinary case study at Alutu volcano. Alutu, identified in the recent Global Facility for Disaster Risk Reduction (GFDRR) of the World Bank [Aspinall et al., 2011] as a ‘high-risk’ volcano, has shown clear signs of unrest, undergoing large pulses of uplift in 2004 (15cm) and 2008 (10cm) [Biggs et al., 2011]. We are assessing the current state of activity at Alutu by both ground deformation (InSAR and continuous GPS), as well as volcanic degassing (portable CO2 gas analyzer). Our knowledge of the historical activity of Alutu will be developed via a combination of geological field mapping as well as remote sensing from satellite (ASTER) and airborne data (LiDAR imagery).

Our initial field mapping and geochemical analysis found that previous eruptions at Alutu have produced a large compositional range of volcanic products from mildly alkaline basalt through to peralkaline rhyolite (spanning 52-74% SiO2). We identified previously unrecognised explosive activity (fallout and pyroclastic flow deposits), as well as evidence of mass flows (lahars). The youngest products of Alutu are a series of obsidian lava flows, often these align with faults, and potentially trace out an older ring-fracture system, linked to an earlier caldera forming event.

Results of the CO2 degassing show that fluxes vary between low background values of 0.1 g(CO2)m-2day-1, on the rift floor away from the volcano, to 2400 g(CO2)m-2day-1

immediately above active faults on the volcano. The results show that volcanic fractures, likely inherited from an earlier caldera forming event, as well as rift related faulting impart significant controls on the degassing pattern.

These measurements will offer some of the first detailed constraints on the eruptive history and current activity of a Main Ethiopian Rift volcano. Through these new data we hope to contribute to the assessment of volcanic hazard at Alutu, and more generally, to expand our knowledge of the role these silicic volcanic centres play in continental rifting processes.

Aspinall, W., et al. (2011) Volcano Risk Study 0100806-00-

1-R. Biggs, J., et al. (2011) Geochemistry Geophysics

Geosystems 12, 1-11.

51


Modelling of magmatic hydrothermal systems: Phlegrean

Fields case study A. JASIM*1, F. WHITAKER F.2, A.C. RUST2

1 School of Earth Sciences, University of Bristol (*[email protected])

2 School of Earth Sciences, University of Bristol

In the last few decades the importance of the hydrothermal system in the behaviour of volcanoes has been increasingly recognised. There are several aspects that make the hydrothermal system important and interesting to investigate: i) it is extremely sensitive to perturbation from magmatic processes; ii) some of the ground deformation attributed to magmatic process could be simply related to hydrological phenomena; iii) the chemical reactions between circulating hot water and country rock change the physical properties (e.g. porosity, permeability, strength) of the rocks and thereby affect the response of the volcano to meteoric precipitation, degassing and ground deformation.

Here we present preliminary results of numerical modelling of the Phlegrean Fields, exploring the importance of heterogeneous rock properties in controlling hydrothermal fluids circulation. Fluid and heat flows are modelled using TOUGH2 which simulates non-isothermal multicomponent and multiphase flows in porous and fractured media (Pruess, 1991). We simulate fluid flow and heat transport in a 2D radial model, centred on a fumarole. The model domain is 1.5 km deep with a radius of 10 km, with three main geological units representing the pyroclastic rocks, underlain by the Yellow tuff formation and the Chaotic tuffites formation (post-caldera deposits). Additionally there is a system of circular faults that dislocates the units. The system is water saturated, there is a constant heat source at the base of the model and a water input at the base of the fumarole (based on Todesco et al, 2010). The system is closed to fluid flow along the base and the side, and the top boundary condition is maintained at atmospheric pressure and temperature. Initial conditions are defined by a pressure gradient of 1*104 Pa/m (hydrostatic pressure) and a temperature gradient of 0.13 �C/m calculated from well data (Rosi & Sbrana, 1987).

Prelimary results reveal the strong connection between rock properties, fluid flow and heat flux. The development of convective cells can be inhibited with a small change in the distributions of rock properties, even keeping constant all the other paramaters, suggesting that discontinuites (e.g. faults) may play a key role in the fluid distribution. The model that is currently running shows a more complex geometry and it aims to understand the role of faults in driving fluid flow in a volcanic caldera setting.

Todesco, M., (2010) Journal of Geophysical Research Vol

115, B09213. Pruess, K., (1991) TOUGH2, Lawrence Berkeley National

Laboratory. Rosi, M., (1987) CNR Quaderni de “La ricerca scientifica”

Vol 114.

Magma mixing, storage and degassing during the 1959 Kīlauea

Iki eruption, Hawaiì I. SIDES*1, M. EDMONDS1, J. MACLENNAN1, B.

HOUGHTON2, D. SWANSON3 1 Earth Sciences Department, University of Cambridge,

Downing Site, Cambridge CB2 3EQ, UK (*[email protected])

2 Dept. of Geology & Geophysics, University of Hawaii, Honolulu, Hawaii, USA

3 Hawaiian Volcano Observatory, United States Geological Survey, Hawaii Volcanoes National Park, HI 96718, USA

We present a study of magma mixing and volatile

degassing during the 1959 Kīlauea Iki eruption (Hawaiì), which was associated with a well-characterised sequence of high fountains. Olivine-hosted melt inclusions from tephra associated with episodes 1-3, 5-8, 10 and 15-16 were analysed for major, trace and volatile elements. The trends in the melt inclusions are consistent with changes in eruption parameters and we propose they record syn-eruptive geochemical changes in melt composition. The melt inclusions show a trend of decreasing MgO content through episodes 1 to 16, and the host olivine compositions become less forsteritic with time. Trace element concentrations in melt inclusions and glasses show that both mixing of heterogeneous mantle melts and fractionation must be operating to produce the range in magma compositions. The melt inclusions record changes in the volatile content of the melts with time, becoming poorer in H2O as the eruption proceeded. The CO2 concentration of the melt inclusions correlates negatively with fountain height; whereas H2O shows no correlation with fountain height. The CO2 concentration of the melt inclusions correlates positively with the Mg# of the matrix glass. We propose a model whereby drained-back lava mixes with both summit-stored, and more primitive magma from depth, causing an overall decreasing melt H2O content with time. Primary magmas supply volatile-rich melts carrying CO2 vapour. The proportion of CO2 over H2O in the melts has a direct impact on fountain dynamics: the higher the fraction of CO2 in the gas mixture, the lower the lava fountain, due to the higher density of CO2 over H2O at a fixed temperature and pressure.

52


Post-caldera explosive activity at Furnas volcano, São Miguel, Azores A. J. JEFFERY*1, R. GERTISSER1, B. O’DRISCOLL1, A.

PIMENTEL2, J. M. PACHECO2, S. SELF3 1 School of Physical and Geographical Sciences, Keele

University, Keele, Staffordshire, ST5 5BG, UK. (*[email protected])

2 Centro de Vulcanologia e Avaliação de Riscos Geológicos, Universidade dos Açores, 9501-801 Ponta Delgada, Açores, Portugal

3 Department of Earth and Environmental Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

Furnas volcano is the easternmost of three active central

volcanoes on the island of São Miguel, Azores. Approximately 3000 people live within the nested Furnas caldera, and several thousand more are within the immediate vicinity [1]. The volcano’s eruptive history comprises a range from effusive, dome-forming activity to, at least, two explosive, caldera-forming events involving pyroclastic density currents (dated at ~30,000 and ~12,000 14C y B.P.) [2]. The younger eruptive history has predominantly been characterised by sub-plinian style activity. Ten post-caldera eruptions (Furnas A-J [2, 3]) occurred at Furnas within the past 5000 years, producing a total of ~0.9 km3 (DRE) of magma. The youngest of these (Furnas J), occurred in A.D. 1630, after human settlement of the island. The deposits comprise interbedded fine ash and pumice lapilli beds that overlie the widespread Fogo A tephra from nearby Fogo volcano, forming the Upper Furnas Group (UFG) [2, 3]. Three UFG eruptions (Furnas E, I, J) produced lava domes following intial explosive phases [1].

Here, we present new whole-rock major and trace element data, alongside mineral chemical data, for the products of the UFG as well as the Povoação Ignimbrite, associated with the formation of the older Furnas caldera. Whole-rock compositions are comenditic trachyte with a limited SiO2 range from 62-63 wt. % and agpaitic indices between 0.73-1.13. Mineral assemblages include anorthoclase, clinopyroxene, Ti-magnetite and occasional biotite and ilmenite. Cognate syenite nodules within the Furnas J deposits contain anorthoclase, clinopyroxene, amphibole and oxides. Whole-rock REE patterns display enrichment of LREE, and, with the exception of the Povoação Ignimbrite, a pronounced negative Eu anomaly (Eu/Eu* = 0.15), as well as minor fractionation between MREE and HREE.

Major and trace element data suggest that the trachytitic magmas are formed predominantly by fractional crystallisation from Furnas basalt. Uniform comenditic trachyte has been erupted from Furnas for several thousands of years, implying a long-lived, evolved magma system capable of producing substantial explosive eruptions. An improved understanding of the longevity and dynamics of this magma system has implications for future hazard assessment. [1] Cole et al. (1999) J Volcanol Geotherm Res 92, 39-53 [2] Guest et al. (1999) J Volcanol Geotherm Res 92, 1-29

[3] Booth et al. (1978) Phil Trans Royal Soc London 288, 271-319

Compositional heterogeneity of the Earth’s convecting mantle:

Constraints from olivine-hosted melt inclusions from a continental

flood basalt setting E.S. JENNINGS*1, S.A. GIBSON1, J. MACLENNAN1, R.N.

THOMPSON2

1 Deptartment of Earth Sciences, University of Cambridge, UK (*[email protected])

2 Department of Earth Sciences, University of Durham, UK

Picrites and ferropicrites from the Early-Cretaceous Parana-Etendeka Large Igneous Province are primary mantle melts associated with the initial impact of the proto-Tristan mantle plume. These high temperature melts provide an excellent opportunity to assess compositional heterogeneity of the convecting mantle. Ferropicritic melts are especially interesting; they are rare on the Earth's surface, and are documented only as small-volume dykes and flows at the base of continental flood-basalts. They are characterised by high FeO (>~13 wt %) and low Al2O3 (< ~10 wt %), with very steep REE profiles ([Gd/Yb]N = 2-3.5), and experiments show that they originate from a garnet pyroxenite source.

Olivine-hosted melt inclusions preserve instantaneous melt compositions, and therefore are key to observing initial melt composition prior to overprinting by mixing, crystallisation and assimilation processes. We have analysed experimentally-quenched olivine-hosted melt inclusions for trace elements using SIMS and have combined this data with mineral chemistry to make a preliminary assessment of their mantle source characteristics and melt evolution. Lithospheric contamination evident in whole-rock chemistry is absent from most melt inclusions in picrites, but crustal contamination is clearly present in a small subset, with combined enrichments in Rb, Ba and Sr. Picrites show a greater range in all elemental concentrations in the olivine-hosted melt inclusions relative to whole rock analyses e.g. [La/Lu]N = 1.5-3 (whole rock); = 0.9-4 (melt inclusions). Ferropicrite olivine-hosted melt inclusions have more diverse chemistries, with a large range in LREE slopes and variably contaminated overprints. Zoned clinopyroxene phenocrysts in some ferropicrite samples also indicate the involvement of more complex petrogenetic processes.

53


Tracking changes of magma transfer beneath Mt. Etna:

Evidence from crystal zonation and real-time gas monitoring

M. KAHL*1,2, S. CHAKRABORTY1, F. COSTA3, M. POMPILIO4, M. LIUZZO5, M. VICCARO6

1 GMG, Ruhr-University Bochum, Germany 2 SEE, The University of Leeds, UK

(*[email protected]) 3 EOS, Nanyang Technological University, Singapore 4 INGV- Sez. di Pisa, Italy 5 INGV- Sez. di Palermo, Italy 6 Università di Catania, Italy

Tracking the pre-eruptive storage and transfer of magmas beneath highly active volcanoes, such as Mt. Etna (Sicily), is one of the goals of modern volcanology Here we show an integrated methodology linking compositional and temporal information recorded in chemically zoned olivines with real-time variations (CO2/SO2) in gas emanations in order to monitor the intermittent input of new magma into sections of Mt. Etna’s plumbing system. We illustrate our approach using the 2006 summit eruptive period of Mt. Etna. We have used data on compositional zoning of olivine in combination with real-time degassing data obtained during the period January 2005 – December 2006 [1] to show how variations in the monitoring signals can be related to changes occurring within the plumbing system at depth. The investigated eruptive products (July & October 2006) contain olivines with different populations of core and rim compositions (Fo70-72; Fo79-82; Fo73-75) and zoning patterns. We explain the diverging compositional and zoning record as the product of magma evolution in at least three distinct environments (labeled M0, containing olivine of Fo79-82; M2 with Fo70-72 and mm1 with Fo73-75) with melt transfer and mixing between them. Modeling the diffusive relaxation of the compositional zoning profiles enabled us to constrain the time the crystals (+ their associated melts) spent in transit between these environments. We can show that the systematic rise of the degassing signal (CO2/SO2) [1] can be related to the gradual (pre- and syn-eruptive) supply of batches of mafic magma into segments of the plumbing system, where mixing with residing (more evolved) magma was promoted. The temporal details of magma supply suggest that some of this new input of mafic magma may have triggered the initiation of different eruptive cycles (July, August).

[1] Aiuppa et al., (2007) Geology 35, 1115-1118.

Timescales of magmatic processes at Mt. Ruapehu, New Zealand: linking

mineral diffusion rates to monitoring data

GEOFF KILGOUR1*, KATE SAUNDERS1, JON BLUNDY1, HEIDY MADER1

1 School of Earth Sciences, University of Bristol. (*[email protected])

Mt. Ruapehu is an andesite cone volcano situated at the

southern end of the Taupo Volcanic Zone, New Zealand. Active vents are submerged beneath a warm, acidic crater lake (~ 0.2 km2) that exhibits a thermal cycle between 12 and 40 °C. The presence of Crater Lake causes the eruption style to be dominantly phreatic or phreatomagmatic. Historical eruptions at Ruapehu were typically small volume (< 0.001 km3) eruptions that effected the summit plateau. Larger eruptions ejected material to > 200 km from the vent. Scoria samples available for this study were erupted in 1969, 1971, 1977, 1995, and 1996.

The monitoring network at Ruapehu has consisted of quasi-monthly Crater Lake temperature and fluid chemistry measurements, and seismic monitoring since the mid-1960’s. A more modern network was installed during and immediately after the last major eruption in 1995-1996, including regular airborne and fluid chemistry measurements and telemetered broadband seismic data. Historical eruptions have been preceded by short-term seismic precursors and/or fluid chemistry changes. Prior to some eruptions, a period of increased seismicity has occcured days to weeks before the event making Ruapehu a very difficult volcano to predict. Here we present mineral diffusion data on zoned pyroxene phenocrysts and microlites in order to constrain the magmatic processes that occurred soon before each eruption.

Recent petrological data has suggested that individual magma batches at Ruapehu interact with other magmas and/or crystal-mush zones during ascent resulting in common crystal (and melt) exchange. In some instances, scoria erupted include phenocrysts (clinpyroxene, orthopyroxene, and plagioclase) that are all foreign to the host melt, implying that a crystal-poor melt intruded into a crystal-mush zone shortly before eruption. In particular, clinopyroxene crystals exhibit common reverse-zoning (Fe-rich to Mg-rich) on the outermost margins of the phenocryst (and microlite). We note that a thin (~2-4 µm) Mg-rich rim is common to most pyroxene phenocrysts and microlites. We interpret these rims to represent a period of heating immediately prior to eruption. Using published Fe-Mg diffusion exchange coefficients for clinopyroxene, we show that the outermost rims record a heating event a matter of weeks before all eruptions analysed. These data are compared to Crater Lake chemistry and seismic data to provide a comprehensive account of the magmatic-hydrothermal system response to magma-magma and magma-mush interactions.

54


Disequilibrium during volcanic eruption: effect of cooling rate on

plagioclase-liquid element exchange M. KLÖCKING*1, M. EDMONDS1, M.C. S. HUMPHREYS2

1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ

(*[email protected]); 2 Department of Earth Sciences, University of Oxford, South

Parks Road, Oxford, OX1 3AN, UK Geothermobarometry is widely applied to volcanic

rocks, based on the equilibrium distribution of elements between melt and crystals. However, often the rapid ascent of magmas means that equilibrium cannot be maintained under the rapidly changing temperature and pressure conditions. This results in errors in the results of common thermometers and barometers that are as yet largely unquantified.

We assess temperatures obtained with the plagioclase-liquid thermometer [1] and the degree of disequilibrium for a range of rocks with different eruption rates erupted from Volcán de Colima, Mexico, and Soufrière Hills Volcano, Montserrat. We compare major element compositions measured in plagioclase and glass by electron microprobe to theoretical models of magma evolution [2], assuming that the degree of disequilibrium in plagioclase increases with cooling rate [3]. Cooling rate is independently estimated using crystal size distribution and other textural features. In addition, the clinopyroxene-liquid thermometer is applied to Colima rocks to test the validity of different temperature models. We also quantify Fe and Mg concentrations in plagioclase phenocryst rims, and attempt to quantitatively deconvolve the effects of heating by mafic recharge and latent heat of crystallisation from disequilibrium caused by quenching. The data from Colima and Montserrat are compared to petrological data from other andesitic arc volcanoes compiled from the literature.

[1] Putirka K., (2008) Reviews in Mineralogy and

Geochemistry 69, 61–120. [2] Gualda G.A.R., Ghiorso M.S., Lemons R.V., Carley

T.L., (2012) Journal of Petrology 53, 875-890. [3] Mollo S., Putirka K., Iezzi G., Del Gaudio P., Scarlato

P., (2011) Lithos 125, 221–235.

Melting events in the Rum layered intrusion

J. LEUTHOLD*1,2, J. BLUNDY1, M. HOLNESS2 1 School of Earth Sciences, University of Bristol.

(*[email protected]) 2 Department of Earth Sciences, University of Cambridge.

The Rum Eastern Layered Intrusion (RELI) Unit 9 is composed of peridotite with poikilitic Plg and Cpx, troctolite with interstitial Cpx and eqigranular gabbro. Gabbro enclaves occur within troctolite. Cpx rims are poorly developped in layered peridotite and troctolite and bulk rock analyses display distinct Eu positive anomalies, evidencing loss of interstitial liquid. Cpx rims are Cr-poor, REE-rich and display low La/Lu (strongly incompatible/moderatly incompatible elements) ratio in regard to associated cores. Troctolite is overlain by gabbro, separated by a wavy horizon. Gabbro Cpx show discrete reverse Cr and REE zoning and a constant high La/Lu ratio. Troctolite interstitial Cpx and gabbro equigranular Cpx progressively get oikocrystic towards the northern edge of an intrusive peridotite sill, forming poikilitic gabbro, with Cpx oikocrysts. Cpx Cr-rich (~1.1 wt% Cr2O3), REE-moderate, high La/Lu anhedral core enclose randomly oriented small (up to 0.5 mm) Ol and Plg inclusions. They are overgrown by a Cr-moderate (~0.7 wt% Cr2O3), REE-poor, high La/Lu anhedral core. Cpx dissolution occurs along inclusions. Poikilitic rims are Cr-poor (~0.2 wt% Cr2O3), REE-rich and display low La/Lu ratio. They enclose large (1 mm) oriented Plg and Ol crystals. The rim thickness increases from the Unit 9 base to the top. The Cpx rim crystallization is synchronous with cumulate pile compaction.

Equilibrium liquids were estimated using Wood and Blundy (1997) partition coefficients. The liquid at the origin of the peridotite, troctolite and poikilitic gabbro Cpx cores display a higher La/Lu ratio than the corresponding rims and the picritic dikes. The high La/Lu ratio of the liquids in equilibrium with the Cpx cores is interpreted as a signature of low degree partial melting. Cpx cores is best explained by 5% partial fractionated melting of a gabbroic assemblage (RELI interstitial crystals or gabbro). Gabbro enclaves within troctolite are relics. The multiple generations of Cpx are witnesses of succesive melting episodes. Cpx rims are best explained by partial melting (5%) of a gabbroic assemblage, mix with picritic liquid (30-50%) and subsequent Rayleigh fractional crystallization (F = 1-0.7).

Known RELI liquids (Upton et al., 2002) fail to model crystallization of the Unit 9 Cpx cores by simple fractional crystallization. We suggest that the Unit 9 gabbro crystallized from aggregated drops from a partially molten gabbroic assemblage deeper in the RELI. Subsequent intrusion of peridotite sills induced partial melting of the Unit 9 gabbro. Once porosity was sufficient, differentiating liquids from the peridotite percolated throughout the crystal mush pile, mixing with interstitial liquid and crystallizing the interstitial crystals. Thus the composition of the percolating liquid and the final metasomatized cumulate differ from simple fractionation products along the Rum liquid line of descent.

Wood, B.J., Blundy, J.D. (1997) CMP, 129, 166-181. Upton, B.G.J et al. (2002) Geol. Mag., 139, 437-452.

55


The origin of the Igwisi Hills kimberlite (Tanzania) constrained

from He and O isotopes F.R. MARKS*1, F.M. STUART2, R.W. BROWN3

1 Geographical & Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK (*[email protected])

2 Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK

3 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK

*Now at: UCD School of Geological Sciences, Science Centre West, University College Dublin, Belfield, Dublin 4, Ireland

Despite decades of study, the depth and source of

kimberlite melts remains enigmatic. The isotopic composition of volatile elements, supply unique information on the melt sources in mantle-derived rocks. However, most kimberlites suffer pervasive post-magmatic alteration that makes them unsuitable for study of their volatile inventory. The Quaternary-aged kimberlites from the Igwisi Hills volcanoes, Tanzania (Brown et al. 2012), have avoided alteration and are amenable to detailed geochemical and isotopic investigation.

The 3He/4He of melt trapped in olivine xenocrysts released by crushing in vacuo (4.1 ± 0.2 Ra; n = 4) are amongst the lowest recorded for mantle rocks. The data rule out a deep mantle plume source that has been implicated in the origin of the Udachnaya (Sumino et al. 2006) and Tertiary West Greenland (Tachibani et al. 2006) kimberlites. Importantly, the 3He/4He are significantly lower than of the value of regional sub-continental lithosphere as measured in peridotite xenoliths from Lashaine, Eledoi and Pello Hill (6.3 - 7.3 Ra (Porcelli et al. 1986)). Phenocryst olivine δ18O (5.1 ± 0.2 ‰; n = 11) are indistinguishable from normal mantle peridotite, with no indication of derivation from an enriched mantle source proposed for Udachnaya kimberlites (Kamenetsky et al. 2008).

The simplest explanation for the isotope data is that the Igwisi Hills kimberlite originates as volatile-rich small volume melts from normal sub-continental lithosphere mantle without the need for thermal perturbation originating at depth. The low 3He/4He in the melt implies that the mantle is heterogenous for U and Th, with early melting phase preferentially enriched relative to bulk mantle values.

Brown, R.W., Manya, S., Buisman, I., Fontana, G., Field,

M., Mac Niocaill, C., Sparks, R.S.J. & Stuart, F.M. (2012) Bulletin of Volcanology. 74, 1621-1643.

Sumino, H., Kaneoka, I., Matsufuji, K. & Sobolev, A.V. (2006) Geophysical Research Letters 33, L16318.

Tachibani, Y., Kanoeka, I., Gaffney, A. & Upton, B. (2006) Geology 34, 273-276.

Porcelli, D.R., O’Nions, R.K. & O’Reilly, S.Y. (1986) Chemical Geology 54, 237-249.

Kamenetsky, V.S., Kamentsky, M.B., Sobolev, A.V., Golovin, A.V., Demouchy, S., Faure, K., Sharygin, V.V.& Kuzmih, V.D. (2008) Journal of Petrology 49, 823-839.

An integrated study of SO2 degassing from Tungurahua

volcano, Ecuador B. MCCORMICK*1, J. YANG2, M. EDMONDS1, T.

MATHER3, S. CARN4, S. HIDALGO5, B. LANGMANN6, M. HERZOG2

1 Department of Earth Sciences, University of Cambridge; (*[email protected])

2 Department of Geography, University of Cambridge;

3 Department of Earth Sciences, University of Oxford; 4 Department of Geological and Mining Sciences and

Engineering, Michigan Technological University;

5 Instituto Geofisico, Escuela Politécnica Nacional;

6 Institute of Geophysics, University of Hamburg.

Tungurahua is a 5023 m-high stratovolcano in Ecuador, with an estimated long-term mean SO2 output of 1458 ± 2026 t/day. Gas emissions from Tungurahua are continuously monitored by UV DOAS spectrometers installed as part of the Network for Observation of Volcanic and Atmospheric Change (NOVAC) programme. The SO2 emissions from the volcano have also been observed and characterised by the satellite-based UV spectrometer OMI (Ozone Monitoring Instrument). Tungurahua is therefore an ideal location for comparing ground- and satellite-based estimates of volcanic SO2 emissions. Although OMI SO2 retrievals for continuous tropospheric degassing are not yet validated, the dataset represents a large and mostly untapped resource for volcano monitoring, particularly in remote or inaccessible regions. This novel study seeks to improve agreement between the DOAS and OMI datasets for Tungurahua, and gain new understanding of why differences in the two estimates of SO2 degassing arise. Uncertainties affecting comparison between the datasets include: the different natures of the quantities measured (flux vs column concentration); the impact of local atmospheric and meteorological conditions (e.g. clouds masking volcanic plumes; humidity and temperature promoting rapid loss of SO2 via oxidation to sulphate or by various wet/dry deposition processes; wind dispersal of plumes); and differences in the spatial and temporal resolution of measurements.

We present a novel numerical modelling-based study of volcanic SO2 emissions from Tungurahua using the atmospheric chemistry/transport model REMOTE, which has already been successfully applied to modelling post-emission SO2 dispersion from volcanoes in Nicaragua and Indonesia. Model input is high time resolution SO2 flux data from the Tungurahua DOAS datasets. The model output is spatial maps of SO2 column concentration for comparison with those produced from OMI data. We calculate total daily atmospheric SO2 burdens for the REMOTE and OMI datasets, and use REMOTE’s treatment of atmospheric chemical reactions, wind dispersal, and cloud cover, as well as additional OMI data products (effective reflectivity and aerosol index) and ground-based observatory records to interpret the variation in agreement between the two datasets. A key aim is to identify whether any of the above sources of uncertainty are dominant, and to investigate potential means of correcting for these. Additionally, we seek to produce a detailed assessment of errors in each dataset.

56


(Mis)understanding bubble growth in magma: Evidence from preserved volatile concentration gradients in

glass I.M. MCINTOSH*1, E.W. LLEWELLIN1, M.C.S.

HUMPHREYS, M.C.S.2, A. BURGISSER3, C.I. SCHIPPER3, A.R.L. NICHOLS4

1 Department of Earth Sciences, Durham University, UK. (*[email protected])

2 Department of Earth Sciences, University of Oxford, UK.

3 Institut des Sciences de la Terre d’Orleans, CNRS – l’Universite d’Orleans, France.

4 Institute for Research on Earth Evolution, JAMSTEC, Japan.

Volcanic eruptions are driven by the nucleation and growth of bubbles that form as volatiles (chiefly water) exsolve from magma. Bubbles grow as volatiles in the melt diffuse down a concentration gradient towards and across the bubble wall; understanding how this gradient evolves over time underpins numerical modelling of bubble growth and is critical to understanding bubble growth mechanisms. We present analytical results of water concentration gradients surrounding bubbles in experimentally decompressed volcanic glasses. These data highlight the hitherto unrecognised importance of hydration and bubble resorption during quench, with implications for both experimental and natural samples. Water speciation data suggest a mechanism for this quench hydration and represent a potential approach for correctly interpreting the cause of hydration profiles observed in natural samples.

Whereas bubble growth theory predicts decreasing volatile concentrations towards the bubble wall, our data instead show increasing water concentrations, of up to 3wt%, in the ~30 microns closest to the bubble wall. Water concentration at the bubble wall must always be the equilibrium solubility value for the given pressure and temperature conditions. Observed hydration profiles result from the increase in equilibrium solubility value as temperature decreases during the quench to glass, and locally overprint the broader concentration gradient resulting from syn-experimental bubble growth. Quench resoprtion resulting from hydration may reduce bubble volumes and sample porosities by as much as a factor of two.

Quench timescales are too rapid (1-5 seconds) for hydration profiles to be explained by diffusivity of ‘total’ water (H2Ot). Speciation data showing molecular (H2Om) and hydroxyl (OH) water concentrations around bubbles reveal that quench hydration is driven by rapid diffusion of H2Om. Speciation data thus offer a methodology for interpreting the pressure and/or temperature causes responsible for hydration profiles surrounding bubbles in natural samples.

H2Ot data are collected using the SIMS-calibrated backscatter SEM technique of Humphreys et al (2008) and are presented as 2D greyscale maps of H2Ot variations and as sample-averaged H2Ot profiles corresponding to different experimental conditions. H2O speciation data are collected using FTIR Stingray imaging and presented as 2D maps of H2Ot, H2Om and H2Om/H2Ot ratios.

Humphreys, M.C.S. et al (2008) EPSL 270, 25-40.

Granitoid magmatism during continental rifting: Preliminary

insights from the Oslo Rift, Norway C. MCLEOD*1, A. BRANDON 1, R. TRØNNES2, T. LAPEN1 1 University of Houston, Dept. Earth and Atmospheric

Sciences, SR1, Houston, Texas, 77204-5007. (*[email protected])

2 Natural History Museum, University of Oslo, Blinderm, 0318 Oslo, Norway.

The rifting of a continent is a crucial step in the Wilson

cycle of supercontinent formation and breakup during which large scale changes are imposed on the architecture of the lithosphere. The Permo-Carboniferous Oslo Rift in southern Norway is a failed intracontinental rift which developed in the Precambrian continental Baltic Shield and produced a large variety of alkaline volcanic and plutonic rock types. The current erosional level is up to 3 km below the original surface and has exposed the plutonic roots of the magmatic plumbing systems. Examination of the granitoid roots within the rift provides the opportunity to 1) evaluate contributions from mantle and crustal sources during continental rifting 2) assess the importance of crustal recycling and 3) investigate the timing of granitoid petrogenesis so that temporal constraints on rift related magmatism can be obtained. These fundamental questions regarding the evolution of a continental rift will be addressed by a thorough microanalytical geochemical study of constituent minerals within the exposed plutonic roots.

The granitic units of the Drammen and Finnemarka intrusive complexes in the Central segment of the Oslo Rift have previously been studied for their whole rock geochemical compositions (TrØnnes and Brandon, 1992)1 and therefore form the basis of this work. Rock types include apilitic porphyry, quartz monzodiorite, rapakivi granite and oligioclase granite (which are not comagmatic). Sampled units are predominantly peraluminous (up to 1.2 Al2O3/(CaO+Na2O+K2O) and exhibit marked differences in age corrected Sr-Nd-Pb isotopic compositions e.g the Finnemarka exhibits εNd values of +3.5 to +4 where as the Drammen ranges from +1 to +1.5. In-situ U-Pb ages of zircons from both intrusive complexes will constrain the timing of magmatism in this segemnt of the rift and identify the role (or not) of crustal recycling. Additional in-situ Hf and O isotopic and trace element analyses will characterise the components of magma source reservoirs and microdrilling of feldspars for their Sr-isotopic compositions will asses the role of crystal recycling by identifying the crystal cargoes of the different intrusive units.

1 Trønnes and Brandon (1992). Mildly peraluminous high silica granites in a continental rift: the Drammen and Finnemarka batholiths, Oslo Rift, Norway. Contributions to Mineralogy and Petrology, v. 109, 275-294.

57


Norwegian larvikites: Colours & origin: An oxygen isotope study

I.G. MEIGHAN *1,2,3,T. FALLICK 1, R. ELLAM1 1 S.U.E.R.C., East Kilbride. (*[email protected]) 2 Geological Survey of N.Ireland, Belfast

3 Geology Department, Trinity College Dublin

Larvikite, a coarse-grained olivine-augite syenite of the Permian Oslo Province and Norway’s National Rock,forms a large intrusive complex extending E and W of Larvik. This comprises both nepheline- and quartz-bearing varieties whose colours are light grey(predominant),black or red. This variation in itself invites O isotopic investigation.

The larvikites of this main body are cumulates with virtually omnipresent igneous lamination (inwardly inclined at 450-900). Their emplacement involved multi-injection by magmatic pulses, each probably coinciding with eruption from an overlying (now eroded) Kilimanjaro-type central volcano. Such an ‘open system’model accounts for a lack of major cryptic layering.

8 samples (7feldspars, 1whole rock) define a relatively narrow delta 18O range of +5.9 to +7.2 per mil (VSMOW: analytical precision +/- 0.2, 1 sigma). Our conclusions are: 1. There is no obvious difference between light and dark

larvikite (these can be interlayered) 2. The only significant variation involves the reddened,

quartz larvikite whose feldspar (6.7)and whole rock(7.2) are elevated relative to the other feldspar values(5.9-6.2). Thus the reddening cannot be attributed to high T, subsolidus meteoric water-rock interaction and probably involved magmatic fluid

3. Excluding the reddened material,the larvikite feldspars have values close to +6 per mil. Accepting these approximate magma compositions, the data support gravity and other isotopic evidence indicating syenite genesis by deep crustal/ Upper Mantle fractional crystallisation of alkali olivine basalt magma.

Exploring volcanic-plutonic connections

V. MEMETI*1, J. DAVIDSON1 1 Durham University, Dept. of Earth Sciences, Science Site,

South Road, Durham, DH1 3LE. (*[email protected])

Understanding the geochemical connection of arc

volcanic rocks with their underlying magma plumbing system remains a great challenge in arc studies. Volcanic and plutonic rocks are often studied separately, but rarely together as an interconnected plumbing system, which has often led to inconsistent interpretations in the past. The challenge is that, in many cases, either the volcanoes are accessible with their subjacent plumbing system buried, or the plutons are exposed with their superjacent plumbing system eroded.

The incentives for studying the entire magma plumbing system of a volcanic-plutonic complex are 1) to examine if we can associate certain volcanic rocks with specific plutonic units and if they are equivalent to one another, complementary, or not related in their geochemical characteristics; 2) to determine if we can pinpoint evidence in the geochemical record of both rock types which magmatic process(es) caused the large ignimbrite eruptions compared to the smaller sized volcanic eruptions, 3) to examine the length and time scales of such processes, and if mixing drives eruptions - as is often suggested – then determine where mixing occurred (magma reservoir or conduit), and 4) to try to reconcile the often disparate time scales and durations of magma processes inferred from both rock types.

We have collected samples from two calderas in the USA, where contemporaneous volcanic and plutonic rocks of the same arc magma plumbing system are juxtaposed, allowing the examination of the physical and geochemical connections between the two realms and the overall evolution of each magma plumbing system through time. One is the 33 Ma Bonanza caldera of the central San Juan Volcanic Field in Colorado and the other is the 36-34 Ma Organ Mountains caldera in New Mexico. Both calderas have been tilted on their sides due to extensional tectonics along the Rio Grande Rift. The compositional changes in these calderas are fairly similar in that they are composed of crystal-rich to crystal-poor dacites, often crystal-poor rhyolite, and minor andesite flows and intrusions. The plutonic rocks range from medium to fine grained diorites to leucogranites and cluster around granodioritic compositions. And most importantly, both systems were magmatically active for only 1-2 myr according to new Sanidine 40Ar/39Ar geochronology (pers. comm. M. Zimmerer, New Mexico Bureau of Geology and Mineral Resources).

We have looked at the structural relationships in the field and are in the initial stages of establishing geochemical characteristics of these rocks using whole rock and single mineral element and isotope geochemistry. We will present some of our initial results from whole rock element geochemistry and cathodoluminescence imaging of minerals from volcanic and plutonic rocks of both calderas to start address some of the questions outlined above.

58


Magma storage and differentiation beneath Dabbahu Volcano, Afar,

Ethiopia H. MILROY*1

1 School of Earth Sciences, University of Bristol. (*[email protected]) Erupted rock types from Dabbahu Volcano, Afar,

Ethiopia have been interpreted as a full fractional crystallisation suite from a single parent basalt. Petrological analysis and 40Ar/39Ar dating show no systematic temporal evolution of erupted products. Evidence from InSAR and seismic data and melt inclusions suggest shallow magma storage, while magnetotellurics has imaged substantial volumes of melt in the upper and lower crust beneath Dabbahu.

We present results from a two-dimensional numerical modelling exercise, which tested the hypothesis that the extreme diversity of compositions found at Dabbahu can be differentiated and stored at shallow crustal levels by the episodic injection of basaltic dykes, at intervals consistent with the known rift spreading rate and the minimum age of Dabbahu.

The full range of compositions observed at Dabbahu was reproduced by the modelled system, and stored simultaneously at shallow crustal levels. Results suggest that the system may have been evolving for ~300 kyr. Estimates of the modelled conductivities are compared with the amounts of melt beneath Dabbahu, estimated by MT imaging studies.

Constraining the pre-eruptive storage conditions for the Pollara

eruptions of Salina, Italy H. MORETTI*1, J. GOTTSMANN1, R. SULPIZIO2, J.

BLUNDY1 1 Department of Earth Sciences, Bristol University, Wills

Memorial Building, Queens Road, Bristol. BS8 iRJ. (*[email protected])

2 Dipartimento di Scienze della Terra e Geoambientali, Universitá di Bari, via Orabona 4, 70125, Bari, Italy.

Salina is the second largest of the Aeolian Islands that

form a complex of faults and volcanic arcs to the north of Sicily, Italy. The explosive eruptions of the Pollara depression, 26.5-13.6 ka, are believed to be the result of the invasion and chaotic mixing of a shallow, partially fractionated, basaltic-andesite by a highly evolved rhyolite. The invasion by a more evolved magma is unusual and is supported by the stratigraphy of the air fall deposits. The volatile content of each body has been calculated by working back from an assumed value for the mafic body, requiring a high volatile input from the silicic magma (4-4.5 wt %).

The depths of the magma bodies are assumed to be in line with those found generally in the Aeolian arc. This presents a paradox where the more evolved melt is deeper (200-300 MPa) than the juvenile mafic melt it later invades (100-200 MPa).

This study will use the volatile content of the numerous trapped melt inclusions to constrain the storage depths and ascent history of each magma body and any deeper parent that may be feeding the shallower chambers. Isotope anlaysis will examine the degree of crustal contamination and the melt history will be experimenatlly reconstructed. Microprobe analysis of the rims of large ubiquitous plagioclase phenocrysts will be used to determine the speed of mixing by constraining the rate of chemical diffusion in banded and streaky pumices.

59


Making granites in the BPIP: How did the evolved rocks in Centre 3,

Ardnamurchan form? J. MURRAY*1, F.C. MEADE2, R.M. ELLAM3, V.R.

TROLL2 1 School of Geography, Earth and Environmental Sciences,

University of Birmingham, UK. (*[email protected])

2 Department of Earth Sciences, Uppsala University, Sweden.

3 Scottish Universities Environmental Research Centre, UK.

The Ardnamurchan Central Complex is part of the British Palaeogene Igneous Province (BPIP) and formed in response to the opening of the North Atlantic Ocean. Ardnamurchan lies within the Northern Highlands Terrane, the basement of which is composed of Lewisian granulite and amphibolite facies gneiss, overthrust by psammites of the Moine Supergroup. These basement country rocks are overlain by Jurassic shale, mudstone and limestone. Centre 3, the youngest of the centres in Ardnamurchan, is dominated by the Great Eucrite, a gabbro lopolith, with a small proportion of tonalite and quartz monzonite in the centre.

Using whole rock Sr and Pb isotope ratios, major, trace and rare earth element (REE) geochemistry, we aimed to settle the long-standing debate over the exact mechanisms by which these evolved rocks formed. Richey & Thomas (1930) proposed a separate stock shaped intrusion, which cross cuts the earlier gabbros. Other studies (e.g. O’Driscoll, 2007) have suggested that Jurassic sedimentary rocks may have melted and pooled above the centre of the gabbro lopolith, mixing and forming tonalite and quartz monzonite; however a full geochemical study has not been previously undertaken.

Rare earth element data shows significant LREE enrichment, indicating a strong crustal component to the magma, but does not differentiate between the similar Moine and Jurassic REE signatures. However, the evolved rocks also have a very strong Lewisian granulite-facies Pb and Sr isotope signature, which indicates that the magma has assimilated large volumes of Lewisian granulite-facies gneiss, deep in the lower crust. Therefore, a Moine influence on the REEs is most likely as the Jurassic rocks are structurally above the gabbro.

Our findings disprove the theory that the evolved rocks of Centre 3 are a cap, formed by in-situ melting and mixing on top of the gabbro lopolith. Instead their formation has been much more complex. We show that these evolved magmas formed by AFC processes, with assimilation of Lewisian granulite-facies basement taking place in the lower crust. This was followed by assimilation of Moine country rock within the upper crust, with eventual intrusion as a central stock, as originally suggested by Richey & Thomas (1930).

Richey, J, E. & Thomas, H. H. (1930) The Geology of

Ardnamurchan, North West Mull and Coll. Memoir of the Geological Survey of Great Britain (Scotland), 393pp.

O’Driscoll, B. (2007) Geol Mag 144 897-908.

Degassing of sulphur from sediments in the Siberian Traps

Large Igneous Province S. NOVIKOVA1*, C. YALLUP1, M. EDMONDS1,

A.V.TURCHYN1, J. MACLENNAN1, H. SVENSEN2 1 Earth Sciences Department, University of Cambridge,

Downing Site, Cambridge CB2 3EQ, UK. (*[email protected])

2 Dept. of Physics, PO box 1048 Blindern, 0316 Oslo, Norway

The eruption of the Siberian Traps igneous province, at

the Permo-Triassic boundary, was synchronous with the largest mass extinction in the geological record. The eruptions likely caused a climatic perturbation, although there is uncertainty regarding its magnitude, as it is unlikely that the magma itself could contain enough volatiles to have as dramatic an environmental impact as observed. The missing component might be the degassing of the country rocks (oil shales and evaporites) during heating by sill intrusion [1]. It has been hypothesized that the fluxes of carbon gases derived during contact metamorphism of sedimentary rocks and intruded LIPs may be sufficient to cause climate change of the right order of magnitude [1]. Furthermore, gas-venting structures (filled with magmatic-sedimentary breccias) have been described in the Lower Tunguska region of the Siberian Traps, which might represent pathways through which the gas escaped to the surface [1]. While there has been much work done in quantifying the potential carbon yield from the devolatilisation of sediments, there has been no attempt to quantify the potential sulphur yield, despite this being perhaps the most important species for dramatic short-term climate change. We present bulk rock sulphur and carbon concentrations, monosulphide, bisulphide and sulphate concentrations and sulphur isotopic compositions, for both igneous (sill, lava flow, tephra) and sediments (shale, evaporates) for the Nepa and Norilsk regions of the Siberian Traps. We also present homogenised olivine-hosted melt inclusion major, trace and volatile data for the lavas. We show that there is evidence for sulphur assimilation from volatile-rich country rocks, consistent with work done on mineralised intrusions in the Norilsk region [2]. We consider the implications of the data for the total volatile budget of the eruptions.

[1] Svensen, H., Planke, S., Polozov, A. G., Schmidbauer,

N., Corfu, F., Podladchikov, Y. Y., Jamtveit, B., (2009) Earth and Planetary Science Letters 277, 490–500.

[2] Li, C., Ripley E. M., Naldrett, A., Schmitt, A., Moore, C. (2009) Geology 37, 259-262

60


The post-Minoan plumbing system behaviour at Santorini Volcanic

field: Implications for the current unrest phase

C.M. PETRONE*1, L. FRANCALANCI2, G.E. VOUGIOUKALAKIS3

1 Natural History Museum, Dept Earth Sciences, Cromwell Road, SW7 5BD London (*[email protected])

2 Dipt Scienze Terra, Universita’ di Firenze, Firenze-Italy.

3I.G.M.E., 3rd Entrance Olympic Village, Athens-Greece.

The post-caldera islets of Palea- and Nea-Kameni formed as result of nine eruptive events from 197 B.C till 1950 A.D. in the center of the Santorini Minoan caldera. These nine eruptive events led to the emplacement of dacitic lava flows and domes also characterised by the presence of basalts to andesites magmatic enclaves. Dacitic rocks have low porphyritic index, which increases with time. Plagioclase is the prevalent mineral phase, followed by clinopyroxene, orthopyroxene, opaque minerals and apatite. Few resorbed xenocrysts of olivine with coronae of pyroxene are also present. Basaltic to andesitic mafic enclaves have variable texture spanning the entire range from cumulate to aphyric. In some eruptive events, porphyritic enclaves with olivine in groundmass are also found. Host lavas show a general decrease of the evolution degree with time, at the same time Mg# of pyroxenes and anorthite contents of plagioclase decrease from mafic enclaves to host lavas. Mafic enclaves have major and trace element compositions falling on the low-silica prolongation of host lavas compositional trends. Sr isotopes systematically increase with time and thus toward the less evolved compositions of lavas and mafic enclaves. The latter, along with mineral separates, generally show slightly more enriched radiogenic compositions in respect with host lavas, with the exception of the 46-47 A.D. products. All data point to mixing/mingling processes between mafic and dacitic magmas. Our data suggest the existence of a shallow layered reservoir where mixing/mingling processes take place at the interface between the upper dacitic magmas and the lower mafic magmas. Cumulus processes, crystal fractionation eventually accompanied by variable degree of crustal assimilation, also characterised the lower part of the plumbing system allowing further layering and evolutionary processes of the mafic magmas which, in turn, generate the complex and variable textures shown by mafic enclaves. Different parts of the layered reservoir were frequently and variably sampled during time, pointing to multiple arrivals of mafic magmas during the post-Minoan activity of Santorini suggesting a still very active magma source in good agreement with the current unrest phase at Santorini Volcanic field.

Geochemical evidence for relict degassing pathways in andesite

M. PLAIL*1, M. EDMONDS2, M.C.S. HUMPHREYS3, J. BARCLAY1, R. HERD1

1 School of Environmental Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK (*[email protected])

2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK

3 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK

The andesitic Soufrière Hills Volcano (SHV), active

since 1995, emits large fluxes of volcanic gases, even during eruptive pauses lasting > 1 year. It has been observed that the flux of gas is largely decoupled from the flux of magma to the surface, indicating efficient magma-vapour segregation, followed by vapour transport to the surface. Evidence for vapour transport through magma is not often preserved in the erupted rocks, perhaps due to overprinting during eruption, or because the transport zones themselves are not usually erupted.

Andesite blocks in deposits from two recent eruptive events from SHV contain narrow shear zones, up to 2 m in length and 2-10 cm in width, with alternating darker fine-grained and lighter coarser-grained bands. Analysis has shown that the dark, fine-grained bands (grain size ~30-70 µm) have very low vesicularity (~1%) with abundant zones of oxides (<8% vol), cordierite, groundmass cristobalite and resorbed plagioclase. The light, coarse bands (grain size ~100-350 µm) consist of broken plagioclase, orthopyroxene, clinopyroxene and sparse large amphibole crystals. Vesicularity ranges from 7 to 19 vol% with reduced abundance of oxides (<2.4% vol) relative to the darker non-vesicular bands. Glass is rare to absent in both types of band. Mineral compositions in the shear zones are identical to the established mineral compositions in the andesite at SHV. However, bulk XRF analyses indicate that some metal concentrations are greatly enhanced relative to the surrounding andesite. For example, copper concentrations are up to four times higher than the andesite host. Copper is typically present as copper sulphide inclusions in Ti-magnetites and plagioclase phenocrysts.

We hypothesise that the elevated metal concentrations and presence of abundant, disseminated metal-bearing grains are evidence that these zones are relic vapour transport pathways in the shallow volcanic system. Rapid shear of andesitic material formed brittle fractures either at the conduit wall or in the shallow dome, along which metal-bearing vapour or fluid was transported. During shearing, frictional heating caused both partial melting, and volatile resorption. These sheared zones may provide the first petrological evidence for vapour transport at SHV, and a window onto the early stages of mineralization at island arc volcanoes.

61


Textural variations of groundmass microlites in the 2006 and 2010

eruptive products of Merapi volcano, Indonesia: Evidence for

magma ascent and shallow conduit processes

K. PREECE*1, J. BARCLAY1, R. GERTISSER2, R. HERD1 1School of Environmental Sciences, University of East

Anglia, Norwich, NR4 7TJ, UK. (*[email protected])

2School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK.

Feldspar microlite textures in scoriaceous and dense block-and-ash flow (BAF) samples from the 2006 eruption of Merapi elucidate short timescale variations of shallow magmatic processes during a typical dome-forming Merapi eruption that can be compared to microlite textures in the rapidly extruded lava dome during the cataclysmic eruption in 2010. Measurements of areal number density (NA), mean microlite size, crystal aspect ratio and groundmass crystallinity (φ), combined with the monitoring record and field observations, allow interpretation of magma ascent processes. Crystal size distribution analysis provides constraints on the timing of crystallisation and identifies that both growth- and nucleation-dominated crystallisation regimes existed during the 2006 eruption, resulting from changing conditions of undercooling (ΔT) during variable magma ascent paths. Stage I of the 2006 eruption (BAFs emplaced between 11th May and 1st June 2006) was characterised by variable magma ascent and extrusion rates, with calculated crystallisation time scales correlating with the monitoring record of magma moving at shallow depths within the edifice. Stage II samples (BAFs emplaced on 14th June 2006) provide evidence of more rapid magma ascent, over a period of a few days, from depths within the amphibole stability field. Stages III (post 14th June BAFs) and IV (remnants of 2006 dome at the summit) indicate that later magma batches stalled temporarily at shallower levels (< ~ 3 km) before being extruded, supporting a previously proposed idea of a shallow, ephemeral magma chamber at Merapi. Despite basaltic andesite whole rock compositions, groundmass glass is rhyodacitic to rhyolitic, with the compositional variation correlating with groundmass crystallinity. Plagioclase microlite compositions show evidence of decompression-induced degassing, often displaying more alkali-rich (anorthoclase) rims. In contrast, analysis of the fast-growing 2010 lava dome preserves evidence of a crystallisation regime more strongly dominated by nucleation, due to higher ΔT during faster ascent.

This work shows that even during an effusive (VEI 1) dome-forming eruption at Merapi, such as in 2006, magma ascends quickly from depth, over a period of a few days. Such behaviour offers little warning time to evacuate the surrounding densely-populated region, inhabited by over 1 million people. During 2010, higher ascent rates, inhibiting magma degassing and causing increasing overpressure, are interpreted to have contributed to the increased explosivity compared to 2006.

Reconciling sulphur dioxide emissions from satellite data with

petrological volatile data for explosive eruptions of Mount Etna,

Italy L. SALEM*1, M. EDMONDS1, B. MCCORMICK1, S.

CARN2 1Department of Earth Sciences, Downing Street, University

of Cambridge, Cambridge CB2 3EQ, United Kingdom (*correspondence: [email protected])

2Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI, USA

The rise of new satellite-based instruments such as the

Ozone Monitoring Instrument (OMI) grant the opportunity to measure SO2 released during volcanic eruptions with greater sensitivity than ever before [1]. Satellites remain the safest and most effective method of assessing eruptive SO2 mass. We present a new OMI dataset of SO2 emissions from the eruptions of Mount Etna, Italy, between October 2004 and December 2008. Known uncertainties in the OMI dataset such as interference from cloud cover, seasonal and altitudinal variations in detection limit, and variable spatial resolution are quantified. OMI data are compared to other satellite and ground-based observations of these eruptions.

The SO2 output during an eruption is dependent on the pre-eruptive concentrations of sulphur in the melt and in the vapour, and on the erupted mass of lava. It has been observed that a large fraction of sulphur exsolves into vapour prior to eruption for oxidized magmas [2]. A direct comparison between gas emissions data and petrological data, combined with data on erupted volumes, will allow assessment of the gas/melt ratio for different eruptions, and for different stages within an eruption, which might be related to eruption style. The formation of a gas-rich “cap” in the magma reservoir prior to eruption, for example, might lead to high gas emissions relative to erupted magma volumes at the beginning of an explosive eruption.

Literature will be mined for measurements of sulphur in olivine-hosted melt inclusions for explosive eruptions between 2004-2008. The inclusion data will be compared to the OMI time series to generate an empirical model for sulphur partitioning between melt and vapour. We will use the model to predict the SO2 output for a large historical eruption of Mount Etna for which no satellite data is available. To this end, we have analysed olivine-hosted melt inclusions for major and volatile species in tephra erupted during the 1669 Monti Rossi flank eruption, which was the largest eruption in the last four hundred years at Mount Etna.

[1] McCormick, B.T., et al. 2012. Remote Sensing of

Volcanoes and Volcanic Processes: Integrating Observation and Modelling. In press. Geological Society of London Special Publication

[2] Wallace, P.J., M. Edmonds, 2011. The Sulfur Budget in Magmas: Evidence from Melt Inclusions, Submarine Glasses, and Volcanic Gas Emissions. Reviews in Mineralogy and Geochemistry 73 (1) pp.215-246

62


Effects of Shear Strain on Deformation and Degassing of

Three-Phase Magmas J. SHIELDS*1, L. CARICCHI2 , D. FLOESS3, H. MADER1,

M. PISTONE4

1 School of Earth Science, University of Bristol, Bristol, UK (*[email protected]).

2 University of Geneva, Geneva, Switzerland. 3 University of Lausanne, Lausanne, Switzerland. 4 Geochemistry and Petrology, ETH Zurich, Switzerland

Simple-shear experiments were performed on synthetic, haplogranitic magmas, with 12-36 initial vol.% CO2-rich bubbles and 0-42 initial vol.% crystals, in a HT-HP Paterson-type pressure vessel, to final strains between γ=0 and γ=10. 3-D imaging and analysis of microstructures was performed using x-ray tomography and Blob 3D software. Bubble elongation and coalescence begins as low as γ=0.3 (φb 20 vol.%) and increases with deformation to produce planar bubble networks at γ=5 (φb 16 vol.%). Bubble connectivity, localization of strain and brittle fracturing of samples increase with crystal content. Progressively lower bubble volume percentages with increasing strain, along with strain-hardening rheological behaviour, suggest significant shear-induced outgassing, which likely occurs through the development of connected bubble or fracture networks. Evidence for formation and subsequent closure of permeable pathways provides an effective mechanism for considerable degassing of samples, which are analogous to volcanic conduits at depth. This could explain the observed transitions from explosive to effusive activity observed at many silicic volcanoes, as well as the formation of flow-banded obsidian.

Effect of fO2 on phase relations and sulphur mobility during magmatic

differentiation of a basaltic andesite S. SKORA*1, J. BLUNDY1

1 School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol, BS8 1RJ, UK. (*[email protected])

New geochemical data suggest that parental magmas

generated in subduction zones have similar base and precious metal concentrations compared to that of mid ocean ridges (1,2). Hence, enrichment of these metals that ultimately lead to the formation of large porphyry copper deposits must be explored in the light of magmatic differentiation processes beneath arcs. A very important parameter to consider is thereby fO2 because of the chalcophile nature of Cu (and many other base and precious metals), coupled to the redox sensitive mobility of sulphur (simplified speaking, sulphur is mobile as sulphate under oxidising conditions, whereas it is immobile as sulphide under reducing conditions). It is further suggested that magmatic crystallization under oxidising conditions (keeping sulphides such as pyrite absent, which would scavenge Cu) is key in pre-enriching Cu to economically valuable concentrations (1).

We have conducted a preliminary set of fO2-buffered, S-doped piston cylinder experiments (5 kbar, 950-1150°C; fO2 = QFM-1.5 and + 0.5; QFM = quartz-fayalite-magnetite buffer), exploring the phase relations of a basaltic andesite that is representing the most mafic compositions erupted by Lascar Volcano (Andes, Chile) (3). The liquidus is located at 1100°C > T > 1150°C. Phases to appear with decreasing temperature are: spinel ± FeTi oxides, followed by orthopyroxene, clinopyroxene & plagioclase. In agreement with previous studies, we find that e.g. phase relations and crystallinities at constant temperatures vary with fO2, (with run products being considerably more crystalline under oxidising conditions). Anhydrite (a sulphate) is the sulphur bearing phase in oxidising experiments, appearing in run products at T ≤ 1050°C. Sulphur phase-in under reducing conditions was not yet found, likely owing to the high melt fraction in these experiments, even at low temperatures (950°C). We speculate that a sulphide will be the sulphur-bearing phase that joins the assemblage at lower temperatures.

Anhydrite, in contrast to e.g. pyrite, is not known to be a good host for Cu. Our preliminary results hence suggest that oxidising conditions inhibit the formation of a sulphide phase, keeping sulphur (and by inference Cu) mobile. In contrast to other studies, we find that sulphate is stable over sulphide at QFM+ 0.5, which is about 1 log unit lower than previously reported. This discrepancy is likely explained by the highly complex behaviour of sulphur as a function of fO2, Fe concentration, Fe/S, melt polymerization, X(H2O), etc. Future experiments will continue to explore phase relations and sulphur solubility and speciation in crystallizing, mafic magmas.

[1] Jenner, F.E. et al. (2010) J. Petrol. 51, 2445-2464. [2] Lee, C.-T. et al. (2012) Science 336, 64-68. [3] Matthews, S.J. et al. (1999) J. Petrol. 40, 1891-1919.

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Timescales of upper crustal residence at Campi Flegrei, Italy

V.C. SMITH*1, K. SAUNDERS2, R. ISAIA3 1 University of Oxford, Research Laboratory for

Archaeology and the History of Art, Dyson Perrins Bldg, Souths Parks Rd, Oxford OX4 1SA (*[email protected])

2 University fo Bristol, Department of Earth Sciences, Wills Memorial Building, Queens Rd, Bristol BS8 1RJ

3 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, via Diocleziano 328, 80154 Napoli, Italy Campi Flegrei caldera, Italy has produced some of the

largest eruptions in Europe, and has been very active in the last 15 kyrs with more than 60 explosive eruptions.

Zoned clinopyroxene crystals within these eruption deposits the provide a detailed record of the magmatic processes. Here we present compositional data, and use diffusion chronometry to assess timescales of crystallisation. These data provide information on range of melts generated beneath Campi Flegrei, magma flux rates, and timescales of upper crustal storage. It is clear that magmatic system is open and that upper crustal residence is short. There is also a clear relationship between vent location, residence time and compositional evolution.

Changes in the composition of fumarole fluids, seismicity, and deformation over the last 20 years indicates that pulses of melt are periodically emplaced in the upper crust (Chiodini et al., 2012). This data is consistent with our data that shows that the recent eruptions have tapped numerous pods of melt that were intuded into the crust between 1 and ~70 years before the eruption.

Chiodini, G., et al. (2012) Geology 40, 943-946.

Triggering of major volcanic eruptions recorded by actively forming cumulates on Tenerife M.J. STOCK*1,2, R.N. TAYLOR2, T.M. GERNON2

1 Department of Earth Sciences, University of Oxford, OX1 3AN (*[email protected])

2 School of Ocean and Earth Science, University of Southampton, SO14 3ZH

Volcanic eruptions are commonly triggered by the

interaction between two compositionally discrete magma batches1. This may occur within either an open or closed system and within numerous geologic environments. However, until now identification of pre-eruptive mixing events has been difficult in explosive systems, limited largely to interpretations based on whole rock analysis. We have recovered partially developed (‘live’) cumulate mush from the pyroclastic deposits of a series of major eruptions on Tenerife2. These samples represent frozen pieces of the actively crystallising magma chamber, which are permiated with the final melt immediatley prior to each catastrophic eruption. Through petrologic and geochemical analyses we have used these cumulate nodules as tools for examining magmatic evolution prior to large-scale explosive volcanic eruptions on Tenerife.

Cumulate material recovered in each eruption is petrologically diverse. The most mafic nodules are wherlites and pyroxenites, which are taken to represent material that originated close to the chamber floor. Compositions range through pyroxene hornblendite, horneblende gabbro, gabbro and syenite, with more evolved samples sourced from progressivley higher layers. The variety of nodules within each deposit indicates that the magma resevoir completely disintigrated on eruption, causing the well-documented caldera collapse events and ocean-island landslides2.

Major- and trace-element zoning profiles across cumulus plagioclase and clinopyroxene crystals record prolonged magmatic evolution through fractional crystallisation, punctuated by periodic mafic recharge events. However, the most striking feature of these zoning profiles is the consistant presence of a chemically distinct zone at the rim of cumulus crystals, which likely grew less than 1 yr before eruption. These mineral zones record the presence of a significantly more evolved melt throughout the cumulate pile. Pre-eruptive mixing with a felsic melt is confirmed through comparison between the REE composition of the intercumulus material and the modelled cumulus clinopyroxene-forming liquids. These felsic-mafic interactions are likely caused by destabilisation and overturn of the stratified magma chamber and represent a potential repeating trigger for large-scale plinian eruptions on Tenerife. This finding is important for hazard assessment on a populated island with on-going felsic volcanic activity.

[1] Sparks S. R. J. et al., 1977, Nature, 267, 315-318 [2] Brown et al., 2003, Geological Magazine, 140, 264-288

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The Alpehué eruption, Sollipulli Caldera, Southern Chile

K. STREHLOW*1, A. FREUNDT2, S. KUTTEROLF2, J.C. SCHINDLBECK2

1 Department of Earth Sciences, University of Bristol. (*[email protected])

2 GEOMAR Helmholtz Centre for Ocean Research Kiel. The ice-filled Sollipulli Caldera is located in the

Southern Volcanic Zone of the Andean Volcanic Arc, where subduction of the Nazca Plate induces volcanism. 2900 years ago, the plinian Alpehué eruption, originating from a vent at the caldera margin, emplaced pyroclastic flow deposits and more than 14 km³ of fallout tephra from eruption columns 16 to 28 km high with mass flow rates up to 3*107 kg/s. In order to better understand triggering and dynamics of this event as a case study for future activity in this developing tourist region, we apply stratigraphic, granulometric, petrologic and geochemical methods to constrain the magmatic evolution and the eruption history of the Alpehué Tephra.

The fallout succession is stratigraphically divided by two prominent ash layers and several lithic-enriched layers, and has a variable amount of crystal-ash matrix causing a bimodal grain size distribution. The eastward dispersal fan is bilobate and asymmetric in shape as well as in lateral grain-size and component distributions. We attribute these lateral variations to an atmosphere profile with a tropospheric northwest wind and a stratospheric southwest wind such that the longer their fall times the more are falling particles deflected southeastward. In addition, interaction with the glacier seems to have variably phreatomagmatically influenced the eruption such that subsequent eruption pulses reached different heights in the atmosphere.

Juvenile components include the vastly dominant, pale-beige trachytic to rhyolitic pumice, gray and banded pumices, very crystal-rich pumices resembling cumulates, and mafic lava nodules, which all together form well-constrained differentiation trends from basalt to rhyolite that are compatible with fractional crystallization of the phenocryst phases. The cauliflower-shaped lava nodules represent mafic replenishing melt that was quenched against the colder trachyte/rhyolite magma. From mafic lava nodules to cumulate-like clasts of intermediate bulk-rock composition through to the most evolved rhyolitic pumice, all share the same rhyolitic matrix-glass composition meaning that variations in bulk-rock compositions merely results from the variable crystal contents. Crystal re-distribution in rhyolitic melt thus was the major cause of compositional zonation in the Alpehué magma chamber, which resided at 4-11 km depth in the crust as deduced from cpx-liq barometry. With H2O contents of 4-6 wt% determined from plag-melt equilibria, the rhyolitic melt was water-saturated at these depths. We thus conclude that the Alpehué magma was ready to erupt and that minor mafic replenishment just served as the last straw to break the camel's back.

Evidence for an open magma system feeding the compositionally

diverse Laacher See (Germany) eruption

E. TOMLINSON*1,2, V. SMITH3, M. MENZIES 2 1 Department of Geology, Trinity College Dublin, Dublin 2,

Ireland (*[email protected]) 2 Department of Earth Sciences, Royal Holloway University

of London, Egham, Surrey, TW20 0EX, UK. 3 RLAHA, University of Oxford, Oxford, OX1 3OY, UK.

The 12,9 ka eruption of the Laacher See volcano (East Eifel, Germany) represents one of the most violent recent eruptions in Europe. Approximaltely 6.3 km3 DRE (Harms and Schmincke, 2010) of volatile rich phonolite magma erupted from what has previously been interprered as a continuously zoned magma chamber (Worner and Schmincke, 1984a).

Here, we present new major and trace element data for volcanic glass and clinopyroxene from the Lower, Middle and Upper Laacher See Tephra (LLST, MLST and ULST). The data challenges the view that the LLST, MLST and ULST are directly related by fractional crystallisation (Tait et al., 1989; Worner and Schmincke, 1984b). There is an abrupt compostional gap between glasses of the MLST and ULST, and ULST glass compositions indicate mixing with a more mafic composition. Co-erupted scoria (not previously reported) is found throughout the LLST and MLST. The scoria is basanitic and its trace element composition is consistent with the ULST mixing end-member. In addition, mingling between ULST and basanite is seen at the top if the ULST unit, implying a second input of basanitic magma into the Laacher See chamber. This is consistent with compositions of pseudo-oscillatory sanidine phenocrysts in th ULST, which imply significant temperature oscillations (Ginibre et al., 2004)

This picture of varying melt composition (and temperature) at Laacher See is consistent with the emerging view that magma systems show temporal variability in temperature, crystallinity and melt composition and also in the gradients of these variables, as well as being subject to extensive open system processes (Bachmann and Bergantz, 2008).

Harms, E. and Schmincke, H.U., (2010) Contrib. Mineral.

Petr. 138, 84-98. Worner, G. and Schmincke, H.U. (1984a) J. Petrol. 25,

805-835. Tait, S.R., Wörner, G., Van Den Bogaard, P. and

Schmincke, H.-U. (1989) J. Volcanol. Geoth. Res. 37, 21-37.

Worner, G. and Schmincke, H.U. (1984b) J. Petrol. 25, 836-851.

Ginibre, C., Worner, G. and Kronz, A. (2004) J. Petrol. 45, 2197-2223.

Bachmann, O. and Bergantz, G.W.. (2008) Rev. Mineral. geochem. 69, 651-674.

65


What lies beneath? A Sr and Pb isotope study of intrusive rocks on

the Isle of Mull M. TURNBULL*1, F.C. MEADE2, G.R. NICOLL3,

R.M. ELLAM4, V.R. TROLL2

1 Midland Valley Exploration, Glasgow, G2 2HG, UK (*[email protected])

2 Department of Earth Sciences, Uppsala University, SE-753 36, Uppsala, Sweden

3 Neftex, Abingdon, Oxfordshire, OX14 4RY, UK 4 Scottish Universities Environmental Research Centre, East

Kilbride, G75 0QF, UK

The Isle of Mull underwent extensive periods of volcanic and intrusive activity 60-55 Ma, forming 3 intrusive centres. Mull lies within the Northern Highlands Terrane, but is in close proximity to two major terrane boundaries, the Moine Thrust and the Great Glen Fault. Using detailed whole-rock geochemistry (major, trace and rare earth elements, Sr and Pb isotopes) the effect of crustal contamination on the intrusive rocks has been investigated and a prominent lack of Pb isotope data, particularly for Centres 1 and 2 has been remedied.

The gabbros analysed appear to correlate with two distinct magma sources representing the REE compositions of the Mull lava fields (Kerr et al. 1999). Centre 1 gabbros show a trend similar to the Central Mull Tholeiites whereas Centre 2 gabbros echo the LREE enriched pattern of the Late Mull magma type. Kerr et al. (1999) used trace elements and Sr isotopes to show that the recorded variations in composition were directly related to magma chamber location and depth. Our new trace element data appears to support these variable sources and our Pb isotope data add weight to the concept that Centre 1 is more contaminated due to the initial intrusion of hot basic material into ‘fresh’ Moine crust. The Pb and Sr data show that both the mafic and felsic rocks of Centre 2 have far less Moine contamination. This may be due to faster ascent in an established plumbing system, reducing interaction times between magma and crust. However, Centre 2 geographically overprints Centre 1, meaning the magmas would have traversed the same crust, which could have lost the majority of its fusible components during the earlier (Centre 1) magmatism. Centre 3 is located 5km NW of the earlier two intrusions in ‘fresh’ crust and shows a comparative increase in Moine contamination. No clear evidence of lower crustal (Lewisian Gneiss) contamination is seen, nor any indication of lateral magma transport from the adjacent Grampian Highlands (Dalradian) terrane.

Kerr, A.C., Kent, R.W., Thomson, B.A., Seedhouse, J.K. & Donaldson, C.H. (1999) Geochemical evolution of the Tertiary Mull volcano, western Scotland, Journal of Petrology, Vol. 40, no. 6, pp873-908.

Degassing of sulphur from shale adjacent to a dolerite sill in Skye:

implications for the volatile budget of large igneous provinces

C. YALLUP1, M. EDMONDS*1, A.V.TURCHYN1 1Earth Sciences Department, University of Cambridge,

Downing Site, Cambridge CB2 3EQ, UK. (*[email protected])

We demonstrate, from a detailed outcrop-scale study,

that the mass of sulphur liberated from sediments during contact metamorphism around a sill intrusion is similar to the mass of sulphur dissolved in the magma. This finding suggests the contribution of sulphur from sediment degassing might be just as significant as magmatic degassing during the emplacement of LIPs. We show that sulphur and carbon have been devolatilised from shales immediately surrounding a 3-metre thick dolerite sill in Elgol, Skye, Scotland. Localised partial melting occurred within a few cm of the contact in the shale, generating melt-filled cracks. Sedimentary pyrite decomposed on heating within 80 cm of the contact, generating sulphur-rich gases (a mixture of H2S and SO2) and pyrrhotite. The remaining pyrrhotite was progressively enriched in 32S, due to the production and loss of SO2, which fractionates 34S, even at high temperatures. Further decomposition and oxidation of pyrrhotite resulted in hematite and/or magnetite within a few cm of the contact. Iron sulphates were produced during cooling and oxidation within 20 cm of the contact. Decarbonation of the sediments due to heating is also observed and is most clear at the upper contact of the sill, where carbon loss correlates with increasing δ 13C, consistent with loss of methane gas. The features observed in the shales are consistent with a short-lived intrusion, emplaced in <5 hours. The dolerite magma contains pervasive pyrite and localized sulphur concentrations greater than the sulphur concentration at sulphide liquid saturation, consistent with late-stage addition of sulphur (perhaps from sediments) at a late stage. The sulphur isotope data does not show unequivocal evidence for sulphur assimilation, but in order to explain the depletion in 34S it is necessary to invoke degassing and/or assimilation of sedimentary sulphur. Our study provides evidence for desulphurisation, as well as decarbonation, of shales adjacent to an igneous intrusion. The liberated fluids, rich in sulphur and carbon, are likely to be focused along regions of low pore fluid pressure along the margins of the sill. This enhancement of the magmatic sulphur budget by sediment degassing has important implications for the climate impact of large flood basalt eruptions that erupt through thick, volatile-rich sedimentary sequences. Questions remain regarding the mechanisms by which the fluids reach the surface, if at all, and whether it is through assimilation into the magma, or migration through fractures.

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MORB like noble gas signatures within Western Antarctic Rift Zone M.W. BROADLEY *1, C.J. BALLENTINE *1, R. BURGESS*1 1 SEAES The University of Manchester, M13 9PL, UK


The Western Antarctic Rift Zone (WARZ) represents a major area of Cenozoic extension between East and West Antarctica. The rift has been exhibiting extensive volcanism since the middle Eocene to present and represents one of the Earths major extensional zones [1]. The cause of this large scale extension is still subject to debate with both active and passive extension being proposed as the likely cause. Noble gas isotopes can provide new information on deep mantle processes which are driving the rifting. The noble gas abundance and iotopic composition within 11 mantle xenolith from Northern Victoria Land have been determined to further our understanding of the WARZ.

The fluid inclusion present within mantle xenolith provide the best medium through which magmatic volatiles can be transported to the surface and still be able to retain a pristine magmatic signature [2]. The mantle xenoliths analysed can be classified into three suites based on location of discovery. All samples were located within alkali basalt dykes and they range in composition from spinel peridotite to olivine – pyroxenite [3]. Samples were analysed for He, Ne and Ar using a VG5400 mass spectrometer. Samples were crushed in vacuo to release gas from fluid inclusion within the crystal and to avoid releasing any radiogenic gas held within the matrix. Sample exhibiting alteration or addtion of cosmogenic nuclide were excluded from this study.

The 3He/4He ratio extracted from the samples ranged from 6.25 – 8.68RA (RA = 3He/4He ratio of Air) with an average of 7.4RA this values is similar to the value attributed to MORB mantle of 8RA ± 1RA. The samples were found to also contain a 20Ne/22Ne ratio ranging from air to 11.85. With 40Ar/36Ar ratio measured ranging from 309 to 1139. All isotopic ratios measured fall within a MORB like mantle signature with the addition of an air derivived component.

This combined noble gas signature greatly differs from that found in Plume source material with the 3He/4He ratio of plumes ranging from 12 - 50RA[4] and therefore the role of plume driven rifting can be ruled out in this case. The ratios within the xenoliths also differs from the value obtained from other rift zones which are thought to have sampled the Sub Continental Lithospheric Mantle (SCLM) with the 3He/4He ratio of 6.1RA ± 0.9RA [5] noticeably differing from the values obtained from the WARZ samples. The data obtained within this study suggestes that the rifting seen within Antarctica has evolved past the point of passive lithospheric melting and is now dominated by active upwelling of MORB like mantle.

[1] Nardini et al. (2009) J. Pet 50 1359 – 1375. [2] Burnard et al (1994) J. Geophy. Res 99 17,709 – 17,715. [3] Perinelli et al (2011). 52 665 – 690. [4] Stuart et al (2003). Nature 424 57 – 59. [5] Gautheron and Moreira (2002). EPSL 199 39 – 47

The Snap Lake Kimberlite; A True Composite Intrusion

R.A. BROOKER*1, R.C. OGILVIE-HARRIS1, T.M. GERNON2, R.S.J. SPARKS1, M. FIELD3


2 School of Ocean & Earth Sciences. University of Southampton.

3 DiaKim Consulting Ltd, Wells, Somerset.

There is some debate in the literature regarding the nature of the Snap Lake kimberlite intrusion (NW Territories, Canada). Field et al. (2009) have suggested it represents two distinct magmas, one rich in ‘xenolithic’ olivine (ORK) and one olivine poor (OPK). These authors suggest the ability to bring different amounts of ‘xenocrystic’ olivine load to the surface could be important as a proxy for diamond grade and size distribution. In contrast, Kopylova et al. (2010) suggested there was a single magma with variable degrees of alteration that impart the false impression of two phases of magmatism. The result of this study (part published in Gernon et al., 2012) not only strongly supports the notion of two magmas, but also that these were intruded ‘simultaneously’ to give the first described example of a true composite kimberlite intrusion. Detailed field mapping, geochemistry and consideration of petrological differences on macro- and micro-scopic scales show how the two magmas intruded whilst still partially molten with ongoing loss of volatiles. Careful analyses of the spatial distribution, texture and composition of phlogopite phenocrysts shows distinct pressure-temperature histories for the two magmas and how they can be seen to mingle but stop short of full mixing. Mixed batches of kimberlite magma may be more common that previously thought, but it is surprising that incomplete mixing of two chemically similar compositions is preserved in the Snap intrusion. This might reflect the rapid timscale of intrusion and solidification.

Field, M. et al., (2009) Lithos 112S, 23-35. Kopylova et al. (2010) Canadian Mineralogist 48, 549-570. Gernon et al., (2012) Journal of the Geological Society 169,

1-16.

67


Vesiculation of a rhyolitic melt: New insights from hot-stage

microscopy experiments J. BROWNING*1, H. TUFFEN1, M. JAMES*1

1 Lancaster Environment Centre, Lancaster University. (*[email protected])

Although pumice is an end-member product of gas-rich

explosive volcanism, the process of bubble growth which leads to the formation of pumiceous textures are not well constrained. Vesiculation in rhyolitic melts is a primary control on some of the largest explosive eruptions. This study presents the results of a series of experiments which have utilised hot-stage microscopy techniques to track vesicle growth in an initially vesicle-poor rhyolitic melt. Using rhyolitic obsidian erupted from Chaiten, Chile in 2008 (containing ~1.38 wt. % H2O), thin wafers were held at atmospheric pressure for periods of between 5 minutes and 2 days in the hot-stage, at temperatures between 575 oC

and 875 oC. In-situ vesiculation was directly observed and the growth of individual bubbles measured using image tracking code in MATLAB. It was found that bubble growth rates increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 oC is ~1.27 mm s-1, compared with the lowest observed growth rate of ~0.02 mm s-1 at 725 oC; below this temperature, no growth was observed. Average growth rate Vr follows an exponential relationship with temperature and melt viscosity where Vr ≈ exp (0.0169T) and Vr ≈ exp (-1.202m). The extent of diffusive degassing from wafer surfaces was estimated with simple diffusion models. Diffusive loss was found to be negligible during brief high-temperature experiments but became increasingly important in slower, lower temperature experiments. Several stages of bubble growth were directly observed, including initial relaxation of deformed existing bubbles into spheres, extensive growth of spherical bubbles, and, at higher temperatures, close packing and foam formation. An advantage of the imaging techniques used here is that bubble-bubble interactions can be observed in-situ at a scale of 2 to 3 microns. Evolving bubble number densities (BND) with time were determined, allowing nucleation rates to be estimated. Maximum observed BNDs were 3.4 × 1012 m-3 with maximum increases of around 160 % observed in samples with lower initial vesicularity (< 5.7 × 1011 m-3). Experimentally determined rates of nucleation, growth and coalescence assist in the reconstruction and vesiculation history of quenched products and in models of magma vesiculation at shallow levels.

Trace element systematics of zircon from I- and S-type granites

A.D. BURNHAM*1,2, A.J. BERRY1,3, I.S. WILLIAMS3, R.B. ICKERT4

1 Department of Earth Science and Engineering, Imperial College London (*[email protected])

2 School of Earth Sciences, University of Bristol. 3 Research School of Earth Sciences, Australian National

University, Canberra.

4 Berkeley Geochronology Center, California 94709, USA.

Zircon is an invaluable accessory mineral because it provides robust determinations of magmatic ages and withstands subsolidus and hydrothermal alteration, thereby recording geochemical and thermobarometric data indelibly. Detrital zircons (e.g. Hadean grains from the Jack Hills) can only provide information about their parental magmas where there are well-characterised natural and experimental samples to compare to. Rare earth element patterns in zircon have been shown to reflect the oxidation states of their host rocks, though Ce and Eu can be decoupled by crystallisation of feldspars, which readily incorporate Eu2+. Bulk rock Fe oxidation state ratios, Fe-Ti oxide systematics and S concentrations in apatite all indicate that S-type granites are generally more reduced than I-type granites (Chappell & White, 2001). This study presents data from zircons separated from granites (sensu lato) of the Lachlan Fold Belt, Australia, to assess the extent to which source and process can be identified from Ce, Eu and other trace element systematics of populations of crystals.

Trace elements in zircons from a range of I- and S-type granites from the Lachlan Fold Belt, Australia (selected to cover a range of ages, major element and isotopic compositions) were analysed by laser ablation mass spectrometry. Simultaneous U-Pb age determination allowed inherited (xenocrystic) crystals to be identified and disregarded.

Ce anomalies in zircons from the I-type granites were larger by a factor of ~10, indicating that the oxygen fugacity of these magmas was ~4 orders of magnitude higher. Despite multiple recent attempts to calibrate this oxybarometer (Trail et al., 2011; Burnham and Berry, 2012; Trail et al., 2012), further work is required to interpret the Ce anomalies fully. Surprisingly, there is a systematic difference between the Eu anomalies in the two granite types (more negative in the S-type granites). This is likely to be due to the relative oxidation states of the melts, and suggests that the Eu anomaly records redox information despite the influence of plagioclase on the Eu budget of a magma. Differences in the abundances of other elements, including U, Th, Li, P, and Hf will also be discussed with reference to the relative contributions of source composition and magmatic differentiation for these variations.

Burnham and Berry (2012). Geochimica et Cosmochimica

Acta 95, 196-212. Chappell & White (2001). Transactions of the Royal Society

of Edinburgh 83, 1-26. Trail et al. (2011). Nature 480, 79-82. Trail et al. (2012). Geochimica et Cosmochimica Acta 97,

70-87.

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A tale of two magmas: Petrological insights into mafic and intermediate

explosive volcanism at Volcán de Colima, Mexico

J.M. CRUMMY*1, I.P. SAVOV1, D.J. MORGAN1, M. WILSON1, C. NAVARRO-OCHOA2, S. LOUGHLIN3

1 Institute of Geophysics & Tectonics, School of Earth & Environment, University of Leeds, Leeds, UK. (*[email protected])

2 Observatorio Volcanologico de Colima, Universidad de Colima, Colima, Mexico.

3 British Geological Survey, Edinburgh, UK.

Volcán de Colima in western Mexico explosively erupts basalt to high-silica andesite magmas. Detailed petrological and geochemical analyses of Holocene tephra fallout deposits reveal two distinct magma types: I. typical calc-alkaline series magmas; and II. mixed calc-alkaline - alkaline magmas. Group I magmas comprise basalt to high-silica andesite (50.7 to 60.4 wt.% SiO2) and typically contain phenocrysts of plagioclase + clinopyroxene + orthopyroxene + Fe-Ti oxides ± hornblende ± olivine. Crystallinity varies from 10-25 vol.% dominated by plagioclase in a groundmass comprising highly vesiculated glass with abundant microlites. Back-scatter electron (BSE) microscope images together with electron microprobe analyses (EPMA) reveal complex zoning patterns and compositional variations in plagioclase and pyroxene phenocrysts which have been interpreted to have resulted from a complex crystallisation history involving multiple magma mixing and decompression events.

Group II magmas comprise basalt to basaltic-andesite (48.3 to 57.5 wt.% SiO2) and contain 10-15 vol.% crystals comprising clinopyroxene + olivine + phlogopite + plagioclase + Fe-Ti oxides ± hornblende ± orthopyroxene. The groundmass comprises highly vesiculated glass with abundant microlites of the same mineral phases. Clinopyroxene and olivine phenocrysts have high-Mg cores (Mg# 88-89) that display strong dissolution with clear resorption and recrystallisation. EPMA analyses reveal large compositional differences with the surrounding growth zone (Mg# 80) indicating recrystallisation and re-equilibration within a compositionally different melt. This composition of the clinopyroxene is similar to that of the Group I magmas.

Whole-rock geochemical and Sr and Nd isotopic analyses reveal strong trends in the Group II magmas towards the composition of monogenetic cinder cones composed of phlogopite-bearing alkaline lamprophyre situated to the north of Volcán de Colima. The alkaline magmas are thought to have formed from partial melting of metasomatically enriched veins within the lithospheric mantle. We suggest the high Mg clinopyroxene cores of the Group II magmas crystallised from such alkaline melts, which then mixed with the parental mantle-derived melts of the Group I magmas.

Dynamics of deforming partially molten regions and the nucleation of

dykes M. DIEZ*1, J. BLUNDY2, A. HOGG3

1 Shcool of Earth Sciences. University of Bristol. (*[email protected])

2 School of Mathematics. University of Bristol

The dynamics of deforming partially molten regions at

different tectonic boundaries is an outstanding problem involving the crossing of many spatial scales, from melt pockets to much larger transport structures. These regions are the source for a large volume of the magmatism occurring in the planet. Thus, their dynamics plays an important role on volcanism and plutonism, specifically through processes leading to dyke formation. In the last two decades or so, field studies of exposed lower crustal migmatitic domains, cooling and deforming intrusive bodies and fossil arcs reveal that partially molten regions organize from pore micron-scale into centimetre-size bands, leading to meter-size transporting dikes. Experiments on deforming synthetic aggregates in simple shear also confirm the organization of partially molten mixtures into shear bands. Even though dyke propagation has been intensively studied, processes governing the previous stage of dike initiation still remain poorly known. These processes control the initial dimensions of the dike and thus will have a large impact on its propagation dynamics once it leaves the source. We look for a theory that accounts for the time and spatial scales inferred from field and experimental studies. We propose that in this theory dike initiation occurs through a nucleation stage governed by melt segregation and compaction within a deforming mixture. This stage would be followed by the comparatively better understood propagation stage, mainly governed by channel flow. In this study we focus on the first nucleation stage. We make use of two-phase theory for mixtures undergoing shear flow, and derive solutions in pure shear. Dyke nucleation and the role of damage is then explored in different conditions, such as stretching host-rocks on top of diapiric upwellings and lithospheric regions undergoing extension.

69


The Red Hills Intrusive System: Easternmost porphyry copper deposit in southwestern North

America A.K. GILMER*1, J.R. KYLE2

1 Virginia Div. of Geology and Mineral Resources. (*[email protected])

2 Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin.

The Red Hills intrusive system hosts the easternmost Laramide porphyry copper deposit in southwestern North America. The Red Hills pluton crops out near the southern margin of the 32-Ma Chinati Mountains caldera. Zircon U-Pb, molybdenite Re-Os, and sericite 40Ar/39Ar analyses yield ages of 64, 60, and 61 Ma, respectively, indicating that the Red Hills magmatism and mineralization are distinctly older than other Cenozoic magmatism (48–17 Ma) in Trans-Pecos Texas. The Red Hills intrusive system is contemporaneous with and genetically related to other Laramide magmatic systems (75–54 Ma) that host porphyry copper deposits in Arizona, southwestern New Mexico, and northern Mexico. These results significantly extend the Laramide magmatic province eastward and suggest that Laramide subduction-related magmatism and deformation are coextensive over a broad area of southwestern North America.

Fluid inclusion studies of the mineralized quartz stockwork in the Red Hills quartz monzonite constrain pressures from 20 to 30 MPa, corresponding to depths of formation of 2 to 3 km. Estimated salinities for the fluid inclusions from quartz veins associated with phyllic alteration range from 33 to 47 wt. % NaCl equiv. The high salinities of these fluids suggest a magmatic source.

Gilmer, A, K., Kyle, J. R., Connelly, J. N., Mathur, R. D.,

Henry, C. D., (2003) Geology Vol 31, page 447-450. Gilmer, Amy K., (2001) Age and characterization of the

Red Hills porphyry copper-molybdenum deposit and its relationship to the Chinati Mountains caldera, Presidio County, Texas. University of Texas at Austin, MS. thesis, 213 pages.

The structure and evolution of shallow magmatic systems emplaced

in fold-and- thrust belts – a case study of Cerro Negro, Neuquén

Province, Argentina D. GÜRER*1, F. CORFU2, O. GALLAND1

1 Physics of Geological Processes (PGP), University of Oslo, Oslo, Norway. (*[email protected])

2 Department of Geoscieces, University of Oslo, Oslo, Norway.

In contrast to the classical concept of magma ascent in

extensional settings, recent studies show that volcanism also occurs in compressional settings. The nature of the interplay between magmatism and tectonics in fold-and-thrust belts however, remains a major question, notably in active margins. The mechanisms of magma transport in such settings and whether magmatism affects tectonic deformation need to be addressed.

Therefore, we carried out detailed structural mapping and sampling of the Cerro Negro intrusive complex, at Tricao Malal, Neuquén Province, Argentina. This intrusive system belongs to a magmatic province in the Agrio fold-and-thrust belt, located between 37°S and 38°S in the Argentinean foothills of the Andes. The fold-and-thrust belt has resulted from intense E-W shortening, and contains tight folds and thrusts, trending N-S. The intrusive complex crops out as a network of sills and dykes around a main intrusion, all of which are of andesitic composition.

The Cerro Negro plumbing system is well exposed so that the relations between the intrusions and the tectonic structures can be studied. We have identified at least two geometries of intrusion: sills that have been folded in an open anticline, and subvertical dykes that strike N-S, i.e. perpendicular to the shortening. According to field observations, the main intrusive body and the dykes have formed in a central anticline, the dykes being close to the hinge, suggesting that there is a structural control on magma emplacement. Furthermore, the dykes locally crosscut the folded sills. No deformation has been observed in the dykes, possibly due to their location close to the anticlinal hinge. This suggests that sills predate or are coeval with deformation, whereas dykes postdate deformation. ��From the structural and temporal relationships between the anticline and the dykes we infer that local stresses controlled the formation of the dykes during outer-arc stretching. This illustrates how tectonic deformation may control magma emplacement. Conversely, the traces of the main tectonic structures curve around the intrusive complex, suggesting that the latter influenced the tectonic deformation.

New U-Pb data for zircons for both dykes and sills yield ages of 11-12 Ma, indicating that the lifetime of the magmatic system was less than 1 m.y. Absolute dating confirms that the dykes were emplaced during a time of active shortening. This study contributes the first robust evidence of substantial regional shortening at least until 11 Ma in the area.

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Water/rock interaction and volcanic behavior

B. HEMMINGS*1, A. JASIM1, F. WHITAKER1, B. BUSE1, J. GOTTSMANN1

1 University of Bristol (*[email protected])

Hydrology is of key importance in volcanic island settings. On the volcanic Caribbean island of Montserrat its importance is evident: storm rainfall can trigger landslides, mudflows and devastating flooding but it also recharges the aquifers of the extinct Centre Hills (CH), which supply drinking water for the entire island. Within the active Soufriere Hills Volcano (SHV), superheating of circulating fluids can produce explosive phreatic eruptions and the chemical interaction of these fluids with the edifice rocks can lead to flank weakening and potentially catastrophic collapse. However, the very presence of an active hydrothermal system provides an attractive target for geothermal energy development. In order to mitigate the hazards and effectively exploit the resources associated with the hydrological and hydrothermal systems it is critical to understand the fundamental physical and chemical interactions that occur in the development and propagation of such systems.

The progression of volcanic activity on Montserrat from north to south over ~2.5 Ma provides a unique insight into the temporal behavior of a system from the building of a volcanic edifice, composed of a lava core surrounded by volcaniclastic aprons (SHV), to the eventual erosion back to the central core of an extinct volcano (Silver Hills, SH). This transition results from an interplay between physical and chemical processes that vary both temporally and spatially. Episodic and intense storms incise and erode the variably consolidated volcaniclastic deposits, transporting material seaward via ephemeral rivers. These storm events also have the potential to trigger volcanic hazards such as dome collapses and pyroclastic flows, as well as lahars.

However, the physical processes are affected by water-rock interactions which modify the physical properties of rocks and deposits, such as hydrothermal weakening of the edifice and reduction of slope stability. Chemical reactions between circulating fluids (both meteoric and hydrothermal) and country rock also have a dramatic effect on permeability and porosity, with important implications for the ongoing maturation of the hydrological system and the eventual distribution of aquifer and aquitard units.

The nature and product of the chemical interactions depend on climatic and topographic controls, but also the thermal and chemical conditions. Chemical processes evolve as the volcanic activity and thus the hydrothermal activity wanes. For example, on the active SHV hydrothermal alteration is dominant, locally leading to complete destruction of the igneous texture, while on the neighboring extinct CH (0.5 Ma older) meteoric weathering dominates. While the distribution of meteoric weathering is potentially controlled by the hydrothermal alteration history, it is also capable of over-printing its signature.

Through field observations, sample examination, reaction experiments and numerical modeling, we explore development and maturation of hydrological systems on Montserrat.

Experimental insights into the formation of amphibole reaction rims: Texture, mineralogy, and

processes of formation S. HENTON DE ANGELIS*1, J. LARSEN1, M. COOMBS2,

A. DUNN3

1 Geophysical Institute, University of Alaska Fairbanks. (*[email protected])

2 Alaska Volcano Observatory, U.S. Geological Survey (USGS).

3 Department of Geology and Geophysics, University of Alaska Fairbanks.

Amphibole is an important mineral present in many

calc-alkaline volcanic deposits. A hydrous phase, volcanic amphibole is only stable at pressures greater than ~100 MPa (approx. 4 km) and in melts containing at least 4 wt % H2O. When removed from their thermal and barometric stability field, amphiboles decompose to form aggregate rims of anhydrous minerals. Reaction rim thicknesses have been used to estimate timescales and rates of magma ascent, an important parameter in determining eruptive style. However, the process of reaction rim formation is complex; numerous magmatic properties exert a control on the reaction process (e.g. magma compositions and viscosities) and multiple forcing factors may be responsible for their formation (e.g. heating, decompression, or changes in magma chemistry). Few studies have performed in-depth, systematic, and quantitative investigations of reaction rim textures and mineralogy: as a result, amphibole reaction rims are poorly understood. We present the results of an in-depth experimental study into the formation of amphibole reaction rims. The experimental series took samples to differing degrees of thermal of barometric instability, over different time scales, ranging from 3 – 144 hours. The resulting reaction rims were analyzed using a variety of analytical imaging and X-ray mapping techniques. We find that a range of different processes can contribute to the formation of reaction rims. A key result is that the mineralogical and textural features of many experimental heating-induced amphibole reaction rims are indistinguishable from natural reaction rims attributed by past studies to decompression (and subsequently used to infer magma ascent rates).

71


Changes in heavy metal distribution and deposition at Poás Volcano,

Costa Rica M. HINRICHS*1, H. RYMER1, S. BLAKE1, M. GILLMAN1

1 Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA


Activity at Poás volcano, Costa Rica, is characterised by periodic cycles of activity. The volcano entered its current phase in 2008, since when there has been a steady increase in the level of activity. There has been a build up in gas flux and this has led to an increased deposition of heavy metals. The deposition of these heavy metal plume components, and their incorporation into soil, is of key interest because soils act as geochemical sinks. Once discharged to the environment heavy metals can accumulate and be a potential source of contamination for plants and animals (Alloway, 1995).

This study investigates the pattern of heavy metal transport, deposition and distribution in Poás soils over a period of two years (2010- 2011). Soils of agricultural and non-agricultural sites at two horizons (0-10 cm and 20-30 cm) were collected and their trace element content analysed. Samples were taken from eleven sites between the active vent and ~6 km downwind, as well as from two control sites that were unaffected by the activity.

None of the metals analysed (Fe, Cr, Cu, Mn, Ni, Zn, Se, As and Hg) showed a concentration above guideline thresholds. However, concentrations increased during the sampling period, suggesting changes in metal distribution and concentration as a result of the increase in volcanic activity. Concentrations generally increased with distance away from the active vent, and were highest in the agricultural soils 4-6 km downwind of the source. Heavy metal concentrations in the soils from the control sites were lower than the sites affected by the active vent, confirming that the plume is a contributing factor to the total metal content. Widespread increases of metal concentrations in agricultural soils caused by long-range transport of contaminants are therefore a risk that needs to be considered.

Alloway, B.J., (1995), Soil processes and the behaviour of

heavy metals. In: Heavy Metals in Soils, Blackie Academic and Professional, London. 11-37.

Platinum group element geochemistry of the Scourie Dykes:

Insights into the Lewisian subcontinental lithospheric mantle

H.S.R. HUGHES*1, I. MCDONALD1,

A.C. KERR1 1 School of Earth and Ocean Sciences, Cardiff University,

Park Place, Cardiff, UK. CF10 3AT. (*[email protected])

Increasing awareness of ‘critical metals’ such as the platinum group elements (PGE) has seen renewed exploration effort for these elements. This research forms part of a larger investigation into the potential for Ni-PGE mineralisation in western Scotland and Northern Ireland, incorporating Archaean, Caledonian, and Palaeogene lavas and magma conduits in an attempt to understand the underlying factors controlling mineralisation.

The NW-SE trending Scourie Dyke swarm comprises a variety of deep-seated mafic and ultramafic dykes, intruded into the Lewisian tonalite trondhjemite granodiorite foreland at the end of the Ivernian deformation, between 2.42 to 2.38 Ga (Davies et al., 2012).

Dyke samples have been collected from across the mainland Lewisian foreland, encompassing a variety of dyke types (Tarney & Weaver, 1987) including picrite, olivine gabbro, and dolerite suites. All 72 samples collected were analysed for major and trace elements, and a representative selection of 32 samples were assayed for bulk PGE. Prior to this investigation, few analyses existed of the Scourie Dykes, with only 5 published PGE analyses (Frick et al., 1994). This research now forms the most comprehensive geochemical database of Scourie Dykes to date.

All dyke groups display normalised rare earth element (REE) patterns that are moderately enriched in the light REE, however the picrite and olivine gabbro dykes are most enriched in light REE. The dolerite group dykes have flatter REE patterns, but the highest total REE concentrations. All dykes are enriched in large ion lithophile elements (some of which may be caused by variable dyke alteration) and depleted in high field strength elements.

Total PGE+Au concentrations range from 38.3 ppb (olivine gabbro group) to 2.7 ppb (dolerite group). All dykes show fractionated PGE trends, enriched in Pd-group PGE (PPGE). Dolerite dykes show a marked depletion in Ir-group PGE (IPGE), indicating a lower degree of mantle melting, leaving a significant residue of Ir-Os alloys in the source. Chalcophile element ratios (e.g. Cu/Pd) are typically higher than estimated primitive mantle, suggesting that the Archaean subcontinental lithospheric mantle underwent low degrees of partial melting thereby contributing low concentrations of PGE to the S-undersaturated parental magmas of the Scourie Dykes. Davies, J.H.F.L., et al. (2012) Goldschmidt Conference. Frick, L.R., et al. (1994) Goldschmidt Conference. Tarney, J. and Weaver, B.L. (1987) Geological Society

Special Publication Vol. 27, pp. 217-233.

72


Fe3+/ΣFe in hydrous glass M.C.S. HUMPHREYS*1, R.A. BROOKER2, D. G.

FRASER1, V.C. SMITH3

1 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK (*[email protected])

2 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK

3 Research Laboratory for Archaeology and the History of Art, Dyson Perrins Building, South Parks Road, Oxford, OX1 3QY, UK

The Fe oxidation state of arc magmas, and the Fe3+

contents of spinels in primitive arc basalts, are typically much higher than in mid-ocean ridge basalts. This has been interpreted by some as a result of the transfer of slab-derived components into the mantle wedge (Evans et al. 2012), and by others as the result of differentiation or late-stage degassing processes within the arc crust (Lee et al. 2010). Recent studies have reported a correlation between Fe3+/Fe2+ and H2O in primitive arc melt inclusions, suggesting a clear link between slab components and oxidation state (Kelley & Cottrell 2009). It is clear that the volatile contents of melts may re-equilibrate very rapidly, but the effect of changing H2O on Fe3+/Fe2+ is unclear.

We present data that suggest that observed correlations between H2O and Fe3+/Fe2+ can be expained in part by considering the acid-base proprties of the melt (Fraser 2005). In particular, basic behaviour of FeO and amphoteric behaviour of Fe2O3, combined with changes in melt basicity relating to dissolution of H2O, can explain increasing Fe oxidation state with increasing H2O. We discuss the implications of these results for using melt inclusions to investigate the oxidation state of the earth’s mantle.

Evans, K. et al. (2012) Geology doi: 10.1130/G33037.1 Fraser, D.G. (2005) Annals of Geophysics 48, 549-559 Kelley, K.A. & Cottrell E. (2009) Science 325, 605-607 Lee, C.T. et al. (2010) Nature 468, 681-685

Depositional and textural characteristics of “dry” maar

volcanoes in northern Tanzania H.B. MATTSSON*1

1 Institute of Geochemistry and Petrology, Clausiusstrasse 25, ETH Zurich, 8003 Zurich, Switzerland. (*[email protected])

Many of the apparent phreatomagmatic landforms in

northern Tanzania lack the classical evidence for phreatomagmatic fragmentation and/or deposition (such as accretionary lapilli, plastering against obstacles, vesiculated tuffs, etc.). Instead, the landforms are dominated by fluidal shaped pyroclasts and dry deposition (e.g., efficient grain-size segregation in the eruptive plume and depositional characteristics indicative of grain-flow). In addition to this, the eruptive centers are predominantly located on top of a horst structure (i.e., the Kerimasi block) in an arid environment where evaporation greatly exceeds rainfall on an annual basis. Thus, the source of water required to drive the phreatomagmatic fragmentation process is unclear in this area of the East African Rift. However, because of the geochemistry of the magmas involved (predominantly melilititic in composition) the vigor of the eruptions and the high degree of fragmentation can be attributed to rapid exsolution of CO2 during ascent of the melilititic magmas. These types of magmas have been shown experimentally to be able to hold up to 18 wt.% CO2 dissolved within the melt structure at upper-mantle pressures (Brooker et al., 2001), and it is likely that delayed nucleation during rapid decompression resulted in massive exsolution of CO2 which may explain the highly explosive character of these “apparently phreatomagmatic” landforms.

Brooker, R., A., Kohn, S., C., Holloway, J., R., McMillan,

P., F., (2001) Chemical Geology 174, 225-239.

73


Temporal geochemical changes in the Miocene Ignimbrite succession

on Gran Canaria: Crustal contamination or mantle

heterogeneity? P. NICHOLLS1, V. TROLL1,2, B. ELLIS3, A. BARKER1, I.

BINDEMAN4

1 Department of Earth Sciences (CEMPEG), Uppsala University, 75236 Uppsala, Sweden

2 Departamento de Física (GEOVOL), Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Canary Islands, Spain.

3 ETH, Swiss Federal Institute of Technology, Zurich, Switzerland

4 Dept. of Geological Sciences 1272 University of Oregon Eugene, OR 97403-1272, USA

Recent work by Bindeman et al (2008) on the

Yellowstone system has postulated the progressive assimilation of crustal volcanic material into the magmas that fed large scale silicic explosive eruptions. This is recorded in oxygen isotopes of the eruptive products. Gran Canaria hosts a long-lived volcanic system which incorporates shield basaltic lavas and an extensive succession of Miocene silicic ignimbrites and lavas (the Mogan and Fataga formations, > 1400km3). These ignimbrites were sampled for oxygen isotopes to see whether the Gran Canarian magmas were influenced by a similar assimilation process or whether temporal changes in the mantle compositions, as proposed by previous workers (eg Cousens et al 1990).

Oxygen isotopes show a progressively more negative excursion up section (14-12 Ma) which confirms that in the late stage of the Miocene cycle a change in the magma composition feeding the eruptions occurred. These more negative values are consistent with similar trends described by Bindeman. However, it is also noted that Strontium isotope ratios decrease at around the same time while Pb isotopes show correlated excursions too. These observations appear to point towards a mantle source change being more plausible than crustal contamination as the latter would likely produce an increase in strontium isotope ratios rather than a decrease. This appears to indicate that crustal contamination played little part in the later stages of Miocene activity on Gran Canaria, and that oxygen-strontium isotope correlations suggest a change in mantle source. This would reflect a change from a mixed source involving EM1, DMM and HIMU to a higher proportion of the HIMU-like component towards the end of Miocene activity , likely a function the waning of the EM1-like component in the supply column.

Bindeman et al. (2008) J. of Petrology, 49, pp. 163-193. Cousens et al. (1990) Earth Planet Sci Lett, 96, pp. 319-335.

The source of A-type magmas in two contrasting settings:

Constraints on processes and tectonics from U–Pb, Lu–Hf and

Re–Os isotopes PANKHURST, M.J.*1,2, SCHAEFER, B.F.1, TURNER,S.P.1

1 GEMOC, Macquarie University, Sydney 2109, Australia.

2 School of Earth & Environment, University of Leeds, UK. (*[email protected])

We observe a convergence of process within two different geodynamic generating post-orogenic, high-temperature, A-type magmas. The production of these distinctive magmas and concomitant formation of stable lithospheric domains is suggested to be the key factor for 1) ending orogenesis and 2) producing stable ‘pinning’ blocks that influence subsequent tectonic evolution. The end of the ca. 514 – 480 Ma Delamerian Orogeny, southeastern South Australia, is marked by ~10 Myr of bimodal A-type magmatism. Mostly felsic products were emplaced at shallow crustal levels, and outcrop in an ~300 km arc coincident with a gravity high, interpreted as voluminous mafic intrusives at mid crustal levels. Here Re–Os and Lu–Hf isotope ratios record a dominantly juvenile lithospheric mantle source, from which mafic parental melts fractionated to produce the granites. Raised lithospheric temperatures were caused by in-welling of aesthenosphere that followed convective thinning of an unstable (thickened) lithospheric column, along the strike length of the orogeny. In contrast, Re–Os and Lu–Hf isotope ratios of the A-type magmas that comprise the ca. 1598 – 1583 Ma Mesoproterozoic Gawler Felsic Large Igneous Province, central South Australia, record a dominant evolved lower crust component mixed with small amounts of juvenile lithospheric mantle. Plume head arrival resulted in a regionally elevated geotherm, driving partial melting of the most fusible portions of the upper lithosphere. This produced rapid, voluminous, bimodal magmatism that lasted for ~15 Myr, and ended the Wartakan Orogeny. These two end member case studies highlight the observation that A-type magmatism is always transient. The contemporaneous fusion of both mantle and crust represents a common, stabilizing influence on the lithospheric column regardless of tectono-magmatic setting.

74


Sm-Nd and U-Pb isotope geochemistry of the Sweetwater Wash and North Piute plutons,

Mojave Desert, California S. PHILLIPS*1, J. HANCHAR1, C. MILLER2

1 Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NL Canada A1B 3X5 (*[email protected])

2 Earth & Environmental Sciences, Vanderbilt University, Nashville, TN 37235-1805, USA

In situ analyses of the isotopic and trace element

composition of minerals at the sub-grain scale have proven to be effective tools for understanding the origins and evolution of magmatic systems. The ability to simultaneously measure Sm-Nd and U-Pb isotopes by LA-ICP-MS allows a high-resolution spatial and temporal snapshot of crystallisation history.

The late Cretaceous Sweetwater Wash Pluton (SWP) in the Mojave Desert, California, provides an excellent oppurtunity to utilise these techniques in order to understand the petrogenesis of continental arc granites. This peraluminous granite is well understood in terms of major & trace element geochemistry (Mittlefehldt & Miller) and accessory phase geochemistry (Wark & Miller). A preliminary study of monazite in the SWP (Fisher) suggest that the εNd signature of the source region is retained (~1700 Ma) yet U-Pb ages show an isotopic resetting during emplacement at ~75Ma. Mineral scale heterogeneity also demonstrates extrememe isotopic disequilibrium in monazite and titanite, whereas REE concentrations are consistant with closed-system fracational crystallisation

The current study is aimed at using these geochemical tools to further constrain the petrogenesis of the SWP and place it in its regional context. Upcoming fielwork involves systematic sampling throuhg a transect of the pluton to examine the spatial changes from the edge to the centre of the pluton as recorded in monazite and zircon.

Mittlefehldt, D.W. and Miller, C.F. (1983) Geochemica et

Cosmochemica Acta, 47, pp. 109-124. Wark, D.A. and Miller, C.F. (1993) Chemical Geology, 110,

pp. 49-67. Fisher, C.M. (2011) Unpublished PhD. Thesis.

The lattice strain model applied to coexisting garnet and clinopyroxene

J. PICKLES *1, J. BLUNDY 1, C.B. SMITH1 1 University of Bristol. (*[email protected])

Understanding the behaviour of elements and minerals subducted into the mantle is important in helping to interpret the behaviour of volatiles and processes in the overlying mantle wedge. Through high-pressure and high-temperature experiments we investigate the behaviour of elements, specifically how elements partition between garnet and clinopyroxene in the subducting slab.

The lattice strain model (Blundy and Wood, 1997) was developed on, and is routinely applied to, the partitioning of elements between a mineral and its equilibrium melt. In this study we investigate the applicability of the lattice strain model when applied to the partitioning of rare earth elements (REE) between garnet and clinopyroxene (cpx). We also investigate the potential of the lattice strain model being utilised as a geothermometer.

The data generated in this work are combined with published experimental data to compare the thermodynamic theory and the experimental data. We show that lattice strain model theory accurately predicts the experimental data. We develop new terms allowing the calculation of the Youngs modulus and ideal cation size through the major element composition without needing to know either temperature or pressure. We show that the lattice strain model can be used to calculate temperature for the experimental data. However, the accuracy of the temperature prediction is highly dependent on the Youngs modulus of the garnet. To circumvent this issue we also present an empirical thermometer based on the REE partitioning between garnet and cpx. Both models have a greater accuracy than the widely used geothermometer of Ellis and Green, 1979.

Blundy, J.D. and Wood, B.J. (1994) Nature 372, 452-454. Ellis, D. and Green, D. (1979) Contributions to Mineralogy

and Petrology 71, 13-22.

75


The role of ice cavities in lava lobe formation

H. REYNOLDS*1, D. WOODCOCK1, J. GILBERT1, S. LANE1

1 Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ. (*[email protected]) Lava lobes are small volcanic features (~ 10 m high)

which have been observed at various locations in Iceland and at Nevados de Chillán, Chile. However, the mechanism for the creation of lava lobes remains controversial. It is plausible that lava was emplaced within pre-formed ice cavities which were themselves generated by volcanic fumaroles. This study investigates the feasibility of this theory using an analogue experimental approach. The generation of ice cavities by fumarolic activity was investigated on a laboratory scale and an experiment was designed to observe and quantify the physical processes which took place. All significant heat transfer processes were considered and a model of the melting process was constructed based on experimental observations. The laboratory-generated cavities were found to most closely resemble a truncated prolate spheroid in morphology. Lava lobes are commonly conical; various processes are suggested to explain the difference between the field observations and those observed in the laboratory. A cavity growth rate of 1.8 x 10-4 m3 s-1 was estimated using experimental data. This equates to a period of 6 x 105 s to generate an ice cavity (4 m in height), which is realistic when compared to the ascent time of rhyolitic magma. This study discusses the major contributing factors which affect cavity growth and morphology including: meltwater drainage; debris and impurities within the ice; fumarole dynamics, and ice deformation. Experimental data obtained during this study could be used to calibrate more sophisticated analogue or numerical models of the physical processes of fumarolic ice melting.

Phreatomagmatic edifices produced by lava-sediment interaction

P. REYNOLDS1*, R. BROWN1, E. LLEWELLIN1, T. THORDARSSON2, K. FIELDING3,

1 Dept. Earth Sciences, Durham University, Science Labs, Durham, UK. DH1 3LE (*[email protected])

2 School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW

3 Level 9, The Adelphi Building, 1-11 John Adam Street, London WC2N 6AG

Rootless cones are formed during the explosive

interaction of pāhoehoe lava and unconsolidated substrate. Previous studies have focused on the architecture of cone groups and proposed generic models for the formation of cones, without a detailed assessment of their lithofacies architecture. This research in progress aims to determine the physical volcanology of a dissected rootless cone, and determine how the preserved deposits can be used to infer lava flow substrate conditions. Field studies in the Columbia River Flood Basalt Province, Western USA, detail transitions from Surtseyan-style tephra jetting to Hawaiian fire fountaining within individual edifices, likely to be the result of decreasing water:magma ratios with time. The explosive brecciation of host rock, sub-flow quenching of pyroclasts and admixture of substrate material are also found to be important facies-forming processes. These processes highlight previously unrecognised complexity during the growth of rootless cones arising from subtle variations in substrate conditions. Intimate mixtures of contact metamorphosed substrate and spatter fall deposits within the cone also have implications for the interpretation of peperitic textures in other volcanic settings. This study forms part of a wider project detailing the architecture of fissure derived products in Flood Basalt settings.

76


Concentrations of critical metals in the Carnmenellis biotite granite,

Cornwall, UK B.S. SIMONS*1, J.C.Ø. ANDERSEN1, R.K. SHAIL1

1 Camborne School of Mines, University of Exeter, Tremough Campus, Penryn, TR10 9EZ. (*[email protected])

Modern technological developments rely increasingly

on resources that have not been the subject of traditional exploration and mineral extraction. Emission reduction and low carbon energy production are particularly strong drivers for the rising demand for a number of rare metals. The term “critical metals” increasingly refer to the metals that are of strategic significance for technological development, and where the supplies are considered to be significantly at risk. This group includes beryllium (Be), gallium (Ga), germanium (Ge), tungsten (W), bismuth (Bi), indium (In), tin (Sn), antimony (Sb) and the rare earth elements (REE), which are particularly significant for components in wind turbines, photovoltaic cells and nuclear power stations.

Many critical metals are concentrated in polymetallic mineral deposits related to granitic igneous provinces, such as the Variscan intrusions of SW England and Germany. However, the geochemical behaviour of the metals in igneous systems remains poorly constrained. The lack of geochemical knowledge leads to poor understanding of the concentration mechanisms within the crust, and ultimately to poor models for their igneous fractionation and subsequent magmatic-hydrothermal mineralisation.

This study investigates critical metals within a carefully selected sample of the Carnmenellis granite (from Holman’s test mine near Troon, Camborne). The granite was selected as an example of the dominant type of coarse-grained porphyritic biotite granite in Cornwall, partly because of the the unweathered and unaltered mineral assemblage, and partly because the granite in this location has not experienced mineralisation.

Whole rock geochemical data indicate that Be, Ga, Ge, In, Sn, W and Bi are present in higher concentrations than average continental crust. Sn and Bi show a 5 times enrichment compared to expected values in crustal rocks with averages of 12.4 and 0.27 ppm respectively. Ge and In show a 2 to 3 times enrichment, with average values of 3.26 and 0.17 ppm whereas Ga shows a 1.5 times enrichment with an average of 24.5 ppm.

The future objectives of the study are to better constrain the concentrations of the critical metals in the different types of granite in SW England, and to establish the partition coefficients between the granitic source magma and their constituent minerals. This information will be used to assess the mechanisms of metal concentration within the host granites, and the significance of the individual granite types as critical metal sources for the magmatic-hydrothermal mineralisation systems.

The nature of deep mantle from Afar plume picrites

F.M. STUART*1, N.W. ROGERS2, I. PARKINSON2, M. DAVIES2

1 Isotope Geosciences Unit, SUERC, East Kilbride G75 0QF (*[email protected]) 2 Department of Earth & Environmental Sciences, Open

University, Milton Keynes MK7 6AA.

Most mantle plumes are widely believed to originate in

the core-mantle boundary. The earliest basalts erupted by mantle plumes are typically hotter than those derived from the convecting upper mantle and offer a window into the composition of the deep mantle. For instance, the earliest picrites erupted by the Iceland plume are strongly enriched in primordial He (3He/4He ~ 50 Ra), indicating an origin in a mantle reservoir that has been isolated from convection for most of Earth history. These basalts have, however, a range in radiogenic isotope and incompatible trace element ratios that overlap MORB and cannot simply be reconciled with pristine primordial mantle dominating the plume head.

In an attempt to provide further constraints on the source of plumes we have analysed the He-Sr-Nd-Pb isotopic composition of the earliest basalts from the ~30 Ma Ethiopian flood basalt province. Picrites from the Dilb section and characterized by high Fe and Ti contents for MgO = 14-15% that implies that the parent magma was derived from a high temperature small melt fraction, most probably from the Afar plume head. The picrites are characterized by a narrow range of 87Sr/86Sr (0.70396–0.70412) and 206Pb/204Pb (18.82-19.01), and 3He/4He of olivine phenocrysts that never exceed 21 Ra. These observations imply that the Afar plume was sourced in a discrete mantle reservoir that is less degassed and more enriched in incompatible elements than the convecting upper mantle. By contrast, the source region is more degassed than the mantle of the proto-Iceland plume and appears to be significantly more homogeneous. This suggests that the largest mantle plumes are not initiated in a single deep mantle domain with the same depletion history, and they do not mix with convecting mantle to the same extent.

77


Recognising mush disaggregation in basaltic systems: The distribution of

olivine compositions in Icelandic basalts and picrites

A.R. THOMSON*1, J. MACLENNAN2 1School of Earth Sciences, University of Bristol, Bristol,

BS8 1DR, UK. (*[email protected]) 2Department of Earth Sciences, University of Cambridge,

Cambridge, CB2 3EQ.

The importance of magmatic mushes in controlling both

the behaviour and compositional evolution of magmas has recently been a topic of widespread interest, espeically in relation to silicic systems. There is also plenty of evidence suggesting that mushes play an important role in the development of basaltic systems. For instance cumulates are an important feature both in the basal wrecks of basaltic volcanos and in ophiolitic sequences worldwide. However, despite this knowledge there has been a surprising lack of effort to identify the compositional and petrographic signature of mush zone processes in the eruptive products of basaltic volcanos. In this study we demonstrate that a number of previously unexplained petrological observations can be understood within the framework of magmatic mush dissaggregation.

A statistical investigation into the forsterite content of Icelandic olivine macrocrysts within individual eruptions, and the relationship to their carrier basaltic liquids was achieved using a large compilation of electron microprobe data. 11 eruptions were examined where glass composition and more than 60 crystal core analyses were available. Kernel density estimates and cluster modelling identified at least one statistically significant peak in olivine distributions. In 10 of the 11 eruptions it is observed that 90% of olivines are too forsteritic to be in equilibrium with the erupted melt. From the 11 eruptions, 8 show unimodal distributions of macrocryst olivine forsterite content, two are bimodal and one is polymodal. In all cases a peak in the distribution occurs at forsterite contents that are 2-3 mol% higher than those expected for olivines in equilibrium with the carrier liquid. These observations combined cannot be explained by simple models of equilibrium or fractional crystallisation.

To account for the observations a three-stage model is required. In the first stage, fractional crystallisation and crystal settling generate a mush pile on the floor of a magma chamber. Compositional stratification is present in this mush, with the olivines at its base being more forsteritic than those at its top, reflecting the evolution of liquid compositions during fractional crystallisation. In the second stage, diffusion occurs in the mush across two different paths. Individual olivine crystals homogenise, removing internal zonation, whilst larger-scale chemical diffusion occurs across the entire mush acting to generate a single peak in olivine compositions close to the mean forsterite content of the olivines in the crystal pile. Finally, the mush is disaggregated throughout the chamber interior shortly before eruption takes place. Quantitative models of this process indicate that the observed offset peak in olivine compositions can be generated after 42-8000 years of diffusion in a mush pile, depending on the mush thickness.

Loading, compaction and injection: Investigating ground deformation

on Mt Etna's Northeast Crater Flowfield A. DAVIES*1

1 Department of Geography, Girton College, University of Cambridge (*[email protected])

Loading by recent lava flows can strongly influence the

siting of flank eruptions by downwarping the surface. This increases stress at the edge of the flowfield, encouraging eruptions. Downwarping can be measured through levelling. However, lava compaction also causes volcanic subsidence. Therefore, downwarping is only confirmed if benchmarks close to, but off the flow subside by a similar amount to those benchmarks on the flow. Vertical movements recorded on the Northeast Crater flowfield on Mount Etna between 1975 and 1980 confirmed downwarping, which progressively decreased. However, there was an abrupt increase in subsidence following the 1981 north flank eruption. Here, the ground deformation was proportional to the thickness of the total flowfield. This indicated downward movement associated with north flank dyke injections (and subsequent eruptions) is controlled by the thickness of the total flowfield, rather than the most recent lava flows (Murray, 1988).

There has been no new lava loading on the Northeast Crater flowfield since 1981. However, in 2008, a dyke was injected beneath the Northeast Crater flowfield and considerable summit subsidence was observed.

This investigation aims to establish if downwarping is continuing, and whether downwarping contributed to the 2008 dyke injection. If downwarping is not evident, it can be reasoned that the lack of downwarping contributed to the lack of subsequent eruption from the injected dyke. A further research aim is to establish if abrupt flowfield subsidence associated with dyke injection is still proportional to the thickness of the total flowfield.

The data being used for this investigation is part of an ongoing annual survey of ground deformation on Mount Etna. Data collected through precise levelling in 2007, 2008, 2011 and 2012 from a traverse set up in 1975 will be used. (I was involved as a fieldworker in 2012). At the time of abstract submission, only very minor data analysis has been performed. Preliminarily, subsidence is known to be ongoing and to have decreased between 2008 and 2012. It is thought that the subsidence associated with the dyke injection is no longer proportional to the thickness of the flowfield, but further analysis is needed to confirm this.

A greater appreciation of the causes of ground deformation will improve understanding of eruptive behaviour. Understanding eruptive behaviour is crucial to minimising hazards, with the aid of hazard mapping, land use planning, and hazard forecasting. Mount Etna's characteristic slow basaltic flows present little risk to human lives, but as Mount Etna is Sicily's main income source, facilitating tourism and agriculture, flank eruptions can have a significant economic impact on the local region. Therefore, the local region would also benefit from improved understanding of the ground deformation.

Murray, JB (1988) J Volcanol Geoth Res, 35, p. 121-139.

78


Detection and categorization of geyser eruption dynamics: Insights

from infrasound monitoring at Yellowstone National Park

P.J. DEMONTE*1, J.B. JOHNSON1, A. QUEZADA-REYES2 1 Boise State University.

(*[email protected]) 2 New Mexico Institute of Mining and Technology.

Volcanic phenomena such as volatile bubble bursts, gas jets, eruption plumes and pyroclastic flows are strong emitters of infrasound, low frequency (0.02-20 Hz) elastic air waves. These surface-to-air pressure perturbations can be easily detected, making infrasound an invaluable accompanyment to seismic monitoring. Gaining direct visual observations of the infrasound signal source mechanisms is difficult at volcanoes. However, geysers at Yellowstone, which are easily accessible and display a wide range of behaviours, provide useful analogues for silicic volcanoes.

The aims of our study are to acoustically detect and characterize the eruption dynamics of individual geysers at Yellowstone in order to better understand multi-phase fluid dynamics in geothermal systems. In addition we are using the geysers to test the effectiveness of electronic condenser microphones (ECMs) and micro-electro-mechanical-systems (MEMS) in the development of low-cost acoustic sensors for volcano monitoring.

Between August 9th to 14th, 2011, three arrays of four MEMS were deployed around Lone Star Geyser (LSG) and Great Fountain Geyser (GFG) in the Lower Geyser Basin at Yellowstone. An infrasound array was also deployed at Sawmill Geyser (SMG) for an hour on 16th August 2011. Data were analyzed for their spectral content, acoustic energy and waveform characterization. To quantify and compare the pressure fields generated during explosive phases, video footage shot at the geysers was synced with the coincident acoustic signal recordings.

Results show that distinct wave forms, eruption durations and inter-eruption periods were detected for the three geysers. SMG was found to generate periodic infrasound dominated by energy in the 1-40 Hz band; its signal source mechanisms are interpreted as: 1) steam-filled bubble oscillations and 2) subsequent bursting at the free surface resulting in violent steam and water discharge. LSG, whose eruptions are characterized by ~18 m/s jets for around 30 minutes, produces higher frequency infrasound and audio-band signal evolving from 20-60 Hz to 40-85 Hz. This is interpreted as phase transition in LSG’s eruptions from mostly water (low acoustic radiation) to steam (high acoustic radiation). During the final stage of an eruption at GFG on August 11, bi-modal infrasound pulses of up to 0.7 Pa-m were detected. Modelling the pulses as volumetric sound sources, we infer that up to 32 m3 of fluid was ejected.

Between October 9th to 21st, 2012, three arrays of eight infrasonic sensors (7 ECMs; 1 MEMS) were deployed at different locations in the Lower Geyser Basin. We are currently applying back azimuth and semblance analysis to the recorded data to spatially and temporally locate multiple geyser sources.

On the lack of InSAR measurements of deformation at

Central American Volcanoes S.K. EBMEIER*1, J. BIGGS1 AND T.A. MATHER2

1 University of Bristol. (*[email protected]) 2 Univeristy of Oxford.

A systematic survey of three years of L-band InSAR

measurements of the Central American Volcanic Arc shows a striking lack of magmatic deformation. We make measurements at 20 of the 26 historically active volcanoes, none of which were deforming magmatically (2007-2010), although we do measure shallow subsidence associated with flow deposits or edifice instability at three volcanoes. We use time series variance to estimate minimum InSAR deformation detection rates for Central America (average of 2.4 cm/yr) and show that the majority (>78%) of deformation events measured at other volcanic arcs would have been measurable at the same levels of noise as observed in Central America. We estimate that if magmatic volcano deformation were spread evenly across historically active volcanoes worldwide, there would be <2% probability of none of Central America's 26 volcanoes deforming.

Central America’s high proportion of basalts to andesites and relative lack of shallow magma storage may contribute to the low number of observations of deformation relative to other parts of the world. Other factors with the potential to inhibit the geodetic expression of magma movement include vertically elongated chamber geometries and high volatile contents.

Ebmeier, S.K., Biggs, J. and Mather, T.A. (in revision) On

the lack of InSAR measurements of deformation at Central American Volcanoes. Journal of Geophysical Research – Solid Earth.

Ebmeier, S.K., Biggs, J., Mather, T.A. and Amelung, F. (in press) Applicability of InSAR to tropical volcanoes: insights from Central America. Geological Society Special Publication, Remote-sensing of volcanoes and volcanic processes: integrating observation and modelling

79


Hydroacoustic, infrasonic and seismic monitoring of the submarine

eruptive activity and subaerial plume generation at South Sarigan,

May 2010 D.N. GREEN*1, L.G. EVERS2, D. FEE3, R.S. MATOZA4,

M. SNELLEN5, P. SMETS2, D. SIMONS5

1 AWE Blacknest, Reading, UK. (*[email protected])

2 KNMI, De Bilt, The Netherlands. 3 Geophysical Institute, University of Alaska, Fairbanks,

USA. 4 Institute of Geophysics and Planetary Physics, Scripps

Institution of Oceanography, University of California, San Diago, USA.

3 Acoustic Remote Sensing Group, Faculty of Aerospace Engineering, Delft University of Technology, The Netherlands.

Explosive submarine volcanic processes are poorly

understood, due to the difficulties associated with both direct observation and continuous monitoring. In this study hydroacoustic, infrasound, and seismic signals recorded during the May 2010 submarine eruption of South Sarigan seamount, Marianas Arc, are used to construct a detailed event chronology. The signals were recorded on stations of the International Monitoring System, which is a component of the verification measures for the Comprehensive Nuclear-Test-Ban Treaty. Numerical hydroacoustic and infrasound propagation modelling confirms that viable propagation paths from the source to receivers exist, and provides traveltimes allowing signals recorded on the different technologies to be associated. The eruption occurred in three stages, separated by three-hour periods of quiescence. 1) A 46 hour period during which broadband impulsive hydroacoustic signals were generated in clusters lasting between 4 and 15 minutes. 85% of the 7606 identified events could be classified into 8 groups based on their waveform similarity. The time interval between clusters decreased steadily from 80 to 25 minutes during this period. 2) A five-hour period of 10Hz hydroacoustic tremor, interspersed with large-amplitude, broadband signals. Associated infrasound signals were also recorded at this time. 3) An hour-long period of transient broadband events culminating in two large-amplitude hydroacoustic events and one broadband infrasound signal. A speculative interpretation, consistent with the data, suggests that during phase (1) transitions between endogenous dome growth and phreatomagmatic explosions occurred with the magma ascent rate accelerating throughout the period; during phase (2) continuous venting of fragmented magma occurred, and was powerful enough to breach the sea surface. During the climactic phase (3) discrete powerful explosions occured, and sufficient seawater was vaporised to produce the contemporaneous 12km altitude steam plume.

Crustal deformation between volcanic segments of the Askja and

Kverkfjöll central volcanoes, Northern Iceland

R. GREEN*1, R.S. WHITE 1 T. GREENFIELD 1 J. TARASEWICZ1, H. SOOSALU2, J. KEY1

1 Bullard Labs, Dept of Earth Sciences, Cambridge.

2 Geological Survey of Estonia, Tallin, Estonia.

Seismicity within Iceland clusters spatially mainly within extensional volcanic systems and is often associated with active volcanic centres and geothermal systems. However in the Northern Volcanic Zone, unusually intense upper crustal seismicity is observed in a region between the fissure swarms of the Askja and Kverkfjöll volcanic systems

This seismicity is persistent through time, and episodically we observe swarms of micro-seismic events locating along linear arrays, believed to be faults. Data presented here from a dense University of Cambridge seismometer network provides evidence of sets of parallel north-easterly striking near vertical faults, with left lateral strike slip motion. Manual refinement and relative relocation techniques enable the micro-seismicity to be well located along a narrow vertical plane striking parallel to the direction of the earthquake slip vectors.

The continuous strike slip motion along these multiple faults would result in the collective rotation of the faults and the crustal blocks between them. This is known as the “bookshelf” mechanism of accommodating crustal extension (Mandl, 1987).

The tectonic deformation probably partitions as it does in these areas because the spreading direction is oblique to the trend of the volcanic rift. There are few fissures or magmatic surface features in the area (Hjartardóttir, 2009) with which to correlate the seismicity, probably because it is frequently resurfaced by volcanic flows. It is possible that high stresses in the region are as a result of its position between volcanic segments within which the main extension and magma input occurs.

Mandl, G., (1987) Tectonic deformation by rotating parallel

faults: the “bookshelf” mechanism, Tectonophysics, 141, 277-316.

Hjartardóttir, A., (2009) The fissure swarm of the Askja volcanic system along the divergent plate boundary of N Iceland, Bulletin of Volcanology, 141, 277-316.

80


Local earthquake tomographic imaging of a magma chamber

beneath Askja Volcano, Iceland

T. GREENFIELD*1, R. GREEN1, J. KEY 1, H.R. MARTENS

1, M.A. MITCHELL1, R.S. WHITE1

1 Bullard Laboratories, University of Cambridge. (*[email protected])

We have used a tomographic travel time inversion to map a pronounced low velocity anomaly interpreted as a magma chamber ~ 7 km below the caldera of the Askja Central Volcano in Iceland. We have operated a high quality seismic network of 25-30 broad-band seismometers around the Askja Volcano in the Northern Volcanic Zone since 2006. Using a subset of ~1100 well constrained earthquakes distributed across the region, P and S-wave arrival times have been used to constrain a 1D velocity model of the Askja region using the program VELEST (Kissling et al 1994). The arrival time picks were then input into a finite difference tomographic inversion program (Roecker et al 2006) and used to invert for a 3D velocity model beneath Askja.

Strong (ΔVp ~ -10%) low velocity anomalies are recorded beneath the most recent caldera within the central volcano and beneath the plain to the west of Herðubreiðartögl. These low velocity anomalies are both interpreted as large magma accumulation bodies. Beneath the strong low velocity anomalies within the mid crust, poorly resolved low velocity regions extend sub-vertically into lower crust and are interpreted as melt channels linking the magma body beneath the volcano with melt ponding in the lower crust or at the Moho. Synthetic model recovery tests show that the shape of the two large low velocity anomalies are well resolved though the magnitude of the velocity anomaly could be underestimated. Additional work using travel time delays and S-wave attenuation from regional and teleseismic arrivals will add further constraints to the size and magnitude of the low velocity bodies.

Kissling, E., Ellsworth, W. L., Eberhart-phillips, D. and

Kradolfer, U. (1994) Initial reference models in local earthquake tomography, Journal of geophysical Reserach 99, 19,635-19,646.

Roecker, S., Thurber, C., Roberts, K. and Powell, L. (2006) Redefining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique Tectonophysics 426, 189-205.

Large-scale ground deformation at Uturuncu volcano: Evidence for magma rise from the Altiplano-

Puna Magma Body J.M. HICKEY*1, J. GOTTSMANN1 & R. DEL POTRO1

1 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK (*[email protected])

This study focuses on the driving mechanism behind a

70 km wide region of ground uplift centered on Uturuncu volcano, in the Altiplano-Puna region of southern Bolivia. We use Finite Element Analysis to test first-order parameters that constrain a viable model for the observed maximum line of sight uplift rate of 1 – 2 cm/yr between 1992 and 2006. Stresses from pressure sources with finite geometries are solved numerically using COMSOL Multiphysics, accounting for both homogeneous and heterogeneous mechanical rock properties in elastic and viscoelastic rheologies. To constrain crustal heterogeneity we invert seismic velocity data which indicates the prevalence of a very large low velocity zone at depths of approximately 17 km below the surface. Combined with other geophysical observations this is deduced to represent the regional Altiplano-Puna Magma Body (APMB). We induce a viscoelastic crustal rheology using the standard linear solid model to account for monotonic time-dependent deformation and a crust with an anomalously high heat flux and thus inelastic conditions. Investigating crustal heterogeneity alongside homogeneity highlights the significant effect of a mechanically weak source-depth layer. This alters surface deformation patterns by absorbing more of the subsurface displacement, thereby acting as a buffering mechanism. As elastic models are unrealistic in this scenario and only account for the spatial component of the observed uplift, their results are used only to guide the source parameters tested in the viscoelastic models. We then demonstrate a range of possible causative source geometries but reject spherical and oblate shapes on the grounds of their depth below the APMB and likely unsustainable pressurisation given the expected crustal mechanics. A prolate shape protruding from the APMB is thus favoured. Our final preferred model suggests that pressurisation of a magma source extending upward from the APMB is causing the observed surface uplift and alludes to a continued increase in this pressure to explain both the spatial and temporal patterns. We also demonstrate how a pressure-time function may play a first order role in explaining the temporal deformation pattern.

81


A comparison of seismically imaged hydrothermal vents with field and

laboratory analogues M. HOGGETT1 N. SCHOFIELD2 S.M. JONES2

1Geography, Earth and Environmental Sciences Dept., University of Birmingham. (*[email protected])

2, Geography, Earth and Environmental Sciences Dept., University of Birmingham.

Intrusion of igneous sills into a sedimentary basin can

cause thermal maturation of organic matter and the generation of significant quantities of greenhouse gases - methane and carbon dioxide. At the same time, pressure can build to the point of rupturing the overlying strata to form a hydrothermal vent, causing an explosive eruption of the gases to the ocean and atmosphere.

Such sudden releases of greenhouse gases hold the potential to cause rapid global climate change. As an example, methane generated next to North Atlantic Igneous Province sills has been postulated to be responsible for the Paleocene-Eocene Thermal Maximum. However, both methane and carbon dioxide have finite residence times in the atmosphere. If these gases were delivered to the atmosphere slowly, they would be removed by natural processes before they could significantly affect climate. Hence, to draw any conclusions as to the importance of sill-vent complexes in causing climate change, we must know the flux of these gasses up the conduit. Little work has been carried out to address this issue.

We present brand new images of hydrothermal vents made by running spectral decomposition on a high resolution 3D seismic dataset from the Bass Basin in southern Australia. This technique has allowed us to image the vents and their conduits to a much higher level of detail than ever before. A comparison of several of these newly imaged vents is made with field studies on the morphology of supra-sill hydrothermal vents and kimberlite pipes, and with laboratory experiments on diatremes. The vent morphologies are used to place bounds on the likely flux of gases through the vent, and also on the duration of explosive activity, in order to shed light on their importance in causing rapid global climate change.

Anatomy of the onset of the current repose period at Volcán de Colima

during July 2011 O. LAMB*1, N. VARLEY2, T. MATHER 1, D. PYLE1

1 Department of Earth Sciences, University of Oxford 2 Facultad de Ciencias, Universidad de Colima. (*[email protected])

After nearly 13 years of continuous activity which included multiple periods of lava-dome extrusion and explosive activity, Volcán de Colima (VdC) in Mexico ceased erupting in July 2011. Historical activity at VdC has been dominated by two century-long cycles, 1814-1913 and 1913-present, with the transition between these cycles marked by a major eruption in 1913, which occurred after a period of quiescence (Luhr and Carmichael, 1980). We have used a suite of statistical tools (including detrended fluctuation analysis and spectral analysis) to analyse the 13 months of continuous volcano-seismic data collected in the period up to and including the beginning of the repose period. These statistical techniques will be used to investigate whether there are any detectable changes in the characteristics of seismic timeseries which may give clues to the process or processes that may have led to the cessation in activity. These statistical techniques have previously been shown to work well when analysing fluctuations in the behaviour of both VdC (e.g. Varley et al., (2006), Lachowycz et al., (2013)) and Soufriѐre Hills, Montserrat (e.g. Nicholson et al., (2013). The aim of this work is to improve our recognition and understanding of the nature of eruptive pauses during long-lived dome-forming eruptions.

Lachowycz, S. et al., 2013. Long-range correlations

identified in time-series of volcanic seismicity during dome-forming eruptions. (In preparation).

Luhr, J.F., Carmichael, I.S.E., 1980. The Colima Volcanic Complex, Mexico: Part I. Post-caldera Andesites from Volcán Colima. Contrib. Mineral. Petrol. 71, 343-372.

Nicholson, E. et al., 2013, Timeseries analysis reveals timescales of cyclical degassing at Soufriѐre Hills Volcano, Montserrat, Earth and Planetary Science Letters (submitted).

Varley, N. et al., 2006. Applying statistical analysis to understanding the dynamics of volcanic explosions. From Mader, H. M., et al., (eds) Statistics in Volcanology. Special Publications of IAVCEI, 1, 57–76. Geological Society, London

82


Modes of volcano growth and linkages to sub-volcanic intrusions determined using seismic reflection

data from the Ceduna Sub-basin (offshore S Australia)

C. MAGEE*1, E. HUNT-STEWART1, C.A.L. JACKSON1 1 Department of Earth Science and Engineering, Imperial

College, Prince Consort Road, London, SW7 2BP, England, UK. (*[email protected])

Temporal and spatial changes in volcano morphology

and internal architecture can influence eruption style and location. However, the external and internal characteristics of volcanoes and their relationship to the sub-volcanic intrusive networks are often difficult to visualise in 3D owing to outcrop limitations. We use high-quality 2D seismic reflection data from the Ceduna Sub-basin, offshore South Australia to quantitatively analyse 48, pristinely-preserved, continental basaltic shield volcanoes, and a genetically-related, upper crustal, sill-complex plumbing system (c. 42 Ma). Detailed seismic mapping has allowed the 3D geometry of each edifice to be reconstructed and the internal seismic facies to be studied. The volcanoes have central summits 0.05–0.76 km high, basal diameters of 1.80–18.89 km, average flank dips of <12° and range in volume from 0.12–66.31 km3. Intra-volcano parallel seismic reflections are interpreted to represent temporally separate eruptions originating from a central vent, and suggest that shield volcano growth typically occurred by a proportional increase in summit height and basal diameter. As the shield volcanoes often overlie lateral sill tips, we suggest that the intermittent eruption phases correspond to the emplacement of incremental magma pulses within the laterally extensive sill-complex. Our observations indicate that the volcanoes are not fed by an underlying, centralised magma chamber. Furthermore, several studies have shown that extensive sill-complexes accommodate a significant proportion of lateral magma flow in the upper crust, implying that sill-fed volcanoes may not directly overlie the lower crustal or mantle source of the magma. These results emphasize the applicability of seismic reflection data to quantifying volcano classification and to understanding the evolution of volcanic systems.

Sill geometries in 3D seismic data: Implications for sill emplacement

B. MANTON *1, J.A. CARTWRIGHT 2,1 1 School of Earth and Ocean Sciences, Cardiff University,

U.K. (*[email protected]) 2 Department of Earth Sciences, Oxford University, U.K.

3D seismic datasets are used to map Palaeogene age sills that intrude Mesozoic sediments, in two regions along the European Atlantic Margin: the NE termination of the Rockall Trough; and between the More and Voring Basins. The sills were emplaced at variable stratigraphic levels, into basins dominated by mudstones.

Sills that are emplaced at depths in the range of 1.5 - 4 km propagate concordantly, before transgressing at approximately 45°, often asymmetrically so that the sill is transgressive along only along part of its margin. Sills emplaced at shallower depths (0.2-1.5 km), into homogenous fine-grained sediments are predominantly transgressive, forming ‘bowl-shaped’ geometries. Sills emplaced in the near-surface (<0.3 km) have lava-like morphologies.

Variations in sill geometry correlate to changes in host rock lithification state, and its composition. Concordant sills at greater depths indicate fractures following bedding planes in well lithified rock. Sill transgression at such depths is primarily caused by forced folding at the seafloor, causing stress rotation in the host rock.

Continuous transgression at shallower levels is indicative that sill geometry is less affected by bedding planes but more affected by continuous changes in the host rock elasticity. Sills at the shallowest levels have morphologies which indicate the host deformed by ductile flow rather than fracturing during emplacement. The host is interpreted to have been unconsolidated for that to be possible.

Sills are also observed to propagate along fault planes within some harder units. Sills transgressing along multiple faults can create stepped geometries. These stepped geometries are observed to limit sill inflation at the sill margins.

83


Causes of continuous activity at Arenal volcano, Costa Rica:

Preliminary results from a volcano-tectonic study

C. MULLER*1, R. DEL POTRO 1, J. GOTTSMANN 1, J. BIGGS1, , M. DIEZ 1, M. PROTTI 2, G. SOTO 3, W.

TAYLOR 3

1 Volcanology Research Group, School of Earth Science, Bristol University, UK. (*[email protected])

2 OVSICORI-UNA, Costa Rica.

3 OSIVAM-ICE, Costa Rica.

Interactions between tectonic and volcanic systems are not well understood. Here we present the initial results of a joint study combining several dozen UNAVCO’s permanent GPS stations, a gravimetric campaign and geological field results at Arenal volcano which showed more than four decades of relatively low-level continuous effusive activity since its reawakening in 1968. Our results indicate that the magmatic system of Arenal volcano is being built in an active extensional area, which may have promoted the low-level yet persistent activity.

Costa Rica is located in the western part of Caribbean Plate; the Cocos Plate subducts along the Middle America Trench (MAT). Low mechanical coupling between the two plates has generated a trench-parallel motion up to 10 mm yr-1 toward the North-West, which creates shear stresses between the MAT and the volcanic arc. Located in the volcanic arc, Arenal volcano, a basaltic-andesitic stratovolcano, grew within a comparable short timescale of a 7ka. The tectonic setting is complex and active seismogenic faults surround the Arenal volcanic edifice. After 440 years of dormancy, Arenal erupted in July 1968. The eruptive period lasted until 2010, during which approximately 0.55 km3 (2 m3 s-1 in 1968 to 0.086 m3 s-1 between 2000 and 2004; of lava and pyroclasts have been erupted.

Gravimetric measurements detect an E-W negative anomaly with 10 mgal amplitude while the GPS velocities shows a centimetric shear strain located within the volcanic arc. These geophysical techniques plus geological field observations suggest the settlement of the volcanic complex on a pull-apart basin. We propose a hypothesis in which the reported long-lived low effusion rate volcanic activity would be a consequence of local extensional tectonics and relatively high heat fluxes typical of active volcanic arcs.

ARGOS: Geophysical study of Alutu

A. NOWACKI*1, J.M. KENDALL1, I. BASTOW2, M. WILKS1, J. BIGGS1, A. AYELE3, S. FISSEHA3, E.

LEWI3, W. HUTCHISON4, D. PYLE4, F. SAMROCK5, A. KUVSHINOV5, A. JACKSON5

1 School of Earth Sciences, University of Bristol., UK (*[email protected])

2 Department of Earth Science and Engineering, Imperial College, London, UK.

3 Intitute for Geophysics, Space Science and Astronomy, Addis Abab University, Ethiopia.

4 Department of Earth Sciences, University of Oxford, UK. 5 Institute of Geophysics, ETH Zurich, Switzerland.

Alutu is a stratovolcano that lies between the Ziway and Langano Lakes of the Main Ethiopian Rift. The aim of the multidisciplinary ARGOS (Alutu Research Geophysical ObservationS) project is to better understand the magmatic and hydrothermal plumbing system of the volcano in an effort to assess hazards and geothermal potential. A small (7.3 MW) pilot geothermal plant has been in operation on and off since 1999, but not all wells have been productive. In this project we bring together satellite measurements (InSAR and GPS), seismicity and seismic imaging, magnetotellurics, and geologic mapping in an effort to understand the geothermal structure and fluid migration patterns.

Many of the best geothermal fields are located on sites prone to magmatic or hydrothermal unrest, and Alutu is no exception; rapid ground deformation has been remotely observed using InSAR (Biggs et al., 2011). Our hypothesis is that the ground deformation is caused by repeated injection of magma beneath the Alutu edifice, which in turn drives a structurally-controlled hydrothermal system. The ARGOS programme is designed to test this hypothesis, determine the nature of the causative source of ground displacement, analyse the imposed stresses within the reservoir and their influence on coupled fluid flow. It will also hopefully reveal why some boreholes are useful sources of geothermally heated water, whilst others are not.

In early 2012 an array of 12 broadband seismometers were deployed around the volcano. These data will be used to help delineate active faults and infer spatial and temporal variations in stress. At the same time, magnetotelluric data were collected at 50 stations and soil CO2 measurements were made across the volcano. The magnetotelluric data will be inverted for the 3D conductivity structure beneath the volcano and the soil CO2 survey will help map out fractures that operate as major gas escape routes. The results will be integrated with existing InSAR and GPS data, and together with detailed geologic mapping will help develop an understanding of the recent volcanic history of Alutu.

Early results from the seismic part of the experiment suggest that most earthquakes beneath Alutu are concentrated along rift valley border faults trending NNE–SSW, at shallow depths (less than 5 km), at the east of the edifice. This correlates with CO2 flux measurements at the surface, suggesting a joint tectonic–hydrothermal driver for seismicity.

84


Seismic and acoustic indices using the registered energies on

Tungurahua volcano, Ecuador P. PALACIOS1,2, H.M. MADER1, J.M. KENDALL1

1 School of Earth Science, University of Bristol. (*[email protected])

2 Geophysical Institute of National Polytechnic School, Ecuador.

Tungurahua volcano, located at the centre of the Eastern

Cordillera of Ecuadorian Andes, reactivated in 1999 generating many episodes of ash falls and pyroclastic flows up to the present. Stronger episodes have occurred since July 2006, with eruptions ranging VEI 2-3, producing moderate damage to surrounding villages. Since 2006, the Geophysical Institute of the National Polytechnic School, in Quito-Ecuador, have installed a broadband network with seismic and acoustic sensors, and collected the data in real time [1].

The construction of the seismic and acoustic indices, based on these main eruptions, might be useful to follow an on-going crisis, and to forecast the volcanic behaviour in term of minutes to hours. An index is a dimensionless and relative measure, and its interpretation depends on the reference values used in its construction. The average energies released in the stronger eruptions are used here as reference values. The interpretation of the volcanic behaviour also depends on the Volcano Acoustic Seismic Ratio (VASR), the ratio between the acoustic and seismic energies [2].

Assuming a stable partitioning of the released energy during the eruptions, it is possible to demonstrate that the expected acoustic and seismic indices become equal for both deterministic and stochastic cases. The data analysis of the stronger eruptions of Tungurahua volcano, and some of its minor crisis periods, appear to be quite consistent with these models.

The difference between the indices (seismic index – acoustic index) suggests that seismicity is greater than infrasound prior to a new pulse and that the converse may indicate the end of an eruption. With the normalized VASR (the ratio between the indices) and the spectrograms of the seismic and acoustic signals, we can identify explosions and pyroclastic flows during the eruptions. In addition, the indices show promise as a possible method for generating a forecast in terms of minutes prior to the generation of pyroclastic flows.

[1] Kumagai H., Nakano M., Maeda T., Yepes H., Palacios

P., Ruiz M., Arrais S., Vaca M., Molina I., Yamashina T. (2010), Broadband seismic monitoring of active volcanoes using deterministic and stochastic approaches, J. Geophys. Res., 115, B08303, doi: 10.1029/2009JB006889.

[2] Johnson J., Aster R. (2005), Relative partitioning of acoustic and seismic energy during Strombolian eruptions, JVGR, 148, 334-354, doi: 10.1016/j.volgeores.2005.05.002.

Monitoring Cascade volcanoes using InSAR

A.L. PARKER*1, J. BIGGS1, T. WRIGHT2 , Z. LU3


2 School of Earth and Environment, University of Leeds. 3 USGS Cascades Volcano Observatory, Vancouver, USA.

Measurements of ground deformation have proven to be a key component of successful volcano monitoring networks. One such technique, Interferometric Synthetic Aperture Radar (InSAR), uses the phase shift between successive satellite radar images to produce ground deformation measurements. A large InSAR dataset has been acquired for the Cascade volcanoes in the western USA, but snow cover, vegetation, steep topography and atmospheric artefacts have limited its application, causing incoherence and compromising the accuracy of measurements. We are working to overcome these limitations by using multi-temporal InSAR techniques to investigate ground deformation at Medicine Lake Volcano, CA.

Levelling surveys throughout the 20th century have revealed caldera-wide subsidence at Medicine Lake Volcano. However, the low spatial resolution of measurements mean little is known about the full extent of the deformation field. Past InSAR studies have offered little improvement due to poor coherence across the caldera. To improve upon these studies, we compare the results of three analysis techniques: stacking, persistent scatterer InSAR (StaMPS) and PI-RATE. We find that all methods indicate slow, steady subsidence of the edifice, which we compare to the results of past geodetic surveys. We use our results as inputs to inverse models of volume loss at depth. By coupling these analytical solutions to a thermal model, we investigate the possibility that present day subsidence is due to cooling and crystallisation of a sill.

85


A photogrammetric feasibility study for DEMs of gulleys in Ecuador

J.J. RATNER*1, D.M. PYLE1, T.A. MATHER1 1 Department of Earth Sciences, The University of Oxford


Laguna Cuicocha in Ecuador is a volcanic lake elevated 600 m above and to the west of a town called Cotacachi. Due to new instances of seismicity and aqueous de-gassing attributed to renewed volcanic activity, the volcanic hazards of the area have recently come under scrutiny. Secondary lahars and similar debris flows of mobilized sediment are interpreted as a main concern, based on historic scarp faults and flow channelization that will be likely directed down gulleys (called quebradas) towards the populated valley to the east.

This study is an exploration of methods for constructing 3D topographic maps, called Digital Elevation Models (DEMs), of the Laguna’s quebradas. The method of choice for this study is photogrammetry- a composite DEM built from correlating recognizable points from a set of digital photographs. Using a basic point-and-shoot camera, hundreds of photographs with overlapping imagery were collected along a section of one quebrada. The method of photo collection was based on previous techniques (James, et al. 2006) with significant deviations intrinsic to the field site. Foliage precluded target deployment and photos were collected from the quebrada floor; this method was selected as preferable to aerial or more complex methods (involving lasers, targets, LiDAR, etc.) in order to serve as a test for citizen-based photogrammetry in un-idealized locations. Similarly, the programs used for photo analysis and DEM construction (Photosynth, MethLab, Bundler) were selected for their availability to the public and relative accessibility to non-experts.

DEMs are critical inputs for lahar and other flow modeling, but must be kept up-to-date in order to provide accurate scenario guesses. In the event of landscape alteration (due to a landslide, slump, human activity, etc.) a new DEM needs to be constructed, but the amount of time needed for appropriate data collection can be an impediment. Should this accessible method of ground-based photogrammetry prove feasibile for this location, it would have potential applications for involving citizens in future data collection. Utilizing local involvement, DEMs can be updated much more rapidly than by relying upon scientists and experts alone, and future methods that utilize collaborative efforts may prove to be useful for hazard management in many locations across the globe. Mike R James et al., “Oblique Photogrammetry with Visible

and Thermal Images of Active Lava Flows,” Bulletin of Volcanology 69, no. 1 (May 30, 2006): 105–108.

Decreases in LP seismicity before the May 2011 eruption of the

persistently restless Telica Volcano, Nicaragua

M. RODGERS*1, H. GEIRSSON2, M. WITTER2, D.C. ROMAN3, P. LAFEMINA2, A. MUÑOZ4, V. TENORIO4

1 University of South Florida, USA (*[email protected])

2 The Pennsylvania State University, USA.

3 Carnegie Institution of Washington, Washington D.C., USA.

4 Instituto Nicaraguense de Estudios Territoriales (INETER), Nicaragua

The occurrence of long-period (LP) events is a

common short-term seismic precursor to eruptive activity. However, at persistently restless volcanoes such as Telica Volcano, Nicaragua, potentially important precursory signals can be missed due to the constant high background rate of seismicity. Seismic observations at Telica have demonstrated the intense background activity over the past two decades. Background seismicity at Telica is characterised by LP events with energy predominantly in the 2 Hz and 4 Hz bands. In March 2010 deployment of the TElica Seismic ANd Deformation (TESAND) network was completed, consisting of six broadband seismometers, ten continuous GPS stations and a pressure sensor. In May 2011 Telica volcano erupted with a three-week-long series of explosions. We analyse 34 months of seismic and geodetic data surrounding this eruption in an attempt to identify precursory changes. The GPS stations show dominantly regional tectonic deformation, i.e. we do not see clear signs of volcanic deformation preceding the eruption. RSAM and spectral analysis of broadband data suggest precursory changes in seismic behaviour at three different time scales. Eight months before the eruption, we observe a one month long swarm of high-frequency (14 Hz) events and a concurrent cessation of the LP (2 Hz) events. Eight to ten weeks before the eruption, we observe a sharp decrease in LP (4 Hz) seismicity. Finally, three to five weeks before the eruption the high-frequency events return. Over 300,000 seismic events have been detected in the 34 month period since installation of the first broadband seismometer. We located the 100 largest amplitude events per month for a nine month period before the May 2011 eruption. These events cluster beneath the main vent and in smaller clusters surrounding the edifice. The dense instrumentation at Telica has allowed us to identify significant decreases in LP activity preceding an explosive eruption. We suggest this decrease may be related to changes in the degassing pathways.

86


Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

H. RYMER*1, E. DE ZEEUW-VAN DALFSEN2 1 Faculty of Science, The Open University.

(*[email protected]) 2 Insitut de Physique du Globe de Paris, Sorbonne Paris Cité,

Paris, France

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground and satellite based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003-2007 we observe a net micro-gravity decrease combined with subsidence and from 2007-2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that here, magma movement and/or a steam cap in the geothermal system of Askja at depth, are responsible for the observed micro-gravity variations. We rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase and suggest magma intruded into the residing magma chamber at Askja. Compressibility of this magma and the magma residing in the magma chamber as well as the compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation. de Zeeuw-van Dalfsen, Rymer, H, Sturkell, E, Pedersen, R,

Hooper, A, Sigmundsson, F & Ófeigsson, B. (2012) Bull Volc (in press).

Rymer, H., Locke, C.A., Ófeigsson, B. G., Einarsson, P & Sturkell, E. (2010) Terra Nova 22(4): 309-313.

Locating the source of volcanic noise, a picture tells a thousand

hertz E. SWANSON *1, M. SCASE 2, D. GREEN3

1 University of Bristol. (*[email protected]) 2 University of Nottingham

3 AWE Blacknest

Volcanic plumes represent a significant hazard both near to and at distance from source. Unfortunately, high temporal resolution monitoring, especially in the case of more remote systems, is often lacking. Due to their turbulent nature, these plumes are a substantial source of sub 10 Hz sound; meaning infrasonic methodology has the potential to fill the monitoring void. Recent studies of volcano infrasound have drawn upon research from the aero-acoustics industry, fitting the large scale jet noise spectrum (LSS) to that observed during column generating eruption events (Matoza et al 2011). However, due to the differences between volcanic plumes and the pure gas jets, from which the aero-acoustics spectra are created, further laboratory studies are required to investigate the true source of the volcanic signals.

In the absence of industrial standard anechoic chambers, and indeed in terms of the field application of experimental findings, successful source localisation methods are paramount. Inverse beamforming, as opposed to the commonly used delay and sum methodology, offers a means to achieve the required spatial resolution of the generated sound field.

In a set of preliminary studies at the University of Nottingham, a 48 microphone array was used to demonstrate the ability of the inverse method in conditions, less than ideal for acoustic investigations. In addition to the sound information, a schlieren data set was gathered, providing details of the plume structure and entrainment rate. Findings from these experiments are now being used to design a thorough laboratory and field study of the sources of sound from sustained volcanic eruptions Matoza,R., et al (2009) Geophysical Research Letters Vol,

36 L08303.

87


Examining Seismic Precursors to Eruptions at Volcanoes in

Extensional Stress Fields Using an Experimental Approach

R. WALL*1, C.R KILBURN1, P. MEREDITH2 1 Aon Benfield UCL Hazard Centre, Dept of Earth Sciences,

University College London, WC1E 6BT, UK. (*[email protected]) 2 Rock & Ice Physics Laboratory, Dept of Earth Sciences,

University College London, WC1E 6BT, UK.

Quantitative analyses of seismic precursors to volcanic eruptions have focussed on volcanoes in compressional background stress fields at subduction zones. We use the results from new laboratory rock-fracture experiments on basalt from Mt Etna, in Sicily, to extend studies to volcanoes in extensional stress fields.

Sequences of volcano-tectonic (VT) earthquakes before flank eruptions on Etna can develop over months-years, during most of which cumulative numbers of events increase exponentially with time. To recreate field conditions, we used the fault jog method to generate extensional stress within a regional compressional stress field. Two parallel slots, 2 mm wide, were cut at 30° to the axis in cylindrical samples, 40 mm across and 110 mm long. The perpendicular offset between slots was held at 10 mm, but the slot overlap was varied from 0 to 10 mm. Water saturated samples were deformed under triaxial stress at a strain rate of 10-5 s-1, 60 MPa confining pressure and 20 MPa pore fluid pressure. Axial strain, volumetric strain and the number of acoustic emissions (AE) and their energy were measured as proxies for the accumulation of crack damage within each sample.

Our first results show exponential increases with time in the cumulative number of AE events (analogues of VT events). The trends are consistent with a new theoretical model for which the exponential trend is characterised by the energy stored in the atomic structure at absolute temperatures and confining pressures above zero. The characteristic stored energy can be calculated using rock composition, temperature and confining pressure. In our experiments, the exponential trends yield values for the characteristic stored energy of 28-36 MJ m-3. These compare well with the calculated values of 32 (+/- 20%) MJ m-3. The good agreement suggests that field precursors can be used as a basis for deterministic forecasts of eruptions.

Digital mapping of accommodation structures associated with

emplacement of the Maiden Creek intrusion, Henry Mountains, Utah

P.I.R. WILSON*1, K.J.W. MCCAFFREY2, R.E. HOLDSWORTH2, J.P. DAVIDSON2, P.J. MURPHY1

1 School of Geography, Geology and the Environment, Kingston University, Penryhn Road, Kingston-upon-Thames, KT1 2EE (*[email protected]).

2 Department of Earth Sciences, Durham University, Durham, DH1 3LE.

High-level sill and laccolith complexes form an

important part of volcanic plumbing systems in which magma is emplaced as a series of sub-horizontal tabular sheets. Most studies of these intrusions concentrate on their geometry and internal architecture, while only a few pay particular attention to emplacement-related deformation structures in the host rock that record how magma is accommodated within the crust, i.e. the ‘space problem’. This research aims to develop a greater understanding of how igneous intrusive bodies are emplaced and accommodated within the shallow crust.

Maiden Creek, a satellite intrusion to the Mount Hillers intrusive complex (Henry Mountains, SW Utah), is a sill-like body with a complex elliptical shape with several finger-like lobes. Traditional field mapping, outcrop studies and detailed data collection of deformation structures have been combined with digital mapping using FieldMove™ and terrestrial laser scanning (TLS) in order to enable 3D modelling of the intrusive bodies and emplacement-related host rock deformation. Kinematic and geometrical studies of emplacement-related structures in the host rocks are supplemented by microstructural and geochemical studies of deformed host rocks, plagioclase feldspar and amphibole phenocryst populations within the intrusions, and the intrusion-host rock contact zone. Fabrics recognised include both solid-state (associated with accommodating structures) and magmatic features (associated with magma flow).

Detailed outcrop studies across two neighbouring lobes have identified a sub-horizontal shear zone which runs along the top contact of each intrusion. This shear zone separates low-/moderately-deformed sandstones above from highly deformed sandstones below and between the two lobes, hence acting as a detachment zone. Strain within the highly deformed sandstones is dominated by compressional faults, fractures and fabrics which point to a ‘bull-dozing’ mechanism for lobe emplacement. Fabrics (stretched plagioclase phenocrysts) within the igneous rock, seen on the upper surface of the intrusive lobes directly beneath this shear zone show that the shear zone was contemporaneous with magma emplacement. The shear zone therefore appears to have played a critical role in accommodating magma emplacement.

88


Comparing predictions of an integral model with observations of

the Eyjafjallajökull 2010 plume M.J. WOODHOUSE*1, L.J. DOWSON2, J.C. PHILLIPS2

1 School of Mathematics, University of Bristol, Bristol, UK. (*[email protected])

2 School of Earth Sciences, University of Bristol, Bristol, UK.

Volcanic eruption columns inject large quantities of

volcanic ash into the atmosphere. The dispersion of ash over large areas results in significant disruption to international transport and infrastructure. The weakly-explosive phase of the 2010 eruption of Eyjafjallajökull, Iceland, demonstrated the severe and extensive consequences of relatively small eruptions to aviation. Accurate modelling of the dispersion of ash in the atmosphere requires, as input, knowledge of the source conditions at the volcanic vent, in particular the source mass flux of material.

It is currently not possible to measure the source mass flux directly. This has led to the development of inversion methods to estimate the source mass flux based on its relationships to the plume height. However, it is difficult to include all atmospheric controls on the plume ascent, such as wind and varying atmospheric stratification, into such relationships.

Integral models of volcanic plumes provide an alternative approach to estimating source conditions. Here we use an integral model of wind-blown volcanic plumes (Woodhouse, Hogg, Phillips & Sparks, 2012) that incorporates detailed meteorological data to estimate the height of volcanic plumes for specified source conditions, and we compare the model predictions to observations of the volcanic plume at Eyjafjallajökull, 2010.

During the Eyjafjallajökull eruption, plume heights were recorded by a weather radar, providing a high temporal resolution time-series of plume heights. The radar data records variation in the plume height on 24-hour time scales. Model predictions can be matched to the observations during the first week of explosive activity with a near-constant source mass flux, with the varying plume height controlled predominately by the atmospheric wind. In constrast, scaling-law relationships would predict abrupt, order-of-magnitude changes in the source mass flux.

In addition, during the Eyjafjallajökull eruption a web-camera captured frequent images of the plume. In contrast to the single-point radar data, web-camera images record additional information on plume structure and dynamics. However, only a portion of the web-camera images provide useful data. Here we analyse the web-camera dataset to determine plume trajectories and plume profiles and compare these to predictions from integral models. We show that our integral model is able to describe the wind-affected trajectory of the plume and the growth of the plume due to the entrainment of ambient air. By matching the model trajectory to the observed trajectory, estimates of the volcanic source conditions can be made.

Woodhouse, M.J., A.J. Hogg, J.C. Phillips & R.S.J. Sparks

(2012) Submitted to Journal of Geophysical Research.

Dispersal and timing of major eruptive events at Ischia (Italy),

insights from distal tephra records P. ALBERT1, E. TOMLINSON*2,1, L. CIVETTA3, S. WULF4,

R. BROWN5, V. SMITH6, G. ORSI3, C. LANE6, M. MENZIES1

1 Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK.

2 Department of Geology, Trinity College Dublin, Dublin 2, Ireland (*[email protected])

3 Instituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy

4 GFZ German Research Centre for Geosciences, Potsdam, Germany

5 Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK

6 RLAHA, University of Oxford, Oxford, OX1 3OY, UK.

Volcanic activity at Ischia is characterised by alternating periods of resurgence, intense volcanic activity and quiescence (Orsi et al. 1996). Volcanism at Ischia began prior to 150 ka, with the largest eruption being the 56 ka, caldera-forming Monte Epomeo Green Tuff (MEGT). Unravelling eruptive history from proximal deposits can be problematic due to burial, resurgent uplift and erosion. In such cases, archives recording distal tephra layers can provide valuable information about eruptive frequencies and repose periods.

Explosive activity on Ischia has produced several important distal tephra markers, including the C-18 (60.3 ka) in the Tyrrhenian Sea (Paterne et al. 1988) and the Y-7 (56 ± 4 ka) in the Ionian Sea (Keller et al. 1978; Kraml 1997). These tephras were linked to the MEGT and to an older eruption, the Unita Monte San Angelo (UMSA), by the original authors. Refining such correlations is important because the Y-7 and C-18 tephras reside close to the marine isotope stage 3/4 climatic transition, which occurs beyond the limit of radiocarbon dating. Consequently, widespread ash dispersals from Ischia enable the assement of climatic leads and lags between different Mediterranean environmental archives.

In this contribution, we provide major and trace element glass data for Ischia tephras from proximal stratigraphies, whilst distally from Lago Grande di Monticchio (Italy) and other key archives. The tephras span 75-20 ka (USMA to the St Angelo Tuff) and represent volcanic eruptions producing widespread ash dispersals in the Mediterranean region. We define diagnostic geochemical fingerprints for key Ischia tephras and provide proximal-distal correlations for a number of important Ischia layers. The correlations are used to constrain the age and repose times of magmatic activity on Ischia.

Orsi G, Piochi M, Campajola L, D'Onofrio A, Gialanella L, Terrasi F (1996) J. Volcanol. Geotherm. Res. 71, 249–257.

Keller, J., Ryan, W.B.F., Ninkovich, D., Altherr, R., (1978). Geol. Soc. Am. Bull. 89, 591–604.

Kraml, M., (1997) Ph.D Thesis Albert-Ludwings-Universitat Freiburg. pp. 216.

Paterne, M., Guichard, F., Labeyrie, J., (1988) J. Volcanol. Geoth. Res. 34, 153-172.

89


Does volcanology work? Evidence from the volcanic fatalities record M.R. AUKER*1, R.S.J. SPARKS1, L. SIEBERT2, H.S.

CROSWELLER1, J. EWERT3

1 University of Bristol, UK. (*[email protected]) 2 Smithsonian Institution (Ret.), Washington, D.C., USA.

3 USGS, Vancouver, WA, USA.

Demonstrating conclusively the efficacy of proactive volcanic disaster risk reduction is of great importance in ensuring the continued use of such techniques. To this end, we firstly explore basic temporal trends in the volcanic fatalities record, and then propose a new measure: the Volcanic Fatalities Index (VFI). The VFI is defined as the number of fatalities divided by the product of the number of recorded volcanic events (fatal and non fatal) and population, in a fixed time period.

If vulnerability to volcanic hazards as measured by fatalities was only controlled by population and numbers of events, then the VFI should be approximately independent of time if vulnerability remained unchanged. However, the VFI declines markedly with time. Values are high and variable in the 17th and first half of the 18th centuries, likely due predominantly to poor data quality. The VFI falls sharply over the latter half of the 18th century, with an approximately steady decrease observed from 1800 onwards. This reduction in the VFI by about an order of magnitude since 1800 is consistent with a real decrease in vulnerability to volcanic hazards. Key explanatory factors include improvements in scientific hazards assessments and early warning systems, timely evacuations, better preparedness, and greater population awareness.

A simple postulate is that had no progress been made in volcanic disaster risk reduction, the VFI would have remained at approximately its 1900 value throughout the 20th century. This value can be used to estimate the fatalities that might have occurred as a consequence of increased exposure due to exponential population growth. We estimate this number as approximately 85,000, compared to the actual number of 31,728 (the eruptions of Pelée and Nevado del Ruiz excluded).

These results provide a numerical indication of the benefits of proactive volcanic risk management, marking an improvement on previous qualitative and anecdotal evidence.

A glimpse into the future – earth science on trial!

R. BRETTON, J. GOTTSMANN, R. CHRISTIE

The United Nations has played a vital role in improving the governance of natural hazards including volcanic unrest and its possible escalation towards a volcanic eruption. The multi-hazard nature of volcanic unrest including seismicity, ground instability, lahar generation and the release of toxic gases epitomises the complexity scientists and decision-makers face amid substantial scientific and technological uncertainty during an unrest crisis.

The International Decade of Natural Disaster Reduction 1990-2000 generated an increased interest in and concern for disaster-related issues and in particular proactive risk prevention. World Disaster Conferences in Yokohama (1994) and Hyogo (2005) and regional conferences, such as those in Latin-America in Cartagena (1994) and Manizales (2004), have led to action priorities, guidelines and policy tools. A Global Assessment Report is now issued every two years.

But little has changed since Kofi Annan, the then Secretary-General of the UN, said in 1999 “Prevention and mitigation are not only more humane than cure: they are also much cheaper… The scientific community understands the importance of the connection between disasters, climate change and policy makers. Prevention policy is too important to be left to governments and international agencies alone. That is a mistake. In order to succeed it must also engage civil society, private sector and the media. We know what has to be done. What is now required is the political and social commitment to do it.”

The driver for "renewed political and social commitment" is likely to come from an unexpected source. Human Rights cases in Strasbourg, France and San Jose, Costa Rica have clarified the rights to life, private and family life, and freedom of information in the context of natural hazards.

90


The structure and emplacement of the Rocche Rosse obsidian lava

flow, Aeolian Islands, Italy L. BULLOCK *1, R. GERTISSER1, B. O’DRISCOLL1

1 School of Physical & Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK


The emplacement kinematics of silicic lava flows are not well understood, due to a paucity of observations on active flows [3]. The measurement of textural and structural features can serve as an important tracer for silicic lava flow emplacement and deformation. The Rocche Rosse obsidian lava flow (1230±40 AD) [1, 2] represents the youngest outpourings of the Monte Pilato pumice cone on the northern coast of Lipari (Aeolian Islands, Italy). The flow surface is characterised by a brecciated flow top, flow ramps and complex folding. In this study, structural measurements on surface features and quantitative textural data are coupled with detailed mapping and cross sectional interpretations of the Rocche Rosse obsidian to gain insight into its rheological behaviour and emplacement history.

We present a new 1:3,000 scale structural map of the Rocche Rosse obsidian. Complex folds and refolded folds (sheath folds) are mapped on a scale of decimeters to tens of metres. Planar (primary banding and fold axial planes) and linear (fold hinges and stretching lineations) structures suggest overall flow-perpendicular foliation and flow-parallel lineation trends (with localised anomalies, where structural observations do not conform to the trends). Small-scale flow folding on a decimeter scale is extensive, as is the generation of larger-scale folding. Spherulites are apparent, with strain analysis, textural observations and crystal-size distribution (CSD) measurements suggesting multiple populations of subcircular to ellipsoidal spherulites exist. A complex range of spherulite textural fabrics and shape profiles points to protracted down-temperature spherulite formation that can be linked to specific stages in the emplacement history and deformation regime of the lava flow.

The dominant foliation and lineation trends are interpreted as a function of flow frontal compression (at shallower topographic gradients) and mid-flow constriction (steeper topographic gradients) occurring during flow emplacement. Initial compression leads to the generation of small scale folding, with increased cooling (leading to crustal thickening) and compression resulting in a second and, possibly, a third generation of larger-scale folding and sheath folding. Variations in structural and textural trends relate to areas of complex deformation, such as at the flow front or flow margins.

The detailed textural and structural features documented here provide important constraints on the rheological properties of the Rocche Rosse obsidian, important for understanding the structural kinematics and emplacement mechanisms of this silicic lava flow.

[1] Arrighi, S. et al. (2006) Phys. Earth Planet. Int. 159,

225-233. [2] Clay, P. et al. (2012) Contrib. Mineral. Petrol. DOI 10.1007/s00410-012-0813-x. [3] Tuffen, H. et al. (2012) Geophys. Res. Abs. 14, EGU2012-11451-2, 2012

A statistical method for determining the volume of volcanic

fall deposits

R. BURDEN*1, L. CHEN2, J.C. PHILLIPS1 1 School of Earth Sciences, University of Bristol.

(*[email protected]) 2 School of Mathematics, United Kingdom

Volumes of tephra fall deposits are difficult to

determine due to the commonly poor preservation of proximal and distal areas of the deposit. Typically these volumes are found by extrapolating exponential or power-law relationships found from isopach maps drawn for the areas of the deposits that are preserved. However the construction of isopach contours is dependent on human interpretation of field measurements and can be highly variable, so imposing subjectivity on the final volumes found.

Here we have investigated the spatial correlation relationships of thickness measurements from fall deposits to the vent location to produce a purely statistical method to objectively determine the volume of a deposit, without the production of isopach maps. Integration of a log linear regression model for thickness measurements with distance from the vent is applied to the field measurements without any prior interpretation. The use of an exponential function in this method is based purely on the statistical requirement that data needs to be transformed to a normal distribution and is not informed by physical processes. At a fixed distance from the vent there can be significant variability in the thickness data from a deposit that can lead to large uncertainties in the volumes found, a feature of the deposit that is lost in the production of isopach maps. To account for this variability in the data a Bayesian approach has been adopted which provides a systematic method for determining the uncertainty in the volume estimated.

Volumes calculated from our method correspond well to those previously determined by alternative approaches. The quantification of uncertainty in field measurements and model error, and the removal of subjectivity incurred by the production of isopach maps, suggests the method presented here can offer benefits in determining the volumes of fall deposits.

91


Ecological impacts of degassing and deposition from recent activity at

Volcán Turrialba, Costa Rica B. BURSON*1,2, R. MARTIN1

1 Department of Geography, University of Cambridge CB2 3EN

2 Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, MK7 6AA (*[email protected])

Volcanic degassing releases a variety of volatiles which

are locally deposited. Biomonitoring has be used to indicate the spatial patterns of deposition1, but the extent to which persistent volcanic degassing pollutes nearby ecosystems has been less well investigated. Since 1996, when the latest phase of degassing began at Volcán Turrialba2, there has been increasing damage to the surrounding vegetation. Using soil pH and element analysis of grasses alongside ecological data, it was found that acidification from Turrialba is the component of volcanic degassing that has the strongest effect on local ecosystems.

Soil pH and ICP-OES analysis of grass (Poa annua) samples were used to identify the spatial distribution of the deposition of volcanic elements, which was statistically assessed using Principle Components Analysis (PCA). Acid deposition decreases with distance downwind (west) from the volcano. This is accordant with results from Masaya (Nicaragua), where acidifying species such as HCl are rapidly scavenged from the plume and deposited nearby3.

The concentration of the volcanogenic elements Cd, Co, Cu, Hg, Ni, Pb and Rb (PC1) in Poa annua is highest several km west of the volcano. These may be deposited after acidifying compounds, and therefore further downwind from the volcano. The concentration of the base cations Ba, Ca and Sr (PC2) in Poa annua is anti-correlated with soil pH. These are probably being leached from acidic soils, which been observed in grasses at volcanoes elsewhere1. Acidifying species that are rapidly deposited from the plume may also be responsible for leaching the elements in PC1 from the soil closer to the volcano, producing an apparent centre of deposition several km from the source.

These results were correlated with data from a vegetation incidence survey using Canonical Correspondence Analysis (CCA) as well as a tree defoliation index. The diversity of understory flora is much lower at sites exposed to the plume, though some plant species are more tolerant to volcanic degassing. According to CCA, the diversity of flora is best explained by the pH of the soil. Defoliation of trees is positively correlated with acidity. Furthermore, since no elements were found at phytotoxic levels in Poa annua, it is concluded that acid deposition is the most important cause of vegetation impacts from degassing at Turrialba.

Martin, R.S. et al. (2010) Bulletin of Volcanology 72(8),

1009-1020. Martini, F. et al. (2010) Journal of Volcanology and

Geothermal Research 198, 416-432. Delmelle, P. et al. Environ Sci Technol 35(7), 1289-1293.

Advances in the construction of volcanic records from marine

sediment cores: A review and case study (Montserrat, West Indies)

M. CASSIDY*1, J. TROFIMOVS2, M.R. PALMER1, W.

SYMONS1 1 National Oceanography Centre, University of

Southampton, European Way, Southampton, SO14 3ZH. (*[email protected])

2 Queensland University of Technology, Brisbane, Australia, 4000

This work tests and reviews current techniques used to

generate volcanic eruption records from marine sediment cores using the volcanic island of Montserrat as a case study. The data is presented using cores sampled both proximally (8-14 km) and distally (55 km) from south and south west offshore Montserrat.

Based on these studies we suggest the following protocol for generating volcanic records from submarine sediment cores: (1) visual sedimentological logging of visible tephras, (2) the use of time-efficient, non-destructive and high spatial resolution techniques that are able to detect and locate potential tephra horizons (e.g., XRF core scanning and magnetic susceptibility), (3) sampling of targeted horizons for microscope analysis to discriminate between primary and reworked volcanic deposits (this has been achieved here by using specific criteria related to clast morphology, compositional maturity, sorting and sedimentological facies indicators), and (4) dating the intervening hemipelagic sediment, by AMS radiocarbon dating on foraminifera (when younger than 47 ka) and, for older marine sediment, using oxygen isotope stratigraphy.

As a result of this procedure, more discrete volcanic events have been recognised than previous marine tephrochronological studies in this region.

92


Landslide processes at Montserrat, Lesser Antilles, and their

implication for tsunami generation M. COUSSENS*1, P. TALLING 1, S. WATT1, M. CASSIDY1,

M. PALMER1 1 National Oceanography Centre Southampton, European

Way, Southampton SO14 3ZH (*[email protected])

The ongoing eruption of Soufrière Hills volcano,

Montserrat, has involved multiple dome collapses, which have produced pyroclastic flows and ultimately led to the offshore deposition of >65% of the total erupted material. The largest collapse in 2003 had a volume of 0.2 km3, and generated a local tsunami 1m high that was recorded 48 Km away on the shores of Guadeloupe. Several much larger landslide deposits have been detected offshore Montserrat via swath bathymetry and seismic surveys, the largest of which involved ~20 km3 of material. Although infrequent, such events may generate regionally significant tsunamis. However, their potential for tsunami generation is strongly dependent on whether these landslides were formed in single or multiple stages, and in where the material was sourced from (i.e. volcanic flank failure versus the incorporation or secondary failure of seafloor sediment). To provide a better understanding of landslide and tsunami hazards around this volcanic island, it is important to constrain landslide magnitude, frequency, and specific emplacement processes. The recently collected cores from IODP Expedition 340 provide samples of multiple landslide, fallout and turbidite deposits spanning the development of Montserrat. Here, we describe the current state of knowledge regarding the eruption and mass-wasting history of Montserrat, and show how the new core samples will be used to advance this understanding. We aim to understand better the emplacement processes of the largest landslides around Montserrat; to investigate the relative frequency, and any episodic behaviour, of the different mass-wasting processes around Montserrat; and to provide a clearer record of the early development of Montserrat by integrating onshore and offshore data.

Interactions between mitigation strategies: Implications for the

scientific bases of mitigation policy S. DAY*1, C. FEARNLEY2

1Aon Benfield UCL Hazard Centre, Department of Earth Sciences, University College London (*[email protected])

2 Institute of Geography and Earth Sciences, Aberystwyth University.

Recent events in natural disasters, such as the

malfunctioning of the Japanese tsunami warning system in the 2011 Tohoku tsunami, reveal a need for a systematic classification of mitigation strategies; an understanding of the scientific uncertainties that affect the effectiveness of such strategies; and an understanding of how the different types of strategy within an overall mitigation policy interact, often destructively, to reduce the effectiveness of overall mitigation policies. This occurs especially where policies deal with multiple hazards and interactions between those hazards (“multihazards”) and their impacts (“multirisks”). The variety and complexity of volcanic hazards means that these issues are particularly important for volcanologists.

We divide mitigation strategies into permanent; responsive; and anticipatory. Permanent mitigation strategies such as coastal and river flood defences or land use restrictions around volcanoes, are both costly and “brittle” in that when they fail they can actually increase mortality by their influence upon peoples’ expectations and their actions during disasters. Such strategies therefore critically depend on the accuracy of the underlying hazard assessments. Responsive mitigation strategies such as tsunami and lahar warning systems rely on capacities to detect and quantify the hazard source events and to transmit warnings fast enough to enable at risk populations to decide and act effectively. Self-warning and voluntary evacuation is also usually a responsive mitigation strategy. Uncertainty in the nature and magnitude of the detected hazard source event is often the key scientific obstacle to responsive mitigation; public understanding of both the hazard and the warnings, to enable their decision making, is also a key factor that hazard scientists can address. Anticipatory mitigation strategies use interpretation of precursors to hazard source events and are used widely in mitigation of volcanic hazards. Their critical limitations, due to uncertainties in time, space and magnitude relationships between precursors and hazard events, are perhaps better understood by volcanologists than by other hazard scientists.

Understanding the interactions between these different types of mitigation strategy, especially in the multirisk environment of a complex natural disaster, requires models of rapid decision making under high levels of both uncertainty and danger. We propose that the Observation-Orientation-Decision-Action (OODA) loop model (Boyd, 1987) may be a particularly useful model, especially in its emphasis that flexibility counteracts uncertainty.

Boyd, J.R. A Discourse on Winning and Losing

[http://dnipogo.org/john-r-boyd/]

93


Eruption and depositional facies of the Stob na Doire Ignimbrite

Member, Glencoe, NW Scotland: Fault-bounded rheomorphic/lava-

like and eutaxitic ignimbrites J. DIETZ*1, D. BROWN1, R. DYMOCK1


Glencoe, NW Scotland, exposes the roots of a late

Silurian-early Devonian (~420-400 Ma) caldera volcano. The caldera is dominated by a succession of silicic pyroclastic rocks, together with intermediate and silicic lavas, and intrusions. The caldera sequence comprises a wide range of ignimbrite lithofacies, which represent major explosive eruptions associated with volcano-tectonic subsidence and piecemeal caldera collapse. Subsidence was incremental and occurred along a series of cross-cutting faults (Moore & Kokelaar 1998). We report on a newly-identified sequence of ignimbrites, previously interpreted as lavas and attributed to the Bidean nam Bian Andesite Member (Moore & Kokelaar 1998), and explain their eruption and depositional history. We propose the name the ‘Stob na Doire Ignimbrite Member’ for these rocks.

The Stob na Doire Ignimbrite Member is located on the flanks of Stob na Doire on the Buachaille Etive Mor massif in eastern Glencoe. Its northernmost exposure is fault-bounded against sandstones and breccias of the Glas Choire Sandstone Member. The lowermost part of the exposure comprises ~40 m of crystal- and lithic-rich andesitic-dacitic rheomorphic/lava-like ignimbrites. These ignimbrites display a strong flow-fabric/parataxitic texture and folds are locally present. The fabric is near vertical in the vicinity of the fault but progressively shallows up sequence and away from the fault, indicative of active subsidence and ponding during the eruption. The high-grade nature of these ignimbrites records a sustained, low-fountaining, ‘boil-over’ type eruption.

The rheomorphic/lava-like ignimbrites pass up into an ~100 m thick sequence of andesitic-dacitic ignimbrites comprising both eutaxitic welded lapilli-tuffs and non-welded lapilli-tuffs and breccias, indicative of a change to more pumiceous Plinian eruptions. The transition is marked by ~5 m of alternating rheomorphic lapilli-tuffs with distinctive strongly eutaxitic lapilli-tuffs, locally with basal vitrophyres. The sequence then passes up in to more typical massive lapilli-tuffs, locally with lithic breccia horizons, and moderately welded eutaxitic zones. These ignimbrites record granular fluid-based pyroclastic density currents, and are indicative of waxing and waning of the eruption due to variations in mass flux. Moore, I., Kokelaar, P., (1998) Bulletin of the Geological

Society of America 110, 1448-1466.

Bentonised silicic pyroclastic fall deposits at the base of the

Palaeogene Skye Lava Field which possess welded ignimbrite like

fabrics S.M. DRAKE*1, A.D. BEARD1

1 Department of Earth and Planetary Sciences, Birkbeck College, University of London, Malet Street, London, WC1E 7HX (*[email protected])

Silicic pyroclastic rocks deposited prior to basaltic lava

fields within the British-Irish Palaeogene Igneous Province (BIPIP) are unrecorded. We report on the first such occurrence from beneath the lowermost Skye Lava Field at An Carnach, Strathaird Peninsula, Isle of Skye, NW Scotland. Here contemporaneous basic and silicic eruptions are recorded by former silicic ash-fall deposits which contain chlorite mica stacks. The presence of these stacks likely indicate basaltic ash settled out in a marine water column (Merriman and Peacor 1999). The silicic-ash fall deposits were subsequently altered to K-bentonite by either intrusion of a later adjacent tholeiitic sill, or by the weight of the overlying lava pile.

The alteration of the protolith to K-bentonite has resulted in a remarkable eutaxitic like fabric which is defined by a strong planar fabric which has frequently been deflected around heterolithic lapilli. This fabric was produced in the cold state since it lacks recognised hot state characteristics (Branney and Sparks 1990). In the field the K-bentonite strongly resembles a silicic welded ignimbrite. Classification to K-bentonite was only possible using an electron microprobe since the Al and Si peaks were much higher and lower respectively than those expected in a silicic welded ignimbrite.

The ability to distinguish between ‘ancient’ bentonite and welded ignimbrite in the field is therefore called into question and may have implications for volume considerations of silicic welded ignimbrite in some volcanic terrains, and the paucity of silicic air-fall deposits in others.

Branney, M.J. and Sparks, R.S.J. 1990. Fiamme formed by diagenesis and burial-compaction in soils and subaqueous sediments. Journal of the Geological Society of London. 147. 919-22

Merriman, R J. and Peacor, D.R. 1999. Very low- grade metapelites: Mineralogy, microfabrics and measuring reaction progress. In: Low grade metamorphism (M Frey and D Robinson. Blackwell Science Oxford Pp160.

94


The importance of conduit erosion J.B. HANSON*1, A.C. RUST1, M.J. PAVIER1, J.C.

PHILLIPS1 1 University of Bristol (*[email protected])

Conduit geometry is known to affect flow processes and critical points such as the fragmentation level. Consequently, conduit flow models are becoming increasingly sophisticated in terms of conduit shape and elastic deformation. Conduit erosion (ie. the permanent removal of material) would modify conduit geometry during eruptions and thus may be influential in terms of eruption dynamics. However, there has been very little modelling of conduit erosion and conduit erosion and conduit fluid dynamic models have yet to be coupled.

Finite element analysis is a powerful tool to elucidate the behaviour of the wall-rock during both dynamic and static loading such as those which occur in volcanic systems, and is capable of dealing with inherently non-linear and complex systems. Our initial investigations have focused on the static effect of the conduit over- and under-pressures determined by steady 1-D models of Costa et al, 2009 (ranging from +30MPa to -106 MPa, with respect to lithostatic). Changing the rheology of the wall rock from elastic to elastic-plastic, using reasonable rock properties (Youngs Modulus = 40 GPa, Poissons ratio = 0.25, and Yield Strength =2-20MPa) has a significant effect both on conduit displacement and the volume damaged (e.g. 50,000 m3). This damage will be particularly important in areas of conduit underpressures, as this material will implode into the conduit; erosion in damaged overpressured areas requires a further mechanism to remove material (e.g. magmatic shear stress or particle collisions).

The amount of damaged material strongly (orders of magnitude) depends on the initial size of the conduit, the yield strength of the material and the applied pressure difference. Lithological variations along the conduit wall (i.e. materials with differing strength properties, e.g. granite to schist, as seen at volcanoes such as Ramadas, Argentina) are very important for conduit erosion and thus conduit geometry evolution and potentially fragmentation level progression.

The FEA work is complemented by field studies at Vesuvius volcano, Italy, on the Pollena, Avellino and Pompeii deposits. Lithics of various lithologies (notably carbonates and older volcanics) indicate the occurrence of conduit erosion along a large portion of the conduit (estimated to be ~5 km long, with a division between the hosting rocks at around 1800m below the summit – Macedonio et al, 1994). Detailed size and shape analysis of the lithic material suggests there is a correlation with the previous fabric of the wall rock material, rather than a distinction based on erosive mechanism.

Costa, A., Sparks, R.S.J., Macedonio, G., and Melnik, O.

(2009) EPSL 288, 455-462. Macedonio, G., Dobran, F., and Neri, A. (1994) EPSL 121,

137-152.

Modelling disaster risk scenarios at La Soufrière, Guadeloupe

S. JENKINS *1, R. SPENCE 2, P. BAXTER 3, J.C. KOMOROWSKI 4, S. BARSOTTI 5, T. ESPOSTI-ONGARO 5,

A. NERI 5 1 University of Bristol. (*[email protected]) 2 Cambridge Architectural Research.

3 University of Cambridge. 4 Institut de Physique du Globe de Paris. 5 Istituto Nazionale di Geofisica e Vulcanologia.

As part of the CASAVA project, potential consequences of an eruption at La Soufrière volcano, Guadeloupe, have been quantified for the local population and infrastructure through development of an impact model. For a given eruption scenario, and on a 250m grid, the model incorporates hazard information about areas potentially affected by ash fall (vertical loading) and/or pyroclastic density currents (peak dynamic pressure, temperature and duration) sourced from numerical modelling able to describe the transient and 3D features of the phenomena. This is combined with vulnerability information derived from engineering and medical analyses of the exposure and response of humans and buildings to these hazards. Model outputs include maps of the numbers and severity of human casualties and building damage for each given scenario. The impact model is further coupled with a dynamic casualty management model that then tracks the rescue, transport and treatment of casualties to estimate the impact of the eruption on the emergency services and on casualty numbers with time following the eruption. This allows dynamic mapping of human survival in space and time following the eruption scenario and can be combined in GIS with emergency management data to support public officials responsible for planning for a crisis. To identify suitable actions that could be taken to reduce the loss of lives and infrastructure, the consequences from different disaster risk scenarios can be tested by varying the crisis conditions for each model run. For example, by incorporating short- and long-term mitigation activities (e.g. partial evacuation, construction of additional road capacity), differing emergency management (e.g. the location of triage centres) or simulating alternative disaster conditions (e.g. reduced rescuing capacity because of ashy conditions). The models are deliberately generic in nature and could be applied to any volcano, providing input hazard scenarios and appropriate exposure and vulnerability data are available.

95


Revised Estimates for the Volume of the Minoan Eruption

E.N. JOHNSTON*1, R.S.J SPARKS1, J.C PHILLIPS 1 1 School of Earth Sciences, University of Bristol, Wills

Memorial Building, Queens Road, Bristol, BS8 1RJ, UK. (*[email protected])

Assessments of erupted volumes are prone to a number

of potentially significant sources of error or omission. Large caldera-forming eruptions are associated with three main types of deposit: intracaldera ignimbrite fill; outflow ignimbrite sheets; and tephra fall-out. Volumes of all three components are rarely estimated which often leads to uncertainties in total volume estimations. The Bronze Age (‘Minoan’) eruption of Santorini is a classic example of this.

Four Minoan eruptive phases have been recognised on Santorini. Interpretations of these phases are straightforward and non-controversial. Difficulties, however, arise regarding the phase 3 deposits; one enigma is how these cold, pyroclastic flows could have been erupted over the steep caldera cliff walls but were deposited on shallow slopes (typically 15° but up to 30°). We propose that the caldera was completely infilled by pyroclastic material during phases 2 and 3. Only when the caldera was filled was it possible for the phase 3 deposits to overflow the caldera rim and produce outflow sheets. Initial calculations suggest an infill volume in excess of 23 km3. We suggest this material has been down-faulted during caldera collapse. Geophysical and seismic data are consistent with a large volume of down-faulted intracaldera fill, both of which indicate several hundred metres of low density deposits on Santorinis caldera floor. This model has implications for estimates of the size of the eruption. Our initial results, in addition to the most recent published estimate of 60 km3

DRE, suggests the volume of the eruption should be raised to 72-93 km3 DRE, making it the largest known explosive Holocene eruption.

Stratigraphy and eruption history of peralkaline welded ignimbrites,

Island of Pantelleria, Italy N. JORDAN*1, M. BRANNEY1, M. NORRY1

1 University of Leicester. (*[email protected])

A revised volcanic stratigraphy is presented for the ignimbrites of Pantelleria, a peralkaline caldera volcano situated in the submerged continental rift between Africa and Sicily. The volcano has been active for ≥325 ka (Mahood & Hildreth, 1986), producing at least six major ignimbrites from large central eruptions, which appear to have alternated with numerous minor pumice falls and lavas from scattered local centres. The main ignimbrites can be traced along superb coastal exposures and have been logged in detail. Eruption-units have been defined by the position of palaeosols and a type section designated. Lithic breccias and pumice fall deposits associated with these major ignimbrites are interpreted as part of the same eruption overcoming correlation problems encountered by previous workers (cf Mahood & Hildreth, 1986).

The ignimbrites are 2 to >20 m thick, welded to rheomorphic and cover most of the island, recording devastating, radial, high-temperature density currents. Five of the six major ignimbrites contain lithic breccias, which have commonly been interpreted as recording caldera collapse events, but the details of individual calderas are not clear. The ignimbrites were erupted between 175 and 50 ka suggesting that the early history of the island (325 to 175 ka) differs from later stages in that only local pumice and lava-producing eruptions have occurred. This means that the amount of erupted magma increased in the later stage as the ignimbrites represent eruptions of many times the volume of the local centres.

Distal peralkaline tephras have been found around the Mediterranean as far away as ~1200 km. With only this volcano erupting peralkaline compositions, it suggests that eruptions from Pantelleria have had a substantial impact on their environment. We infer that there were few Plinian events on the island, and that the distal tephras may be co-ignimbrite ashfall deposits.

Mahood, G.A., Hildreth, W., (1986) Bulletin of Volcanology

48, 143-172.

96


Drilling into a super-eruption caldera? Initial report of the

proximal rhyolites revealed by the Snake River deep drill hole, Idaho

T. KNOTT*1, M. BRANNEY1, M. REICHOW1, M. MCCURRY2 AND THE HOTSPOT TEAM

1 Dept. of Geology, University of Leicester, University Road, Leicester, UK. (*[email protected])

2 Dept. of Geosciences, Idaho State University, Pocatello, USA.

Project HOTSPOT seeks to understand voluminous

bimodal volcanism and magma generation in the Snake River-Yellowstone LIP, USA. A 1.9 km-deep drill hole in the Snake River Plain, southern Idaho has cored the mid-Miocene rhyolitic succession marginal to the putative Twin Falls explosive centre and provides a unique window into otherwise concealed proximal deposits. Four rhyolitic eruption-units occur separated by basalt lavas, palaeosols and volcaniclastic sediments. Whole-rock, glass, and mineral chemical data (XRF, EMP) from the top and base of each facilitate correlation with units exposed distally, and aid volume calculations of some of the most catastrophic, hot super-eruptions in Earth history. Rhyolite 1, the lowest unit, is >1322.78 m thick and extends beyond the base of the well. Its pyroclastic origin is indicated by a eutaxitic foliation with abundant obsidian chips (2–5 mm). Its thickness suggests ponding in the margin of a caldera. However lithic mesobreccias, as characterise caldera fills elsewhere, are not seen. Lake sediments (18 m) separate it from overlying Rhyolite 2 (159.41 m thick), which has a non-brecciated glassy base, a lithoidal centre also with 2–5 mm obsidian chips and an upper glassy rheomorphic autobreccia. It is probably a proximal outflow correlative of an ignimbrite exposed further south. Rhyolite 3 (8.84 m thick) is glassy, with upper and lower autobreccias 5.7 m and 1.5 m thick. It is separated from Rhyolite 2 by a 2.7 m-thick zone of glass clasts in clay, which may represent an altered zone within a single eruption-unit. Laminated lake sediments 64 m thick, basalt lavas, 67 m thick, 23 m sediments, and 30.5 m basalt lava with an upper palaeosol are succeeded by Rhyolite 4 (127.1 m thick). This has a glassy basal autobreccia (4.5 m), well-developed flow banding and no visible pyroclasts, and may be part of the 6.3 Ma, <200 m thick Shoshone rhyolite lava [1]. [1] Armstrong et al., (1975) American journal of science 275, 225-‐251

Glacial modulation of eruptive activity at Volcán Sollipulli, Chile

S.M. LACHOWYCZ*1, D.M. PYLE1, T.A. MATHER1, K. MEE2, J.A. NARANJO3

1 Department of Earth Sciences, University of Oxford, UK. (*[email protected])

2 British Geological Survey, Keyworth, Nottingham, UK. 3 SERNAGEOMIN, Santiago, Chile

There is growing evidence that changes in ice cover on stratovolcanoes may influence the timing and rate (e.g., Watt et al. [2011]) as well as the style (e.g., Mee et al. [2009]) of eruptive activity. These effects could potentially be enhanced at those volcanoes with ice-filled large craters or calderas, which also pose particular hazards in the event of renewed activity and with changing climate. One such edifice is Volcán Sollipulli in Chile, located at ~39°S between Llaima and Villarrica in the Southern Andean Volcanic Zone. This little-studied volcano has a ~4 km wide summit caldera containing ~6 km3 of ice, which is believed to have formed without any explosive eruption [Gilbert et al., 1996].

In this study, we examine how ice cover and intra-caldera ice has influenced the style of late Quaternary eruptive activity at this volcano, and possibly the timing of caldera formation. We present preliminary results from study of the field relationships, textures, and geochemistry of the eruption stratigraphy exposed around the caldera wall, as well as some more distal eruption deposits. A wide range of volcanic ‘lithofacies’ are observed, probably reflecting varying degrees of interaction between glacial meltwater and the erupting lava, in addition to the extent of alteration and remobilisation of these eruption products. These ‘lithofacies’ appear to range between various end-members, which we use as the basis of a novel, simple classification scheme that relates to the inferred formation processes.

From these eruption deposit sequences, we also infer the palaeo-ice depth through time. Indications of a short time period between the latest pre-caldera and first post-caldera units constrain caldera formation to during or shortly after a period of deglaciation. This is consistent with the occurrence at nearby Llaima and Villarrica of assumed caldera-forming, ignimbrite-forming eruptions during deglaciation following the Last Glacial Maximum [Lohmar et al., 2005], and contrasts with recent modelling of the potential influence of ice loading on caldera formation [Geyer & Bindeman, 2011]. Geyer, A., & I. Bindeman (2011) J. Volc. Geotherm. Res. 202, 127–142. Gilbert, J.S., et al. (1996) Bull. Volc. 58, 67–83. Lohmar, S., et al. (2005) 6th International Symposium on

Andean Geodynamics Extended Abstracts, 442–445. Mee, K., et al. (2009) Bull. Volc. 68, 363–376. Watt, S.F.L., et al. (2011) Quat. Int., 246, 324–343.

97


Development of a database of volcanic ash layers from ocean

drilling cores as a record of global explosive volcanism

S.H. MAHONY*1, R.S.J. SPARKS1, N.H. BARNARD3 1 University of Bristol. (*[email protected])

Ocean drilling is a largely untapped source with the potential to generate a more complete record of global volcanism through time. Creation of a global dataset of ash layers through time is the initial aim of this work, these layers will then be translated into volcanic eruptions of varying magnitude, essentially forming a global time series of explosive volcanic eruptions. This will then be used to examine magnitude-frequency relationships, look to see how rates of activity change through time and space, link with tectonic, glacial/interglacial and climate cycles.

The huge potential for information to be gained by

observing ocean drilling cores can be complicated by the uncertainties associated with many physical processes. These uncertainties occur during eruption, through transport, to deposition, burial, coring and the eventual identification and measurement of ash layers. If these uncertainties can be quantified then it is possible to analyse the record by stochastic ensemble models.

The cores we have focused on are those from the

Integrated Ocean Drilling Program (IODP), Ocean Drilling Program (ODP) and the Deep Dea Drilling Program (DSDP). Data are initially being gathered from literature generated by IODP, with a series of ground truthing campaigns to rigourously test the quality of this data. Alternative approaches to data collection are being attempted to use the physical properties data to identify ash layers. This indends to make ash layer identification and measurement more consistent, and create a more complete ash layer record.

Lithofacies architecture of the Stallachan Dubha Ignimbrite

Member, Ardnamurchan, NW Scotland: valley-filling ignimbrites

and the incursion of pyroclastic density currents into a lake

C. MCLEAN*1, J. BUCHANAN1, P. REYNOLDS1, 2, P. NICHOLLS1, 3, R. DYMOCK1, C. PATMORE1, D. BROWN1


2 Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE (present address)

3 Department of Earth Sciences, Uppsala Universitet, Villavägen 16, Uppsala, Sweden (present address)

The Palaeogene Ben Hiant Member of Ardnamurchan,

NW Scotland, comprises a thick sequence of breccias and conglomerates interpreted as debris flow deposits (Brown & Bell 2007). These rocks are unconformably overlain by a recently identified sequence of pyroclastic rocks, which we interpret as ignimbrites. We propose the name the ‘Stallachan Dubha Ignimbrite Member’ for these rocks.

Silicic explosive eruptions generated pyroclastic density currents that deposited a range of ignimbrite lithofacies. Lateral thickness variations in the ignimbrites record the localised filling of the palaeotopography. Periodically, the pyroclastic density currents entered small lakes and the ash was subject to aqueous reworking.

Three phases of eruption have been recognised in the sequence. Phase 1 was marked by the emplacement of rheomorphic tuffs and breccias. The rheomorphic tuffs display a strong flow-fabric/parataxitic texture and folds are locally present. The tuffs coarsen up in to a massive lithic breccia which contains boulders of the rheomorphic tuff and local country rock, before fining to rheomorphic tuff. The massive lithic breccia records a significant increase in mass flux, perhaps related to a high-energy vent clearing event, and/or vent collapse, and/or erosion of the substrate.

Phase 2 is marked by the emplacement of a sequence of stratified tuffs, lapilli-tuffs and breccias. The tuffs and lapilli-tuffs are planar- to cross-stratified and alternate with massive lapilli-tuffs and breccias. In the finer units, convolute laminae and ripples are present. These rocks record the entry of a dilute pyroclastic density current into a small lake and the aqueous reworking of finer pyroclasts.

Phase 3 is marked by the emplacement of a valley-filling crystal-rich massive lapilli-tuff. This unit unconformably overlies both Phase 1 and 2 rocks, including siltstones interpreted as lacustrine deposits associated with Phase 2. The lapilli-tuff comprises up to 50% crystals of plagioclase feldspar in an ashy matrix. Locally, breccia lenses, dominated by clasts of Phase 1 rheomorphic ignimbrite are present. Brown, D., Bell, B., (2007) Bulletin of Volcanology 69,

847-868.

98


The respiratory health hazard of volcanic ash: Factors affecting the

formation and toxicity of cristobalite

C. NATTRASS*1, C. HORWELL1 1 Institute of Hazard, Risk & Resilience, Department of

Earth Sciences, Durham University, Science Labs., South Road, Durham, DH1 3LE, UK (*[email protected])

Concern of the potential respiratory health hazard posed

by exposure to volcanic ash was raised after the 1980 eruption of Mount St Helens, due to high concentrations of crystalline silica in the ash. The toxic potential of crystalline silica is well-known and the silica polymorphs quartz and cristobalite are known carcinogens. Volcanic ash can contain up to 23 wt.% crystalline silica (Horwell et al., 2010), however toxicological studies have shown that the ash produces a much lower toxic response than would be expected for a dust containing this proportion of crystalline silica.

Volcanic cristobalite is formed metastably in volcanic domes by vapour-phase deposition and the devitrification of glass. Temperatures in the volcanic dome setting are typically <850 °C, much lower than the 1470 °C needed to form stable cristobalite. Collapse of these unstable domes generates highly respirable, cristobalite-rich volcanic ash. Substitutions of up to 3 wt. % Al2O3 into the cristobalite structure have been observed (Horwell et al., in review) and we hypothesise that these impurities allow the formation of cristobalite within the dome environment and affect the toxic potential of the cristobalite within the ash.

Laboratory studies synthesising crystalline silica from an amorphous colloidal silica sol have shown that with the addition of Al and Na impurities a pure cristobalite phase can be formed at temperatures lower than the 1470 °C threshold. Under the same conditions pure silica, with no added impurities, formed tridymite, demonstrating the addition of impurities alone can alter the silica polymorph produced. However, further work is needed to constrain conditions needed for cristobalite formation at temperatures found within volcanic dome systems. Future toxicological tests on these synthetic samples will help gain an insight as to whether impurities can affect the toxicity of crystalline silica and the implications this has for the volcanic ash respiratory hazard. Horwell et al. (2010) Bulletin of Volcanology 72:249–253 Horwell et al. (In review) Particle and Fibre Toxicology

Exploring the mechanisms of basaltic fragmentation: Insights

from textural analysis E.J. NICHOLSON*1, K.V. CASHMAN1, A. RUST1

1 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol (*[email protected])

Explosive volcanism can generate large volumes of

volcanic ash, with potentially significant economic and environmental implications. Recent eruptions, such as that of Eyjafjallajokull in 2010, have highlighted the need for improved understanding of both the mechanisms involved in ash generation and the controls on the size distribution and morphology of erupted pyroclasts. Mafic eruptions, in particular, exhibit a wide range of eruption styles that generate pyroclasts with variable sizes, shapes, vesicularities and crystallinities. We are investigating links between eruption style and pyroclast characteristics, with the goal of using these data both to gain insight into the processes governing fragmentation and to develop characteristic source parameters for different eruption conditions

We are examining tephra samples from basaltic eruptions ranging from magmatic to phreatomagmatic and with crystallinities from negligible to >50%. Preliminary analyses show that fragmentation in these samples cannot be simply explained by expansion and interaction of exsolving bubbles (that is, the fragmentation mechanism commonly attributed to silicic magmas). For example, ash from the 2011 subglacial eruption of Grimsvötn, Iceland, consists largely of broken, dense glass, with less abundant irregularly shaped glass shards formed from the interstices of a population of large bubbles exhibiting a relatively narrow size distribution (bubble diameters of 10-50 um). These data contrast with the moderately to highly vesicular clasts produced by Hawaiian-style fire fountains. They also contrast with pyroclasts from magmatic eruptions of mafic arc volcanoes, such as Fuego, Guatemala, which are highly crystalline and poorly to moderately vesicular. Together this small survey raises interesting questions about both where, how, and when gas loss occurs in mafic eruptions and the role of crystals in the fragmentation process.

99


Deforming a volcano by surface deposit loading: How loading may

mislead classic deformation analyses

H. ODBERT*1, B. TAISNE2 , S. TAIT3 1 School of Earth Sciences, University of Bristol, UK.

(*[email protected]) 2 Earth Observatory of Singapore, Singapore.

3 Institut de Physique du Globe de Paris, France.

The deformation of a volcanic edifice before, during and after an eruption is often interpreted as a response to pressurisation or depressurisation of parts of the magmatic system below ground (e.g. magma reservoirs). Typically, surface displacements are compared to numerical models to estimate the elastic response of the crust to such pressurisation. Model inversion techniques are routinely used to infer the size, shape and volume or pressure change of pressure sources using these kinds of models. A ‘best fit’ solution is obtained by minimising the misfit between the modelled and observed ground displacements. This allows the system parameters of interest to be constrained. Such approaches require a number of simplifying assumptions and typically neglect the influence of complicating factors, such as compressibility of magma in the system and the crustal response to surface loading of erupted material (i.e. lava domes, pyroclastic density current deposits, etc.). Here, we explore the effects a deposit load on ground displacement, and how they compare to deformation from classic burried pressure point source models.

Over 1 km3 of lava has been erupted since 1995 by the Soufrière Hills Volcano, Montserrat. Much of this material has moved offshore but there remains a substantial subaerial and submarine deposit around the volcano’s flank. We measure the distribution of deposits, and hence the load, around the flanks of the volcano using recent topographic survey data. We then model the elastic crustal response of the deposit load across Montserrat, in terms of expected associated ground deformation. We discuss our results in the context of deformation source model inversions and show how such inversions may be mislead when loading is neglected.

Pele’s tears and spheres – insights into the fragmentation of low

viscosity magmas L.A. PORRITT*1, 2, J.K. RUSSELL2, S.L. QUANE3

1 University of Bristol, Bristol, UK. (*[email protected]) 2 University of British Columbia, Vancouver, Canada. 3 Quest University Canada, Squamish, Canada.

Pele’s tears are a well known curiosity commonly associated with low viscosity basaltic explosive eruptions. However, these pyroclasts are rarely studied in detail and there is no full explanation for their formation. These intriguing pyroclasts have smooth glassy surfaces, vesiculated interiors, and fluidal morphologies tending towards droplets and then spheres as they decrease in size to <2 mm. We present a detailed characterisation of Pele’s tears from the 1959 fire-fountaining eruption of Kilauea Iki involving size and density measurements. Using thin section and SEM analysis we also consider their internal and external morphologies, porosity and bubble size distributions, and surface textures. Finally we consider the mechanisms of magma fragmentation, timescales of relaxation, and cooling rates that are responsible for their formation.

Porritt, L.A., Russell, J.K., Quane, S.L. (2012) Earth and

Planetary Science Letters 333-334, 171-180.

100


The atmospheric habit of fine volcanic ash

G.S. PRATA*1, B.E. REED2, T.A. MATHER1, D.M. PYLE1, D.M. PETERS2

1 Dept. of Earth Sciences, University of Oxford (*[email protected])

2 Department of Atmospheric, Oceanic and Planetary Physics (AOPP), Clarendon Laboratory, University of Oxford

Lack of information on the physical, compositional and

optical characteristics of volcanic ash and properties that affect its residence time, aggregation and interaction with other atmospheric species (e.g. ash-nucleated ice) represents a major gap in basic knowledge and understanding. Measurements of the physical, compositional and optical properties of fine ash are needed as input for radiative transfer models and dispersion models (e.g. Stohl et al., 1998; Costa et al., 2006). A comprehensive database of information for dispersion models will provide useful data for environmental impact assessments (e.g. human, animal and plant health impacts) and at a fundamental level provide new information for studying volcano-climate interactions (e.g. Watson, 1997; Robock, 2000; Durant et al., 2010). These new measurements will also be very valuable for satellite retrievals of volcanic ash that rely on assumptions of the optical properties, size distribution and morphology of ash particles (e.g. Prata, 1989; Wen & Rose, 1994; Prata & Grant, 2001). The overall aim of this research is to fill an important gap in our understanding of the atmospheric habit of fine volcanic ash and processes governing its sedimentation. Costa A., Macedonio G. & Folch A. (2006) Earth and

Planetary Science Letters 241, 634-647. Durant A., Bonadonna C. & Horwell C. (2010) Elements 6,

235-240. Prata A. J. (1989) Geophysical Research Letters 16, 1293-

1296. Prata A. J. & Grant I. F. (2001) Quarterly Journal of the

Royal Meteorological Society 127, 2153-2179. Robock, A. (2000) Reviews of Geophysics, 38, 191-219. Stohl A., Hittenberger M. & Wotawa G. (1998)

Atmospheric Environment 32, 4245-4264. Watson A. J. (1997) Nature 385, 587-588. Wen S. & Rose W. (1994) J. Geophys. Res 99, 5421-5431.

The sky’s the limit: Mapping volcanic deposits using kites

J. STONE*1,3, J. BARCLAY1, P. COLE2, S. LOUGHLIN3, P. SIMMONS1

1 University of East Anglia. (* [email protected]) 2 Montserrat Volcano Observatory.

3 Britisth Geological Survey.

Aerial photographs are very useful for mapping the extent and variability of volcanic products. However the acquisition of these images is often expensive and infrequent, requiring the use of a helicopter or light aircraft. Kite Aerial Photography (KAP) can be a cheaper alternative, allowing for low altitude, high-resolution images to be rapidly and frequently taken. A camera can be attached to a kite and flown over a deposit, with vertical images stitched together to form a mosaic. Using KAP in Montserrat, a group of volunteer scientists from the local Community College took a series of aerial photographs of a river valley at various times before and after lahars to document geomorphic change. The method produced some very promising results and has acted as a conduit for public involvement in monitoring. This project has demonstrated the low cost and relative ease of this method in a volcanic setting. Depending on the type of camera used, future applications could vary from photographs to photogrammetry and the generation of DEM’s.

101


Strombolian to phreatomagmatic explosive eruption: The Cova de Paúl Crater eruption on Santo

Antão, Cape Verde Islands R.W. TARFF *1, S.J. DAY2

1 Department of Earth and Planetary Sciences, Birkbeck University of London. (*[email protected])

2 Aon Benfield UCL Hazard Research Center, Department of Earth Sciences, University College, London.

Episodes of hazardous phreatomagmatic explosive

activity, often prolonged and complex, occur within eruptions at high-elevation vents on many oceanic island volcanoes. The water driving these explosions is sourced from freshwater aquifers within the volcanic edifices. An understanding of how groundwater enters the eruption conduits is key to effective warning and mitigation of the resulting hazards, including an understanding of how long such eruptions may last. Here we describe near-vent deposits from a single prehistoric eruption at the large Cova de Paúl crater on the island of Santo Antão, Cape Verde Islands that included both mild Strombolian magmatic and violently explosive phreatomagmatic activity. The phreatomagmatic sequence includes extensive low-temperature, lithic-rich phreatomagmatic pyroclastic flow and surge deposits, indicating a prolonged series of more and less violent explosive episodes whose characteristics are indicated by variations in clast size distributions and clast assemblages. The underlying Strombolian deposits, which grade up from welded spatter to unwelded, often blocky scoria, contain distinctive strongly flow-banded angular sub-glassy clasts. These also form a distinct bed of large clasts at just below the phreatomagmatic deposits. Comparison of these clasts with the Strombolian scoria indicates that they are fragments of chilled margins from the walls of the eruptive conduit. Disintegration of the chilled margin allowed a sudden increase in groundwater flow into the conduit causing the onset of the phreatomagmatic explosive phase of the eruption. Subsequent variations in groundwater influx rates as different parts of the groundwater aquifer drained into the conduit may explain the many explosions recorded by the complex phreatomagmatic sequence.

Towards quantifying the arc-scale and global magmatic response to

deglaciation S. WATT*1, D. PYLE2, T. MATHER2

1 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK (*[email protected])

2 Department of Earth Sciences, University of Oxford

There is a growing body of evidence that the retreat of ice sheets after the last glacial maximum (LGM) resulted in temporarily enhanced levels of volcanism. This has been postulated on the scale of individual edifices, and on regional scales in intraplate and rift settings. It has been proposed that this pattern was of global significance in contributing to rising atmospheric CO2 concentrations, and thereby formed a feedback process for global warming. However, the impact of deglaciation on volcanic arcs has been incompletely explored. Volcanic arcs account for 90% of present-day subaerial volcanic eruptions, and for volcanically-sourced volatiles they are of first-order significance. Without understanding fluctuations in arc volcanic output, an assessment of global changes in volcanic activity cannot be made.

Here, we present the first systematic assessment of the response of glaciated volcanic arcs to deglaciation. By using comprehensive compilations of eruption records from southern Chile, augmented by records from the Cascade and Kamchatka arcs, we show that the post-glacial increase in volcanism was relatively small in comparison to non-arc volcano-tectonic settings. Where ice unloading was at its greatest, eruption frequency approximately doubled for ~5 kyr, but this pattern is at the limit of statistical significance. The same period coincides with a few notably large explosive eruptions. In less heavily glaciated regions, no pattern can be deduced at the resolution of available data. While eruption patterns are commonly episodic, the timing of increases in activity does not always show a clear link to deglaciation.

In light of the above, we critically examine available eruption records in an effort to constrain global-scale changes in volcanic output. Great caution must be exercised when attempting to quantify variation in volcanism from such data. Due to extremely sparse sampling (i.e. highly incomplete data), temporal and spatial biases must be corrected. Spatial variation in sampling rates is particularly significant. In some highly active volcanic regions, such as Indonesia, as few as 1 in 20,000 VEI ≥2 eruptions have been identified during the 5–20 ka time period. Globally, >99% of all eruptions of VEI ≥2 have not been identified. Because of this, variations in eruption rate between glaciated and non-glaciated regions cannot be precisely quantified. We suggest that, at most, global eruption rates may have doubled after the last glaciation, from 13–7 ka. Although volcanism may have been an important source of CO2 in the early Holocene, it cannot have been a dominant control on changes in atmospheric CO2 after the LGM. To improve our ability to constrain global-scale patterns in magmatic processes, there is a need for improved records of past volcanic activity, particularly from several low-latitude regions, where data are extremely sparse.

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Constraints on the physical characteristics of volcanic activity

on Venus M.W. AIREY*1, T. A. MATHER1, D. M. PYLE1

1 University of Oxford, UK. (*[email protected])

Volcanoes and their deposits are ubiquitous on Venus, being some of the most recognisable geological phenomena in the surface record. Clues to their origin, mode of formation, composition, and style of eruption can be gathered on inspection of the wealth of data acquired over more than 50 years of modern Venus observation, from the Russian probes and landers of the 60s, 70s, and 80s, through Magellan’s mapping cycles in the 90s to the highly successful Venus Express spacecraft operational today.

The style of volcanic activity on Venus differs from that on Earth due to the very different prevailing conditions into which they erupt; Venus has a surface temperature of ~730 K and a surface pressure of ~92 Earth atmospheres (Kliore, et al., 1986). When conducting an investigation into what may characterise the eruptive style of volcanoes under these conditions, a broad array of environmental, chemical, and physical factors must be considered.

The core of this investigation was based on the linking of two previously developed models of volcanic processes that can be parameterised for Venusian conditions. The conduit flow model ‘Conflow’ (Mastin, 2011) in order to simulate processes below the surface and the volcanic plume model described in Glaze & Baloga (1996) in order to simulate processes above the surface. The approach taken here is to conduct a comprehensive study of the mass flux threshold (by way of vent diameter as a proxy) between buoyancy and collapse using realistic assumptions of initial physical and geological conditions to obtain a best estimate of the initial vent parameters with which to calculate the threshold boundaries at various temperatures, volatile contents, and elevations. These findings were then compared with case studies derived from the Magellan radar data set.

When selecting case studies, localities were sought that provide examples of deposits that suggest a range of styles of volcanic activity upon which to test the models and impose constraints upon volcanic processes. Once selected, the available satellite data (primarily from the Magellan data repository) from the subject region was acquired and, from those, the values of various measured/derived surface properties could be extracted upon which to base the interpretation of the corresponding volcanic terrains in question.

Results from the explosivity/buoyancy modelling study were then used in conjunction with the interpretation of the terrains to provide constraints on the eruption parameters suggested by the various observed deposits.

Glaze, L. S., and Baloga, S. M. (1996) Sensitivity of

buoyant plume heights to ambient atmospheric conditions: Implications for volcanic eruption columns, J. Geophys. Res.-Atmos., 101, 1529-1540.

Kliore, A. J., Moroz, V. I., and Keating, G. M. (1986) The Venus International Reference Atmosphere, Pergamon Press, Oxford.; Mastin, L. G. (2011) Conflow v1.0.5, http://vhub.org/resources/453.

Effusive activity at Somma-Vesuvius: lava flow-field

characteristics from 1631 to 1944 S.K. BROWN*1,2, M.C. SOLANA1, C.R.J. KILBURN3

1 School of Earth and Enivironmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, PO1 3QL. (*[email protected])

2 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ.

3 Aeon-Benfield Hazard Research Centre, UCL, Gower St., London WC1E 6TP.

Vesuvius in southern Italy is among the most densely

populated active volcanoes. Although famous for its destructive Plinian eruptions, its recent activity, between 1631 and 1944, was primarily effusive and Strombolian, producing numerous lava flows that inundated the populated lower slopes.

Here we present data compiled for 183 eruptions during the last eruptive period, with additional data regarding all major flank events. These eruptions fed the emplacement of lava flow-fields which on average reached 4km in length, with a maximum length identified of <8km. Both simple and compound flow-fields are recognised, with typical surface areas of less than 5km2, and approximately two-thirds achieved their maximum lengths after more than one week.

Although most activity was focussed at the summit, flank vents also developed with lava flow-fields affecting correspondingly distal areas. Eccentric vents formed in the southern edifice at approximately 300m above sea level at a mean distance of 4.2km from the summit. A submarine pit crater, newly correlated with the 1861 eruption, highlights the potential for vent development at low altitudes.

Flank vents normally developed when the summit crater had filled with lavas, and eccentric vents were preceded by three days of increased felt seismicity. Such events can thus potentially be forecast with at least a 24-hour warning, and even after the onset of eruption, several days may be available before a settlement is threatened. The design of an effective emergency response to future effusions is thus a feasible objective.

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Lava channel networks H.R. DIETTERICH*1, K.V. CASHMAN 1,2

1 Department of Geological Sciences, University of Oregon, Eugene, Oregon, USA. (*[email protected])

2 School of Earth Sciences, University of Bristol, Bristol, UK.

A crucial question for assessing lava flow hazard, as

well as understanding flow emplacement at volcanoes around the world and on other terrestrial planets, is what controls the lengths of lava flows. John Guest sought to answer this question by investigating the emplacement of lava flows at Mt Etna that, despite very different eruptive rates and durations, yielded flows of the same final length (Guest et al., 1987). This observation can also be made of Hawaiian lava flows, many of which have a length of ~ 25 km regardless of effusion rate, except where the longer flows have been topographically confined into single channels. We hypothesize that both flow length and advance rate of Hawaiian lava is modulated by the development of complex channel networks.

To analyze the creation of lava channel networks, we focus on the bifurcations and confluences that create them. We hypothesize that flow interaction with topographic obstacles will result in flow splitting, while flow confluences will be driven by topographic confinement or the merging of parallel lobes due to lateral spreading. Our analysis of the Hawaiian channel networks reveals that greater pre-eruptive slopes correlate with more parallel channels, supporting that flow height, which is reduced on steeper slopes, could control bifurcation formation. However, in analogue experiments, flows are able to overtop obstacles at high velocity when a bow wave thickens the flow upslope of the obstacle. We also document how cooling influences confluence formation after a bifurcation with molten basalt experiments that show that levee formation inhibits the formation of confluences between parallel flow lobes. Further analysis of lava channels as interconnected networks, combined with an investigation of flow thickness, yields insight into the evolution of channel networks during emplacement. This work has implications for flow prediction, diversion barriers, and the interpretation of terrestrial and planetary lava channel networks and flow morphology.

Guest, J.E., Kilburn, C.R.J., Pinkerton, H., and Duncan,

A.M., (1987) Bulletin of Volcanology 49, 527-540.

Kalkarindji - the forgotten volcanic province

P.E. MARSHALL*1, M. WIDDOWSON1 1 Volcano Dynamics Group, Envinronment, Earth &

Ecosystems, The Open University, Walton Hall, Milton Keynes, UK. MK7 6AA. (*[email protected]) Kalkarindji, the oldest Phanerozoic continental flood

basalt province (CFBP), covers a large part of the northern Australian desert, being well preserved due to its location on the stable Australian craton. The province enjoys an anonymity not usually afforded to structures of this size, due mainly to this remote location. Dated at 505-510 Ma, the province now consists of scattered basaltic suites across northern and central Australia, which if connected indicate a minimum eruptive volume of 1.5 ϰ 105 km3.

The most extensive of these suites, the Antrim Plateau Volcanics (APV), lies SE of Lake Argyle, stretching for 400,000 km2 with thicknesses of 0.7 - 1.1 km. They are predominantly evolved, low-Ti tholeiitic basaltic andesites exhibiting extreme crustal contamination signatures. Therefore, the composition of the Kalkarindji lavas are quite distinct from other tholeiitic CFBP successions.

Field reconnaissance reveals flow units to be thick (40 - 60 m) sheet-like aphanitic basalt with vesiculated or rubbly flow-tops. Four different flow types, seen throughout the province, are identified, based on geochemistry, morphology and volcanolgy. We present new data from boreholes and field samples, building up a volcanostratigraphy across the APV.

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Levee control on the evolution of lava flow fields

M. NOLAN*1, M.C. SOLANA2, C.R.J. KILBURN3 1 SEES, Burnaby Building, Burnaby Road, Portsmouth, PO1

3QL 2 Aeon-Benfield Hazard research Centre, UCL, Gower St.,

London WC1E 6TP

Traditional lava flows studies (e.g. Guest et al 1987, Kilburn and Lopes 1988, Kilburn and Lopes 1991) established five mechanisms controlling the growth of flow-fields: New flows originating from the vent, Breaching of levees, Breaching/ reactivation of fronts, Bifurcation of channels and

Overtopping. An analysis of the frequency of these mechanisms showed that breaching is one of the most important process for the growth of flow fields. Breaches from the medial sides of lava channels on Etna frequently occur where a pulse of lava stops and inflates and is clearly favoured by topography. Even slight topographical irregularities favour the deformation and breaching of channels. On slopes with a lower gradient the sinuosity in lava channels increases and in 57% of the cases examined, curvature of the channel led to breakouts or over-topping. In the majority of these instances, the breakout occurred on the outside bend of the flow.

Rock mechanic tests on levee samples confirmed that the strength and mode of failure varied substantially between levee types and even between different orientations within the same levee. The alignment of crystals and vesicles also had a significant impact in reducing the strength of the rock. As expected, “massive” levee types with low vesicularity were found to be the most resistant and “rubbly” the weakest.

Inflation, drainage and lava-water interaction during the emplacement

of the Nesjahraun, Iceland J.A STEVENSON*1, N.C. MITCHELL2, M. CASSIDY3, H.

PINKERTON4

1 School of Geosciences, University of Edinburgh. (*[email protected])

2 S.E.A.E.S. University of Manchester 3 National Oceanography Centre, University of

Southampton 4 Lancaster Environment Centre, University of Lancaster

This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake Þingvallavatn, SW Iceland. It combines LiDAR, sidescan sonar and Chirp seismic data with field observations to map the flow field and investigate the varied behaviour of the lava.

The lava in the central part of the flow field has a platy-ridged surface, similar to other lava flows in Iceland and on Mars. This is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into plates. The ridges of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings. Along the shoreline, stacks of thin (5–20 cm thick), vesicular, flows rest upon and surround low (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Offshore from the pāhoehoe lavas, the gradient of the flow surface steepens, suggesting a change in flow regime and the development of a talus ramp.

Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly, flowing as ‘a‘ā lava that eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and rafts of compound shelly pāhoehoe lava and leaving a network of drained channels and tubes in the upstream part of the flow. The ‘a‘ā channel split into individual flow lobes 20–50 m wide along the shore. ‘A‘ā clinker is exposed on the water’s edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava bodies. Mounds consisting predominantly of scoria lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The ‘a‘ā flow can be identified underwater over 1 km offshore, and the sidescan data show that the flow lobes remained coherent flowing down a gradient of <10°.

We suggest that formation of this type of platy-ridged lava, where the plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the surface crust is broken into plates in situ. The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and that pāhoehoe flows are much more likely to break up on entering water than ‘a‘ā flows, which, with a higher flux and shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km.

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VMSG 2013 – List of Delegates

Delegate List Name E-mail Institution Martin Airey [email protected] University of Oxford Paul Albert [email protected] Royal Holloway Jens Andersen [email protected] University of Exeter Paul Anderson [email protected] University of Birmingham Catherine Annen [email protected] University of Bristol Chris Arkwright [email protected] The Open University Willy Aspinall [email protected] University of Bristol Melanie Auker [email protected] University of Bristol Jenni Barclay [email protected] University of East Anglia Dan Barfod [email protected] SUERC Heidi Barnes [email protected]

Andrew Beard [email protected] Birkbeck Samuel Bewick [email protected] Open University Juliet Biggs [email protected] University of Bristol Stephen Blake [email protected] The Open University Jon Blundy [email protected] University of Bristol John Bowles [email protected] University of Manchester Julie Boyce [email protected] Monash University Mike Branney [email protected] University of Leicester Richard Bretton [email protected] University of Bristol Michael Broadley [email protected] University of Manchester Richard Brooker [email protected] University of Bristol David Brown [email protected] University of Glasgow Sarah Brown [email protected] The University of Bristol John Browning [email protected] Lancaster University David Budd [email protected] Uppsala University Liam Bullock [email protected] Keele University Rose Burden [email protected] University of Bristol Anthony Burnham [email protected] University of Bristol Bethan Burson [email protected] Open University Luca Caricchi [email protected] University of Geneva Kathy Cashman [email protected] University of Bristol Michael Cassidy [email protected] University of Southampton Janet Catchpole [email protected]

Ana Carracedo [email protected] SUERC David Chester [email protected] University of Liverpool Paul Cole [email protected] Plymouth University Frances Cooper [email protected] University of Bristol Maya Coussens [email protected] Southampton University Paul Cragg [email protected]

Elizabeth Cramer [email protected] The Open University Annabel Crowther [email protected] Cambridge University Julia Crummy [email protected] University of Leeds Börje Dahren [email protected] Uppsala University Jon Davidson [email protected] University of Durham Alistair Davies [email protected] University of Cambridge Simon Day [email protected] University College London Sarah Henton De Angelis [email protected] University of Alaska Fairbanks Alexandra De Joux [email protected] University of Edinburgh Rodrigo Del Potro [email protected] University of Bristol Philippa Demonte [email protected] Boise State University Hannah Dietterich [email protected] University of Oregon Jonathan Dietz [email protected] University of Glasgow Mikel Diez [email protected] University of Bristol Kate Dobson [email protected] University of Manchester Amy Donovan [email protected] University of Cambridge Simon Drake [email protected] Birkbeck Angus Duncan [email protected] University of Bedfordshire Susanna Ebmeier [email protected] University of Bristol Marie Edmonds [email protected] University of Cambridge Holly Elliott [email protected] University of Southampton Ben Ellis [email protected] ETH Zurich Fran Entwistle [email protected] University of Leeds Laura Evenstar [email protected] University of Bristol Peter Fawdon [email protected] The Open University Carina Fearnley [email protected] Aberystwyth University April Fitzgerald-Hudson [email protected] Keele University Anne Forbes [email protected] The Open University Heye Freymuth [email protected] University of Bristol Raffaella Fusillo [email protected] University of Bristol Thomas Gernon [email protected] University of Southampton Ralf Gertisser [email protected] Keele University Robin Gill [email protected] Royal Holloway Amy Gilmer [email protected] University of Bristol Joachim Gottsmann [email protected] University of Bristol Tanya Gray [email protected] University of Bristol Robert Green [email protected] University of Cambridge David Green [email protected] AWE Blacknest Tim Greenfield [email protected] University of Cambridge

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Clayton Grove [email protected] University of Durham Leanne Gunn [email protected] The Open University Derya Gurer [email protected] University of Oslo Jonathan Hanson [email protected] University of Bristol Margaret Hartley [email protected] University of Cambridge Ben Hayes [email protected] Cardiff University Brioch Hemmings [email protected] University of Bristol James Hickey [email protected] University of Bristol Kelby Hicks [email protected] University of Cambridge Anna Hicks [email protected] University of East Anglia Melanie Hinrichs [email protected] The Open University Murray Hoggett [email protected] University of Birmingham Marian Holness [email protected] University of Cambridge Eoghan Holohan [email protected] University College Dublin Adrian Hornby

University of Liverpool

Martin Hughes [email protected] Mineralogical Society Sion Hughes [email protected] The Open University Hannah Hughes [email protected] Cardiff University Madeleine Humphreys [email protected] University of Oxford Will Hutchinson [email protected] University of Oxford Evgenia Ilyinskaya [email protected] Bristish Geological Survey Paul Jarvis [email protected] University of Bristol Alia Jasim [email protected] University of Bristol Adam Jeffrey [email protected] Keele University Susanna Jenkins [email protected] University of Bristol Eleanor Jennings [email protected] University of Cambridge Lisa Jepson [email protected] University of Manchester Dougal Jerram [email protected] Dougal Earth Limited Emma Johnston [email protected] University of Bristol Adrian Jones [email protected] University College London Rosie Jones [email protected] The University of Edinburgh Nina Jordan [email protected] University of Leicester Maren Kahl [email protected] University of Leeds Jessica Kandlbauer [email protected] University of Bristol Michael Kendall [email protected] University of Bristol Jackie Kendrick [email protected] Ludwig-Maximilan University Christopher Kilburn [email protected] University College London Geoff Kilgour [email protected] University of Bristol Marthe Klocking [email protected] University of Cambridge Tom Knott [email protected] University of Leicester Stefan Lachowycz [email protected] University of Oxford Oliver Lamb [email protected] University of Oxford Yan Lavallee

University of Liverpool

Phillip Leat [email protected] British Antarctic Survey Julien Leuthold [email protected] University of Bristol David Litchfield [email protected] University College London Marion Louvel [email protected] Institut Neel John Maclennan [email protected] University of Cambridge Heidy Mader [email protected] University of Bristol Craig Magee [email protected] Imperial College Sue Mahony [email protected] University of Bristol Ben Manton [email protected] Cardiff University Freya Marks [email protected]

Peter Marshall [email protected] The Open University Tamsin Mather [email protected] University of Oxford Hannes Mattsson [email protected] ETH Zurich Brendan McCormick [email protected] Queens College Dave McGarvie [email protected] The Open University Iona McIntosh [email protected] Durham University Charlotte McLean [email protected] University of Glasgow Claire McLeod [email protected] University of Houston Sorcha McMahon [email protected] University of Bristol Fiona Meade [email protected] Uppsala University Ian Meighan [email protected] SUERC Vali Memeti [email protected] University of Durham Hilary Milroy [email protected] University of Bristol Helena Moretti [email protected] University of Bristol Dan Morgan [email protected] University of Leeds Duncan Muir [email protected] University of Bristol Cyril Muller [email protected] University of Bristol Irving Munguia [email protected] University of Bristol Laura Murdoch [email protected] University of Strathclyde Joanne Murray [email protected] University of Birmingham John Murray [email protected] The Open University Claire Nattrass [email protected] University of Durham David Neave [email protected] University of Cambridge Peter Nicholls [email protected] Uppsala University Emma Nicholson [email protected] University of Bristol Graeme Nicoll [email protected] Neftex Svetlana Novikova [email protected] University of Cambridge Andy Nowacki [email protected] University of Bristol Henry Odbert [email protected] University of Bristol

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Julie Oppenheimer [email protected] University of Bristol Pablo Palacios [email protected] University of Bristol Kris Palubicki [email protected] The Open University Matthew Pankhurst [email protected] University of Leeds Amy Parker [email protected] University of Bristol Chiara Maria Petrone [email protected] The Natural History Museum Stacy Phillips [email protected] Memorial University of Newfoundland Jeremy Phillips [email protected] University of Bristol Joe Pickles [email protected] University of Bristol Mattia Pistone [email protected] ETH Zurich Melissa Plail [email protected] University of East Anglia Lucy Porritt [email protected] University of Bristol Gemma Prata [email protected] University of Oxford Katie Preece [email protected] University of East Anglia Jacqueline Ratner [email protected] University of Oxford Heather Rawcliffe [email protected] University of Glasgow Marco Reichow [email protected] University of Leicester Peter Reynolds [email protected] University of Durham Hannah Reynolds [email protected] Lancaster University Jenny Riker [email protected] University of Bristol Teal Riley [email protected] British Antarctic Survey Elspeth Robertson [email protected] University of Bristol Mel Rodgers [email protected] University of South Florida Alison Rust [email protected] Bristol University Hazel Rymer [email protected] The Open University Lois Salem [email protected] University of Cambridge Christopher Satow [email protected] Royal Holloway Kate Saunders [email protected] University of Bristol Andy Saunders [email protected] University of Leicester Nick Schofield [email protected] University of Birmingham Anne Schopa [email protected] University of Bristol Thomas Sheldrake [email protected] University of Bristol Jessica Shields [email protected] University of Bristol Isobel Sides [email protected] University of Cambridge Bethany Simons [email protected] Cambourne School of Mines Susanne Skora [email protected] University of Bristol Jean-Francois Smekens [email protected] Arizona State University Vicki Smith [email protected] University of Oxford Kate Smith [email protected] University of Exeter Carmen Solana [email protected] University of Portsmouth Stephen Sparks [email protected] Bristol University Charlotte Stamper [email protected] University of Bristol John Stevenson [email protected] University of Edinburgh Michael Stock [email protected] University of Oxford Ellen Stofan [email protected]

Jonathan Stone [email protected] University of East Anglia Karen Strehlow [email protected] University of Bristol Finlay Stuart [email protected] SUERC Elizabeth Swanson [email protected] University of Bristol Yoshihiko Tamura [email protected] JAMSTEC Bob Tarff [email protected] Birbeck College Andrew Thomson [email protected] University of Bristol Emma Tomlinson [email protected] Trinity College Dublin Valentin Troll [email protected] Uppsala University Marie Turnbull [email protected]

Marit Van Zalinge [email protected] University of Bristol Geoff Wadge [email protected] University of Reading Richard Wall [email protected] University College London Matt Watson [email protected] University of Bristol Sebastian Watt [email protected] Southampton University Daniel Weidendorfer [email protected] ETH Zurich Bradley West [email protected] University of Bristol Robert White [email protected] Cambridge University Rebecca Williams [email protected] University of Leicester Lionel Wilson [email protected] Lancaster University Penelope Wilson [email protected] Kingston University Mark Woodhouse [email protected] University of Bristol Christine Yallup [email protected] University of Cambridge

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Volcanic and Magmatic Study Group (2013) Bristol

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