1
2. Field observations Dunites in both ophiolites host podiform chromitite (>60 vol.% Cr-spinel) seams. Chromitite seams are 0.01-5 m thick and at the outcrop scale may appear complexly deformed, forming stock-work veins and patches. The largest chromitites (3-5 m thick) always occur within ~100 m of the petrological Moho The mantle sections of both ophiolites are compositionally heterogeneous at the cm-to-m scales; noticeably manifested by dunite lenses and layers in dominantly harzburgitic host rock. Dunite Harzburgite Layered harzburgite-dunite at Cliff, Shetlands Intrusive dunite Layered dunite/wehrlite Layered Series, Kvaløya, Leka 'Podiform' chromitite at Hagdale, Shetlands Rhythmic chromitite layering, Kvaløya, Leka 4. Mineral-scale observations: Siting the HSE 4.1 Shetland Ophiolite Complex 4.2 Leka Ophiolite Complex 10 μm 5 μm Very little primary silicate remains due to serpentinisation. Dunites contain variable sulphide (pentlandite) abundances, whilst chromitites contain minor Ni-sulphide and platinum-group minerals (PGM) At Cliff, 1-2 vol.% Ni-arsenide is present in chromitites 100 μm Os Ir Ru Pt Pd Re 0.0001 0.001 0.01 0.1 1 10 100 1000 Chromite Pentlandite Viels Mineralized Dunite Cr-spinel Ni-arsenide Serpentinite The Viels dunite Os Ir Ru Rh Pt Pd Re Au 0.001 0.01 0.1 1 10 100 1000 10000 100000 Chromite (n = 15) Whole-rock NiAs grains Sample/CI-Chondrite Ni-arsenide Sperrylite Sperrylite Cliff chromitite Sample/CI-Chondrite LA-ICP-MS analyses show that sulphides, arsenides and PGM site the highly siderophile elements (HSE). HSE abundances are below detection limits in Cr-spinel, except at Cliff locality Pt and Ir anomalies indicate the types of HSE-rich alloy present, supported by petrography (above) In the mantle section, the HSE site in sulphides (and possibly PGM) that concentrate in clinopyroxene-rich veinlets and in cleavage planes in large orthopyroxene(?) pseudomorphs Olivine Clinopyroxene veinlets Cr-spinel Ferritchromit Ferritchromit Pentlandite Both the mantle section and Layered Series are remarkably fresh compared to the Shetland rocks, with only <20 vol.% serpentinisation in dunites, harzburgites and pyroxenites in some cases Above the petrological Moho, chromitite seams and sulphide (pentlandite)-rich dunites become more abundant, and control the HSE budget of the Layered Series portion of the ophiolite Olivine Ferritchromit Relict orthopyroxene(?) cleavages with ferritchromit/sulphide inclusions Olivine domains Serpentine domain Clinopyroxene Sulphide and ferritchromit inclusions PGM in ferritchromit rims References and acknowledgements Furnes, H., Pedersen, R.B., Stillman, C.J. 1988. Journal of the Geological Society, London 145, 401-412. Dunning, G.R., Pedersen, R.B. 1988. Contributions to Mineralogy and Petrology 98, 13-23. O’Driscoll, B., Day, J. M. D., Walker, R. J., Daly., J. S., McDonough, W. F., Piccoli, P. M. 2012. Earth and Planetary Science Letters 333-334, 226-237 (and references therein). Derbyshire, E.J., O'Driscoll, B., Lenaz, D., Gertisser, R., Kronz, A. 2012. Lithos, in press. Spray, J.G., Dunning, G.R. 1991. Geological Magazine 128, 667- 671. Walker, R.J., Prichard, H.M., Ishiwatari, A.,Pimentel, M. 2002. Geochimica et Cosmochimica Acta 66, 329-345. This work is funded by a NERC New Investigator Grant (NE/J00457X/1) as well as additional support from the Mineralogical Society of Great Britain and Ireland, the Geological Society of London, the Geological Society of Edinburgh and the Royal Society 6. Discussion points Some Shetland harzburgites reveal evidence that Neoproterozoic-aged cm-m scale melt depletion domains survived subsequent (492 Ma) supra-subduction zone melt extraction Leka and Shetland harzburgite HSE concentrations both compare well to the range of Gakkel Ridge abyssal peridotite compositions, suggesting that variable degrees of serpentinisation do not obfuscate primary HSE signatures Relatively radiogenic γOs in Leka and Shetland dunites (compared to harzburgites), support their formation in upper mantle melt channels. Such channels have been effective at homogenising Os isotope signatures from the surrounding mantle (e.g., γOs ranges in the Shetland chromitites) Scales of preservation and root causes of heterogeneities in the convecting upper mantle Corresponding Author: b.o'[email protected] of the Iapetan Ocean Paper Number: V53A-2808 Keel e Pet r ol ogy Gr oup KEELE PETROLOGY GROUP Brian O'Driscoll , Richard J Walker , James MD Day , J Stephen Daly 1 2 3 4 School of Physical and Geographical Sciences, Keele University, Keele, United Kingdom, Department of Geology, University of Maryland, College Park, MD, United States, Geosciences Research Division Scripps Institution of Oceanography, La Jolla, CA, United States, UCD School of Geological Sciences, University College Dublin, Dublin, Ireland 1 2 3 4 3. Mantle melting regime and tectonic setting Unaltered ophiolite Cr-spinel compositions, especially those from podiform chromitite, are a useful petrogenetic tool for determining Cr-spinel parental melt compositions and tectonic setting 0.01 0 10 1.0 0.1 45 40 35 30 25 20 15 10 5 Cr-spinel TiO wt.% 2 MORB OIB LIP ARC 50 Cr-spinel Al O wt.% 2 3 Chromitite Cr-spinel compositions are highlighted in red and support field observations (e.g., dunite layering) that suggest mantle melting in a supra-subduction zone (island-arc) setting Shetland Cr-spinel Leka Cr-spinel × Cliff Cr-spinel localities Nikka Vord Hagdale The Viels Harolds Grave Cr-spinel localities Altered compositions (ferritchromit) Refertilisation trend? Kvaløya Kvaløya-moen Lauvhatten Kvaløya High-T (equilibration with melt) and low-T (ferritchromit) alteration of Cr-spinel is also evident Cr-spinel Al O wt.% 2 3 Cr-spinel TiO wt.% 2 1. Introduction and Geological Setting The Shetland (UK; ~492 Ma) and Leka (Norway; ~497 Ma) ophiolites each comprise a portion of early-Paleozoic oceanic lithosphere with well-exposed upper mantle sections Ophiolites allow an assessment of the timing, causes and extent of processes that operate in the mantle, facilitating the coupling of field-based investigations with geochemical analysis of these otherwise inaccessible rocks Siberia Baltica Gondwana Laurentia Iapetus Ocean Tornquist Sea SOUTH POLE Scotland Shetland Isles 100 km 1 W o 60 N o Tornquist Line Great Glen Fault Walls Boundary Fault Unst North Sea Bergen Trondheim Leka Karmøy Norwegian Sea Oslo Norway Harzburgite (with dunite) Layered dunite and wehrlite Layered gabbro/metabasalt dykes Metabasalt dykes Pillow basalts and volcaniclastics Cover sequences: Skei Group (Leka) and Muness Phyllites (Shetlands) Solsemøyene Group (Leka) Petrological Moho 5 km 55 Balta Sound 65 15 10 05 00 N The Viels Hagdale Harolds Grave Unst Fetlar Yell 95 90 Aithbank Fault Bluemull Fault Lamb Hoga Fault North Sea Muckle Flugga Skaw Granite 418m 376m Raudberget Solsem Madsøya Husby Skei Lauvhatten Skeisnesset Storøya Leknesøyan Kvaløya Steins Branden Vattind Lekafjorden N Langdraget Halin 3 km Late Cambrian-Early Ordovican ca. 490 Ma Proterozoic metasediments (Shetlands) Harzburgite (Upper Nappe, Shetlands) Skaw Granite (Unst) Inset maps show regional (geographic) and tectonic setting for the Shetland and Leka Ophiolite Complexes. Corresponding units are coloured the same for each ophiolite in the geological maps. Sampling focused on the mantle section (harzburgite) and around the petrological Moho in both ophiolite complexes. Nikka Vord Kvaløya- moen Sampled sites Sampled traverse Cliff 5. Os isotope systematics and HSE abundances 0 1 10000 1000 100 10 -25 Os (ppb) 15 5 -5 -15 γOs Shetland chromitites Hagdale Harolds Grave Cliff Shetland dunites Shetland harzburgites Leka dunites Leka harzburgites 0 2 10 8 6 4 0.110 0.140 0.120 0.130 0.150 0.160 0.170 Re/ Os 187 188 Os/ Os 187 188 γOs calculated at 492 Ma for Shetland samples and 497 Ma for Leka samples Dunites from both ophiolites have more variable and absolute abundances of the HSE than harzburgites: Late-stage Re addition Late-stage Re addition γOs 3.90-3.95 γOs 0.48-1.00 γOs 1.34-1.37 Individual Shetland chromitite localities have very restricted ranges in initial Os isotopic composition (see figure above) All harzburgites have relatively unradiogenic initial Os isotope compositions (γOs of -5.3 to +2.6) and low Re/ Os (0.02-1.3) 187 188 Shetland and Leka dunites range to more radiogenic γOs (-3.3 to +4.4) with higher Re/ Os (0.02-2.8) 187 188 The Shetland chromitite data point to significant Os isotope heterogeneity being present at 492 Ma Harzburgites: 2.6-4.1 ppb Os, 3.5-12.1 ppb Pt, 0.03-0.94 ppb Re, broadly similar to the primitive mantle composition estimate (PM) Dunites: 0.4-41.7 ppb Os, 0.03-407 ppb Pt, 0.01-14.8 ppb Re A pyroxenite from below the Leka petrological Moho has 17.8 ppb Os, 1056 ppb Pt and 2.09 ppb Re Shetland chromitites reveal a significant range in HSE concentrations: 0.09-2.94 ppm Os and 0.02-96.6 ppm Pt Leka harzburgites Leka dunites (and pyroxenite) Shetland dunites Shetland harzburgites Shetland chromitites Range of Shetland harzburgites
