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Carbonatite Magmatism of North East Africa& Implications for the East African Rift

James Sean DicksonPhoto Credit: Cawsey, 2011

Carbonatite Magmatism of North East Africa

• Why study carbonatites?

• Carbonatite characterisation, classification and features

• Setting

• Carbonate melts

• Genesis

• Implications

Belton, 1998

WHY STUDY CARBONATITES?

• Source of REE, Nb, U & Ta

• Wider implications

• East African Rift

• Mantle geochemistry

• Academic study (particularly as these rocks are so petrologically distinct)

DEPOSIT RESERVES AND GRADE COMMENTS

Oka Carbonatite, Quebec 112.7 Mt at 0.44% Nb2O523.8 Mt at 0.2-0.5% REO

Hydrothermal REE mineralisation especially

pyrochlore

Phalaborwa, South Africa600 Mt at 7% P2O5 286 Mt at 0.69% Cu

2.16 Mt REO

Banded carbonatite contains Cu sulphides, magnetite and

baddeleyite

Bayan Obo, Inner Mongolia 37 Mt at 6% REO1Mt at 0.1% Nb Largest mined REE deposit

Amba Dongar, India 11.6 Mt at 30% CaF2

Ore associated with fenite units between carbonatite and

country rock

Panda Hill, Tanzania 113 Mt at 0.3% Nb2O5Disseminated pyrochlore,

apatite, magnetite in sövite plug

Jones et al. 2013

Nelson, 2011

CARBONATITE CHARACTERISATION• Part of the alkaline igneous suite (Na2O +

K2O high relative to SiO2)

• Comprised of more than 50 modal percent primary carbonate minerals (Le Maitre, 2002)

• Less than 20 modal percent SiO2 (less common in literature) (Le Maitre, 2002)

• Rare in nature - only ~527 occurrences of carbonatites are known, 49 of which are extrusive (Woolley and Church, 2005; Woolley and Kjarsgaard, 2008)






Ol Doinyo Lengai natrocarbonatite. Mainly comprised of gregoryite and nyerereite.






Compiled analyses in Jones et al. 2013

CARBONATITE CLASSIFICATION

• Protracted history of classification with numerous models proposed involving inaccessible & obscure rock names

• Modern classifications like those suggested by Jones et al. (2013) focus on self-explanatory compositional names

CalciocarbonatiteCaO/(CaO+FeO+MgO >

0.80

Ferrocarbonatite (FeOT + MnO) > MgO

Dolomite carbonatite (Ca,Mg)-rich

Magnesiocarbonatite MgO > (FeO + MnO)

Rare Earth Carbonatite RE2O3 > 1% wt

NatrocarbonatiteNa2O + K2O) > (CaO

+MgO+FeO)

CARBONATITE CLASSIFICATION

• Protracted history of classification with numerous models proposed involving inaccessible & obscure rock names

• Modern classifications like those suggested by Jones et al. (2013) focus on self-explanatory compositional names

CalciocarbonatiteCaO/(CaO+FeO+MgO >

0.80

Ferrocarbonatite (FeOT + MnO) > MgO

Dolomite carbonatite (Ca,Mg)-rich

Magnesiocarbonatite MgO > (FeO + MnO)

Rare Earth Carbonatite RE2O3 > 1% wt

NatrocarbonatiteNa2O + K2O) > (CaO

+MgO+FeO)

Jones et al. 2013

CARBONATITE FEATURES• Heavily enriched in LREE compared to the Bulk Earth (CI

Chondrites)

• High Ce/Yb ratios

• Accessory minerals: forsterite, enstatite, aegirine-augite, melilite, phlogopite, biotite, apatite, magnetite, pyrochlore and Zr-Ti garnets

• Greater electronic conductivity than hydrated mantle - 5 orders of magnitude (Gaillard, 2008)

• Very low density of ~ 2000kg m-3 @ 0.1 GPa (Jones et al. 2013)

• Hygroscopic - rapidly absorb water

• Eg. Nyerereite → Pirssonite / Gaylussite Na2Ca(CO3)2 → Na2Ca(CO3)2•2H2O / Na2Ca(CO3)2•5H2O

Electio, 2014

CARBONATITE FEATURES• Heavily enriched in LREE compared to the Bulk Earth (CI

Chondrites)

• High Ce/Yb ratios

• Accessory minerals: forsterite, enstatite, aegirine-augite, melilite, phlogopite, biotite, apatite, magnetite, pyrochlore and Zr-Ti garnets

• Greater electronic conductivity than hydrated mantle - 5 orders of magnitude (Gaillard, 2008)

• Very low density of ~ 2000kg m-3 @ 0.1 GPa (Jones et al. 2013)

• Hygroscopic - rapidly absorb water

• Eg. Nyerereite → Pirssonite / Gaylussite Na2Ca(CO3)2 → Na2Ca(CO3)2•2H2O / Na2Ca(CO3)2•5H2O

Electio, 2014

Jones et al. 2013

HYGROSCOPIC PROPERTIES

Photo Volcanica

Nyerereite → Pirssonite / Gaylussite Na2Ca(CO3)2 → Na2Ca(CO3)2•2H2O / Na2Ca(CO3)2•5H2O

SETTING

• Over half of all known carbonatites are found in Africa (Jones et al. 2013; Bailey, 1993)

• The known extrusive alkaline rocks of Kenya, Tanzania and Ethiopia have a collective volume greater than the rest of the world combined (Wooley, 2001)

• Usually found in association with alkaline silicate rocks - not the case in ~20% of occurrences (Woolley and Kjarsgaard, 2008)

Johnson, 2006

FIELD RELATIONSHIPS

• Intrusive carbonatites are always emplaced after alkaline silicates (if they exist in association)

• Often this will manifest itself in veins that cross cut the original alkaline silicate rocks

Genge, 2014

FIELD RELATIONSHIPS

• Intrusive carbonatites are always emplaced after alkaline silicates (if they exist in association)

• Often this will manifest itself in veins that cross cut the original alkaline silicate rocks

Chakhmouradian

CARBONATITE MELTS• Very low viscosity melts - only an order

of magnitude higher than water in PaS

• Carbonatites almost completely degassed at the surface (Teague et al. 2008)

• Very low temperature melts at surface - 491–593°C at Ol Doinyo Lengai, Tanzania (Zaitsev et al. 2009)

• Essentially ionic melts with no polymeric structure

Fluid Dynamic Viscosity in PaS

Olive Oilº ~84

Waterº 8.9*10-4

Nitrogenº 1.8*10-5

Rhyolitic Lavas (Giordano 2008) < 1015

Basaltic Lavas @ ~1100ºC(Pinkerton & Norton, 1995)

~150 - 3000Calciocarbonatite Lavas @

800ºC (Wolff, 1994)8*10-2

Natrocarbonatite (@ 800ºC Wolff, 1994; @ 491-593ºC Zaitsev et al. 2009)

8*10-3; 0.3-120

CAUSE OF LOW VISCOSITY

• Lack of polymerisation is the cause of the low viscosity

• Silica melts have polymeric chain structures

• Carbonatite viscosity is derived almost solely from coulombic interaction between the component ions with no local melt structure

Photo Volcanica

Strekeisen

POSSIBLE SOURCES• Melting of crustal carbonates by ascending plutonic rocks

• Primary mantle carbonatite melt

• Secondary melt

• Separation due to carbonate-silicate phase immiscibility

• Residual melt left from crystal fractionation of carbonated nephelinite, ijolite or melilitite melts (all types of alkali rich, silica poor rocks)

• Mixture of the above

CRUSTAL LIMESTONE MELTING

• Marine carbonates derived from 87Sr/86Sr = 0.70916 ocean water (Palmer & Edmond, 1989)

• Lacustrine carbonates derived from 87Sr/86Sr = 0.7119 river water (Palmer & Edmond, 1989)

• Ol Doinyo Lengai fumarole gasses have the same 3He/4He ratios as local mantle xenoliths (Teague et al. 2008) Carbonates

Johnson, 2006

CRUSTAL LIMESTONE MELTING

• Marine carbonates derived from 87Sr/86Sr = 0.70916 ocean water (Palmer & Edmond, 1989)

• Lacustrine carbonates derived from 87Sr/86Sr = 0.7119 river water (Palmer & Edmond, 1989)

• Ol Doinyo Lengai fumarole gasses have the same 3He/4He ratios as local mantle xenoliths (Teague et al. 2008) Carbonates

Johnson, 2006

DEFINITE MANTLE SOURCE• Ol Doinyo Lengai fumarole gasses have

the same 3He/4He ratios as local mantle xenoliths + mantle Nd/Sr (not affected by partial melting or fractional crystallisation)

‣ Sub-continental lithospheric mantle source ✓ (Teague et al. 2008; Ernst & Bell 2009)

‣ He should partition well into CO2 - the source signature will be retained regardless of whether the melt is primary or secondary (Teague et al. 2008)

206Pb/204Pb Initial

Jones et al. 2013

PRIMARY MANTLE MELT• Adding CO2 + H2O to mantle peridotites

(lherzolite) allows for low degree partial melts to be created (Harmer & Gittins, 1998)

• Experimental petrology confirms that natrocarbonatite and magnesiocarbonate primary mantle melts are viable (Harmer & Gittins, 1998)

• Calciocarbonatite produced by carbonatite melt metasomatism of mantle wehrlite peridotite (Harmer & Gittins, 1998)

Genge, 2014

(Harmer & Gittins, 1998)

IMMISCIBILITY - GEOCHEMISTRY

• Experimental petrology confirms that carbonate and silicate melts can become immiscible depending on phase concentrations, temperature and pressure

• Dashed tie-lines opposite represent experimentally demonstrable liquids that can exist in equilibrium

• Ijolite or nephelenite mantle melts that are CO2 saturated provide the mechanism

Johnson, 2006

IMMISCIBILITY - FIELD RELATIONSHIPS & PETROLOGY

• Petrological evidence of this happening at Ol Doinyo Lengai

• Unusual carbonatite lava flow in 1993 gave evidence of nepheline-phenocryst containing silicate melt droplets existing in a carbonatite melt

• Field relationships - carbonatites come later

• Conclusion: EAR carbonatites are formed through carbonatite phase immiscibility

Church and Jones, 1995

Jones et al. 2013IMPLICATIONS - MANTLE SOURCE

• Geochemistry suggests the source is not ‘DM’ depleted mantle

• HIMU means a mantle budget contribution by old, altered oceanic crust

• EMI means a mantle budget contribution by delaminated lithosphere

• Certainly fits with the model of local ancient cratonic crust

206Pb/204Pb Initial

IMPLICATIONS - MANTLE SOURCE

• Geochemistry suggests the source is not ‘DM’ depleted mantle

• HIMU means a mantle budget contribution by old, altered oceanic crust

• EMI means a mantle budget contribution by delaminated lithosphere

• Certainly fits with the model of local ancient cratonic crust Genge, 2014

IMPLICATIONS - LITHOSPHERE

• Geophysical constraints on lithospheric thickness are well known through seismics

• Geochemical confirmation - thickened lithosphere plays an important role in the production of CO2-rich melts (Bailey 1993)

• Ugandan lithosphere demonstrably thicker with potassic natrocarbonatite magmatism in association with ultrapotassic silicate magmas Genge, 2014

Ernst & Bell, 2009

IMPLICATIONS - PLUME BEHAVIOUR

• Carbonatites provide independent, non-geophysical, confirmation of EAR plume existence and the extent of its effects through geochemical understanding (Bailey 1993; Ernst & Bell, 2009)

• Geophysics less confident on extent of mantle metasomatism - carbonatite surface expression is useful here (Ernst & Bell, 2009)

• Combining the two allows for even greater precision, Gaillard et al. (2008) suggest that electrical conductivity of the mantle can indicate < 0.1% vol carbonatite melt existence

Johnson, 2006





Kelbert et al. 2009





IMPLICATIONS - PLUME HISTORY

• Do plumes exist in pulses?

• Carbonatites are uniquely sensitive to ‘thermal pulsation’ because they require so little thermal input to melt

• More precise dating may allow for the identification of pulses in the future (Ernst & Bell, 2009)

• Another constraint on mantle rheology?

Ernst & Bell, 2009

PROBLEMS REMAIN• How do we get extrusive calciocarbonatites?

• We know they exist!

• But CaCO3 → CaO + CO2 @ 1atm!

• Natrocarbonatites → Calciocarbonatites @ Kerimasi?

• What about elsewhere, are all melts secondary?

• ‘Usually found in association with alkaline silicate rocks - not the case in ~20% of occurrences (Woolley and Kjarsgaard, 2008)’

Genge, 2014













Genge, 2014

CONCLUSIONS• East African Rift carbonatites are derived from an immiscible melt that separated from

volatile rich mantle nephelinite and ijolite melts

• Local mantle is enriched - likely by both delamination and plume

• Independent confirmation of thickened lithosphere in southernmost rift area

• Carbonatite existence can give an indications as to the extent of the effects of a plume on the mantle and its metasomatism

• Research into conversion to Ca/Mg carbonatites from natrocarbonatite lavas is needed

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File:OldoinyoLengaiAir.jpgChakhmouradian, A. N/A. Carbonatites. http://www.umanitoba.ca/science/geological_sciences/

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