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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)
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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.
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)
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
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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
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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
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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
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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
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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
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
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Kelbert et al. 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
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
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)’
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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
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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33214090/1998%20ExpeditionCawsey, A. 2011. Flying over Oldoinyo Lengai. http://commons.wikimedia.org/wiki/
File:OldoinyoLengaiAir.jpgChakhmouradian, A. N/A. Carbonatites. http://www.umanitoba.ca/science/geological_sciences/
faculty/arc/carbonatite.htmlChurch, A. A., & Jones, A. P. 1995. Silicate—Carbonate Immiscibility at Oldoinyo Lengai. Journal of
Petrology, 36(4), 869-889.Ernst, R. E., & Bell, K. 2010. Large igneous provinces (LIPs) and carbonatites. Mineralogy and
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electrical conductivity in the asthenosphere. Science, 322(5906), 1363-1365.Genge. 2013. Igneous 2: Continental Rift Magmatism. Imperial College London.Giordano, D., Russell, J. K., & Dingwell, D. B. 2008. Viscosity of magmatic liquids: a model. Earth
and Planetary Science Letters, 271(1), 123-134.Harmer, R. E., & Gittins, J. 1998. The case for primary, mantle-derived carbonatite magma. Journal
of Petrology, 39(11-12), 1895-1903.Hay, R. L. 1989. Holocene carbonatite-nephelinite tephra deposits of Oldoinyo Lengai, Tanzania.
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www2.ess.ucla.edu/~ejohnson/ess103a/10_2_ContAlkalineforweb.pdfJones, A. P., Genge, M., & Carmody, L. 2013. Carbonate melts and carbonatites. Reviews in
Mineralogy and Geochemistry, 75, 289-322.Kelbert, A., Schultz, A., & Egbert, G. (2009). Global electromagnetic induction constraints on
transition-zone water content variations. Nature, 460(7258), 1003-1006.Le Maitre, R. W. 2002. Igneous Rocks: A Classification and Glossary of Terms: A Classification and
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Nelson, S. A. 2011. General Classification of Igneous Rocks. http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm
Palmer, M. R., & Edmond, J. M. 1989. The strontium isotope budget of the modern ocean. Earth and Planetary Science Letters, 92(1), 11-26.
Photo Volcanica. N/A. http://www.photovolcanica.com/VolcanoInfo/Oldoinyo%20Lengai/Oldoinyo%20Lengai.html
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St. John, J. N/A. Carbonatites. http://www.newark.osu.edu/facultystaff/personal/jstjohn/Documents/Cool-Rocks/Carbonatites.htm
Strekeisen, A. Volcanic Rocks. http://www.alexstrekeisen.it/english/vulc/index.phpTeague, A. J., Seward, T. M., & Harrison, D. 2008. Mantle source for Oldoinyo Lengai carbonatites:
Evidence from helium isotopes in fumarole gases. Journal of Volcanology and Geothermal Research, 175(3), 386-390.
Wolff, J. A. 1994. Physical properties of carbonatite magmas inferred from molten salt data, and application to extraction patterns from carbonatite–silicate magma chambers. Geological Magazine, 131(02), 145-153.
Woolley, A.R. and Kempe, D.R.C. 1989. Carbonatites: nomenclature, average chemical compositions, and element distribution. Carbonatites: Genesis and Evolution (K. Bell, Ed.). Unwin Hyman, London, 1-14.
Woolley, A. R. (Ed.). 2001. Alkaline Rocks and Carbonatites of the World: Africa. Geological Society of London.
Woolley, A. R., & Church, A. A. 2005. Extrusive carbonatites: a brief review. Lithos, 85(1), 1-14.Woolley, A. R., & Kjarsgaard, B. A. 2008. Paragenetic types of carbonatite as indicated by the
diversity and relative abundances of associated silicate rocks: evidence from a global database. The Canadian Mineralogist, 46(4), 741-752.
Zaitsev, A. N., Keller, J., Spratt, J., Jeffries, T. E., & Sharygin, V. V. 2009. Chemical composition of nyerereite and gregoryite from natrocarbonatites of Oldoinyo Lengai volcano, Tanzania. Geology of Ore Deposits, 51(7), 608-616.