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

James Sean Dickson

What are carbonatites?Carbonatites are a member of the alkaline igneous suite of rocks (where Na2O + K2O are high relative to SiO2), and are comprised of > 50 modal per cent primary carbonate minerals (like calcite, dolomite, and ankerite). They also contain less than 20 modal percent SiO2. (Le Maitre, 2002)

Only one, natrocarbonatite-erupting volcano exists today. Carbonatite melts have very low viscosities and densities compared to ‘normal’, silicate igneous melts. They are typically highly enriched in Rare Earth Elements; REE are metals of geopolitical and technological importance.

Calciocarbonatite CaO/(CaO+FeO+MgO > 0.80

Ferrocarbonatite (FeOT + MnO) > MgO

Dolomite carbonatite (Ca,Mg)-rich

Magnesiocarbonatite MgO > (FeO + MnO)

Rare Earth Carbonatite RE2O3 > 1% wt

Natrocarbonatite Na2O + K2O) > (CaO+MgO+FeO)

Map of African CarbonatitesJohnson, 2006

In Africa carbonatites are heavily associated with the East African Rift area. It is probable that they are spatially and temporally associated with the edge of the thermal effects of the mantle plume plume. They also cluster around cratonic areas - where the crust is very old, and very thick.

In ~80% of cases, carbonatites are found in association with rocks like nephelenite and ijolite, both silica poor alkalic silicate rocks.

Where are they found?

206Pb/204Pb Initial

Jones et al. 2013Carbonatites are sub-continental lithospheric mantle derived, as is proven by their mantle strontium, neodymium, lead and helium isotope ratios.

Carbonatites probably have multiple genesis methods, but the melts associated with the East African Rift are all almost certainly secondary melts. Carbonate melts are immiscible with silicate melts, and so if the carbon and volatile concentration of a silica poor melt is high enough, the carbonate melt will separate. The melts in question are silica poor nephelenite and ijolite in composition - and they are r ich in feldspathoids. Feldspathoids are silica undersaturated feldspar ‘equivalents’. The melts are created in the asthenospheric mantle through very small degrees of partial melting, beneath the lithosphere.

How are they formed?Johnson, 2006

Carbonatite occurrence can be used to spatially and temporally map the extent of the thermal effects of the EAR mantle plume. Critically, this method is fully independent of the most common geophysical methods of understanding the plume.

Their isotope signatures indicate a mantle source that ranges from EMI to HIMU. This means that the mantle beneath the EAR is enriched; the HIMU component being added through the plume bringing up ancient oceanic lithosphere from the core-mantle boundary, and the EMI component existing due to lithospheric delamination.

Finally, they confirm that south of the Afar triangle, the local African lithosphere is very thick - 100+ km.

What can they tell us?

Church and Jones, 1995

Jones et al. 2013

Right to left: an immiscible droplet of silicate melt containing a crystal of nepheline, surrounded by carbonatite rock; two charts showing the mantle isotope ratios of North East African carbonatites.

Genge. 2013. Igneous 2: Continental Rift Magmatism. Imperial College London.Ernst, R. E., & Bell, K. 2010. Large igneous provinces (LIPs) and carbonatites. Mineralogy and Petrology, 98(1-4), 55-76.Johnson, E. 2006. Continental Alkaline Magmatism: The East African Rift. http://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.Photo Volcanica. N/A. http://www.photovolcanica.com/VolcanoInfo/Oldoinyo%20Lengai/Oldoinyo%20Lengai.html

References

Genge, 2014. Partial melting of crystals & the pathway for

these melts - beginning in the asthenospheric mantle.

Natrocarbonatite eruption at Ol

Doinyo Lengai. The white discolouration is due to hydration.

Photo Volcanica

Ernst & Bell, 2009. Diagram of plume effects on the mantle.