Journal of Petrology | Volume 39 | Number 11-12 | Pages 1895-1903 | 1998
© Oxford University Press 1998
The Case for Primary, Mantle-derived Carbonatite Magma
1 Council for Geoscience Private Bag X112, Pretoria 0001, South Africa
2 Geology Department, University of Toronto Toronto, Ont., M5S 3B1, Canada
Received September 30, 1997; Revised typescript accepted June 16, 1998
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Abstract |
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There is much debate about whether carbonatite magmas are derived in ‘secondary’ fashion through the advent of liquid immiscibility operating in the crust on evolved nephelinitic magma, or whether they are derived in the mantle by direct partial melting of a carbonated peridotite. This paper briefly summarizes the 
Sr–Nd data fo carbonatites in general and evaluates the isotopic relationships between carbonatites and alkaline silicate rocks in several well-studied complexes from Africa. Available data for carbonatites younger than 200 Ma have a range in Sr–Nd that is less than that found in oceanic basalts despite the fact that carbonatites traverse lithospheres that ar much more complex than those in the oceans. By contrast, for the Napak, Kerimasi, Shombole, Dorowa, Shawa and Spitskop complexes the alkaline silicate rocks show greater variability and have more enriched Sr–Nd (higher Sr, lower Nd) values than their associated carbonatites. In general, the carbonatites hav isotopic compositions that are closer to the more primitive silicate rocks, such as melilitites a olivine nephelinites, than to more evolved nephelinites and phonolites. In the case of the Napak Complex the enriched component was introduced from the lower crust whereas for the Dorowa and Shawa complexes of SE Zimbabwe, the component was derived from the sub-continental lithospheric mantle. These relationships indicate that the carbonatites must have existed as discrete magmas i themantle and argue against a derivation by liquid immiscibility in the crust. Although a contrast i isotopic composition does not rule out an immiscibility relationship at mantle depths and early in the evolutionary history of a melilititic or nephelinitic magma, there is little experimental support for it. Existing experimental data indicate that immiscibility between carbonate an silicate liquids is favoured at low, crustal, pressures but that immiscibility is unlikely to occur in realistic mantle melts or their derivatives at mantle pressures. Many experimental data exist to show that magnesian carbonatite liquids form as the near-solidus melts of carbonated mantl peridotite at depths in excess of 75 km. We conclude that the calcitic and dolomitic carbonatite magma discussed in this paper are best considered as being derived from primary carbonatite magmas generated in the mantle by partial melting of carbonated peridotite.
KEY WORDS: carbonatitec; liquid immiscibility; isotopes; petrogenesis
* Corresponding author. Telephone: +27-12-8411378. Fax: +27-12-8411278. e-mail: jock{at}geoscience.org.za
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