Geochemistry of magnesian ilmenite megacrysts from Southern African kimberlites

Ilmenite megacryst suites from 20 kimberlites in southern Africa have been analyzed for trace elements (Ni, Zn, Cu, Ga, Nb ,Ta , Zr, Hf) using the proton microprobe; major and minor elements have been analyzed by electron probe. These data, and those from other suites, suggest that ilmenite macrocrysts in kimberlites represent a minor phase, crystallizing late in the fractionation history of mafic magmas at depth. Nb behaves as an incompatible element in the magma throughout the crystallization history, and the Nb content of ilmenite can serve as an index of the degree of fractional crystallization. For many ilmenite suites, plots of other trace- and major elements against Nb content define smooth curves, with breaks in slope corresponding to changes in the assemblage of minerals crystallizing together with ilmenite. Relationships between major- and trace elements, previously regarded as evidence against a fractional-crystallization origin for the ilmenites, are shown to be consistent with such an origin. Ilmenite suites from different kimberlites show broadly similar crystallization histories, but differ in detail. Two groups of kimberlites may be defined, on the basis of the maximum Zr content of ilmenite reached during fractionation. One group, defined by essentially constant Zr (500-600 ppm) with increasing Nb, may be derived from metasomatized peridotites. All other suites show a positive correlation between Nb and Zr up to > 1000 ppm Zr, where zircon apparently began to crystallize. The ilmenites in many kimberlites, especially those of the latter group, appear to have crystallized from a single batch of magma. These magmas were broadly similar, and picritic in composition; small differences in initial composition have been magnified by extreme fractional crystallization, to produce the distinctive characters of ilmenite suites from individual kimberlites. These observations suggest a genetic relation between the megacryst magma and the host kimberlite in each pipe, but the nature of this relation is not clear.