Petrology, mineralogy and geochemistry of oxide minerals in polymict xenoliths from the Bultfontein kimberlites, South Africa: implication for low bulk-rock oxygen isotopic ratios

Abstract.

Polymict mantle xenoliths from the Bultfontein kimberlites, South Africa, contain abundant ilmenites (30% in BD2666, 15% in JJG1414, 3% each in BD2394 and BD344). These ilmenites occur as disrupted veins or layers, coarse discrete grains, small segregations interstitial to other silicate minerals, and tiny irregular grains disseminated in the subgrains of enstatites. The vein-like ilmenite usually shows a textural zonation across the vein, in rare cases along veins. This textural zonation is coincident with chemical and oxygen isotopic variations, with the margins being finer in grain sizes and richer in incompatible elements. The chemical and isotopic compositions also vary between different occurrences of ilmenite grains. In general, the smaller grains are richer in Cr, LREE and LILE and lighter in oxygen isotopes. Thus, chemical and oxygen isotopic disequilibria are well preserved in these ilmenites, which are also seen in the silicate minerals. These features suggest that ilmenites from the polymict xenoliths formed by magmatic and/or metasomatic processes. The invasion of the Fe-Ti-Cr-rich melt with low oxygen isotopic ratio can account for the observed low bulk oxygen isotopic ratios in the polymict xenoliths. This Fe-Ti-rich melt with high ilmenite normative could be produced by melt immiscibility during the migration of an initially homogeneous high-Ti silicate melt.