Fe-RICH DUNITE XENOLITHS FROM SOUTH AFRICAN KIMBERLITES: CUMULATES FROM KAROO FLOOD BASALTS

Abstract

Fe-rich dunite xenoliths within the Kimberley kimberlites comprise olivine neoblasts with minor elongated, parallel-oriented ilmenite, and rarely olivine porphyroclasts and spinel. Compared with typical mantle peridotites, olivines in the Fe-rich dunites have lower forsterite (Fo87-89) and NiO contents (1300-2800 ppm), which precludes a restitic origin for the dunites. Chrome-rich spinels are remnants of a metasomatic reaction that produced ilmenite and phlogopite. Trace element compositions differ between porphyroclastic and neoblastic olivine, the latter having higher Ti, V, Cr and Ni and lower Zn, Zr and Nb contents, documenting their different origins.The dunites have high 187Os/188Os ratios (0.11-0.15) that result in young model ages for most samples, whereas three samples show isotopic mixtures between Phanerozoic neoblasts and ancient porphyroclastic material. Most Fe-rich dunite xenoliths are interpreted to be recrystallized cumulates related to fractional crystallization of Jurassic Karoo flood basalt magmatism, whereas the porphyroclasts are interpreted to be remnants from a much earlier (probably Archaean Ventersdorp) magmatic episode. The calculated parental magma for the most primitive olivine neoblasts in the Fe-rich dunites is similar to low-Ti Karoo basalts. Modelling the crystal fractionation of the inferred parental magma with pMELTS yields element fractionation trends that mirror the element variation of primitive low-Ti Karoo basalts.