Evolution of South African Cratonic Peridotites Based on Micro-XRF Mapping and Reconstruction of Bulk Rock Compositions

The bulk composition of cratonic peridotites can reveal useful information about the igneous protolith. However, many peridotites are coarse-grained, making it challenging to obtain representative sampling on the thin section scale. Destructive bulk rock analysis would require sacrificing too much of the sample. Here, we examine samples from eight coarse-grained peridotite xenoliths from the Bultfontein kimberlite pipe in the Kaapvaal craton. The samples were mapped by micro-XRF over areas ranging from approximately 25-70 cm2; clinopyroxene, orthopyroxene, olivine, garnet, and spinel were identified based on major element compositions and comparison with hand samples. Mineral modes were calculated based on area percentages, and bulk rock compositions were reconstructed by combining these with elemental abundances in major minerals, measured by electron microprobe. Most of the samples in this study display a harzburgitic composition, with oxide weight percentages and Mg# which appear consistent with measurements in the literature for Archean cratonic peridotites from South Africa, particularly those formed in the garnet facies. Bulk FeO and MgO contents suggests that the peridotite protoliths examined here formed by melt extraction at temperatures ranging from 1600-1700°C with melt fractions up to 40%. This study will seek to further reconstruct the P-T history of the peridotites by examining subsolidus equilibration temperatures and pressures. The temperature and pressure at which the igneous protoliths formed is derived from the bulk rock composition. Thermobarometry on minerals, combined with consideration of diffusion kinetics, will be used to determine rates of sub-solidus P-T change.