Origin and Age of Modal Heterogeneities in Kaapvaal Cratonic Harzburgites

Evidence for the nature and timing of processes responsible for the formation and consolidation of the ancient mantle roots to continents is commonly obscured by more recent mantle events. In this study we apply a number of analytical approaches to a suite of highly depleted garnet harzburgite xenoliths from the Bultfontein kimberlite, South Africa that were selected for their apparent lack of late-stage metasomatic features. Distinct textural varieties include harzburgites with large (up to 5 cm) garnet clots, enstatite poor garnet-chromite dunites, and samples with veins or irregular blobs rich in enstatite. Coarse, primary mica is commonly present. Mineral compositions are correlated with garnet texture and are highly refractory, including several samples with subcalcic garnets. Sample BFT137 is a 30 cm-sized garnet harzburgite bounded by a 2 cm wide selvage of orthopyroxenite, containing abundant phlogopite and sulfide. Mineral compositions are the same in vein and host and are highly refractory (Fo94 olivine, subcalcic garnet). Sulfide is Ni- and PGE-rich, but many grains are affected by secondary serpentinization, recording kimberlite-like Pb-isotope compositions. Garnet trace element analyses by LA-ICP-MS indicate very low Ti and Zr contents, and bell-shaped to sinusoidal REE patterns rich in MREE. Phlogopite is similarly HFSE-poor but has high Ba and Cl contents, contrasting with a typical kimberlite- metasomatism signature. Petrographic and chemical evidence suggests that orthopyroxene was introduced into the highly depleted host rock by a mobile phase rich in Si, K, H, Cl and S, but extremely poor in Ca, Fe, HREE and HFSE. Garnet-dunites and other subcalcic garnet-bearing harzburgites without Si-enrichment show similar trace element compositions and abundance of primary phlogopite, suggesting pervasive infiltration of these rocks by a hydrous fluid poor in typical basaltic components. The large scale homogeneity of the BFT137 host-vein system indicates substantial equilibration, which at the relatively low temperatures(~1000 C) indicated by geothermometry suggest long mantle residence times, currently under investigation by isotopic studies.