The origin and evolution of the Kaapvaal cratonic lithospheric mantle

A detailed petrological and geochemical study of low-temperature peridotite xenoliths from Kimberley and northern Lesotho is presented to constrain the processes that led to the magmaphile element depletion of the Kaapvaal cratonic lithospheric mantle and its subsequent re-enrichment in Si and incompatible trace elements. Whole-rocks and minerals have been characterized for Re-Os isotope compositions, and major and trace element concentrations, and garnet and clinopyroxene for Lu-Hf and Sm-Nd isotope compositions. Most samples are characterized by Archaean Os model ages, low Al, Fe and Ca contents, high Mg/Fe, low Re/Os, very low (< 0.1 x chondrite) heavy rare earth element (HREE) concentrations and a decoupling between Nd and Hf isotope ratios. These features are most consistent with initial melting at similar to 3.2 Ga followed by metasomatism by hydrous fluids, which may have also caused additional melting to produce a harzburgitic residue. The low HREE abundances of the peridotites require that extensive melting occurred in the spinel stability field, possibly preceded by some melting in the presence of garnet. Fractional melting models suggest that similar to 30% melting in the spinel field or similar to 20% melting in the garnet field followed by similar to 20% spinel-facies melting are required to explain the most melt-depleted samples. Garnet Nd-Hf isotope characteristics indicate metasomatic trace element enrichment during the Archaean. We therefore suggest a model including shallow ridge melting, followed by metasomatism of the Kaapvaal upper mantle in subduction zones surrounding cratonic nuclei, probably during amalgamation of smaller pre-existing terranes in the Late Archaean (similar to 2.9 Ga). The fluid-metasomatized residua have subsequently undergone localized silicate melt infiltration that led to clinopyroxene +/- garnet enrichment. Calculated equilibrium liquids for clinopyroxene and their Hf-Nd isotope compositions suggest that most diopside in the xenoliths crystallized from an infiltrating kimberlite-like melt, either during Group II kimberlite magmatism at similar to 200-110 Ma (Kimberley), or shortly prior to eruption of the host kimberlite around 90 Ma (northern Lesotho).