Multi-stage kimberlite evolution tracked in zoned olivine from the Benfontein sill, South Africa

Abstract

Olivine is the dominant mineral present in kimberlite magmas; however, due to the volatile-rich nature of most kimberlites, they rarely survive late-stage serpentinisation. Here we present major and trace element data for a rare example of ultra-fresh olivine in a macrocrystic calcite kimberlite from the Benfontein kimberlite sill complex. Olivines are characterised by xenocrystic cores surrounded by multiple growth zones representing melt crystallisation and late-stage equilibration. Two distinct core populations are distinguished: Type 1) low Fo (88–89), Ni-rich, Ca- and Na-rich cores, interpreted here to be the result of carbonate–silicate metasomatism potentially as part of the earliest stages of kimberlite magmatism, and Type 2) high Fo (91–93), Ni-rich, low-Ca cores derived from a typical garnet peridotite mantle source. In both cases, the cores have transitional margins (Fo89–90) representing equilibration with a proto-kimberlite melt. Trace element concentrations, in particular Cr, of these transition zones suggest formation of the proto-kimberlite melt through assimilation of orthopyroxene from the surrounding garnet peridotite lithology. Trace element trends in the surrounding melt-zone olivine (Fo87–90) suggest evolution of the kimberlite through progressive olivine crystallisation. The final stages of olivine growth are represented by Fe-rich (Fo85) and P-rich olivine indicating kimberlite evolution to mafic compositions. Fine (<60μm), Mg-rich olivine rims (Fo94–98) represent equilibration with the final stages of kimberlite evolution back to Fe-poor carbonatitic melts.We present a step-by-step model for kimberlite magma genesis and evolution from mantle to crust tracked by the chemistry of olivines in the Benfontein kimberlite. These steps include early stages of metasomatism and mantle assimilation followed by direct crystallisation of the kimberlite melt and late-stage equilibration with the evolved carbonatitic residual liquids. The Ca contents of the Type 1 xenocrystic olivines are the highest yet measured for mantle olivines, and do not overlap with any known mantle xenolith lithologies. These olivines likely represent an important stage of metasomatism directly related to the early stages of kimberlite melt ponding at the base of the lithospheric mantle.