Archean subcontinental mantle is commonly enriched in Si with respect to other regions of the upper mantle and is also the virtually exclusive source of natural diamond. It has been proposed that both these phenomena are the result of metasomatism, yet the nature and origins of the metasomatic agents and the details of their reaction with the mantle lithosphere remain poorly understood. The proposed research will attempt to quantify the chemical and mineralogical reactions accompanying the deposition of Si and C into Archean mantle peridotite and constrain the compositions, origin and timing of the metasomatic fluids. The specific objectives of this work are to determine the spatial variations in chemical composition of veined harzburgites and to characterize the mineralogy, texture and chemical features of carbonaceous harzburgite xenoliths. These results will be used to calculate the stoichiometry of chemical reactions producing enstatite-rich veins in subcalcic garnet harzburgite, to assess the petrologic context of carbon deposition in refractory harzburgite, and to test the hypotheses that the two processes are genetically related and responsible for metasomatism on a craton-wide scale. The problem will be approached by the analysis of mantle xenolith samples from the Bultfontein kimberlite, Kimberley, South Africa. These include refractory harzburgites selected for their lack of evidence of Mesozoic overprinting and unusual veined examples interpreted as persuasive textural evidence for fluid reaction. Mineral major and trace element compositions will be determined by EMP and LA-ICP-MS analysis and bulk compositions calculated from chemical maps of large thin sections. Compositions of added components will be determined by digital spatial analysis of chemical maps of veined samples. Mineralogical evidence for carbonate will be examined using EMP, SEM, XRD and TEM. Abundance and distribution of diamond in subcalcic garnet harzburgite will be evaluated by X-ray tomography. A young age for diamond-forming metasomatism will be tested by examining garnet for trace element heterogeneity. Protolith ages will be constrained using Re-Os isotope analysis and source of metasomatizing fluids by mineral trace element, and Sr and Nd isotope compositions. Quantifying metasomatic effects allows a clearer window into the history of Archean mantle and an increased understanding of petrogenesis that is the key to continent stabilization in the early Earth. It is anticipated that the results of this study will have important implications for the Archean craton formation and evolution, mechanisms of volatile and trace element storage and transport in the mantle, and the evolution of the global carbon cycle.
