Mantle Oxidation States and the Solubility of Oxygen in Diamond

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The nature of mantle fluids and relative stabilities of carbonaceous phases (diamond, graphite, carbonate, fluid) are partly determined by oxidation state. Diamonds are of particular interest in understanding how oxygen fugacity varies in the upper mantle because while most appear to have been transported from relatively oxidized parts of the Earth, some contain extremely reduced mineral inclusions (e.g., iron metal, moissanite). These inclusions and the diamonds that contain them must record a different oxidation state than and be petrogenetically unrelated to the kimberlites that brought them to the surface. Published data and a pilot experiment indicate that measurable quantities of oxygen dissolve in diamond. Therefore, diamonds equilibrated under a range of oxygen fugacities may contain measurably different amounts of oxygen. The pints of this investigation are to: (a) determine the solubility of oxygen in diamond; (b) determine if there are systematic variations in diamond oxygen contents that can be used as qualitative indicators of oxygen fugacity; and (c) search for relationships between diamond oxygen contents and other characteristics, such as inclusion assemblages, nitrogen contents, isotopic compositions and localities. Among the questions to be addressed are, do diamonds with reduced inclusions contain less oxygen than those without such inclusions; what is the significance of the occurrences of moissanite, a mineral which should be stable only under much more reduced conditions than thought to exist in the mantle; might variations in oxygen fugacity in part control the variability in nitrogen contents and carbon isotopic compositions of diamonds. Oxygen will be analyzed at Los Alamos National Laboratory.