Oxygen fugacity constraints on the southern African lithosphere |
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| Authors: | Leon R M Daniels John J Gurney |
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| Institution: | (1) Department of Geochemistry, University of Cape Town, 7700 Rondebosch, South Africa |
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| Abstract: | Oxygen fugacities are calculated for olivine—spinel ±orthopyroxene assemblages recovered from diamonds and the concentrate
of the Dokolwayo kimberlite, Swaziland. In addition thermobarometric oxygen fugacities are obtained for chrome spinel-garnet
peridotites and diamonds from several other southern African kimberlites. The southern African lithosphere appears to be laterally
homogeneous with respect to oxygen fugacity. Vertically the oxygen fugacity of the lithospheric upper mantle decreases with
an increase in pressure. Locally, oxygen fugacities calculated for Dokolwayo mineral assemblages are indicative of an upper
mantle characterised by diverse redox conditions within the range FMQ-IW. Reduced oxygen fugacities, calculated for the majority
of the Dokolwayo samples, suggest that CH4 may be the dominant carbon volatile species in the lower lithosphere. These reduced conditions also suggest that the Dokolwayo
kimberlite is unlikely to be a product of redox melting, but may be the product of a thermal anomaly. Calculated equilibrium
temperatures for olivine-spinel pairs from Dokolwayo diamonds and concentrate indicate that the upper mantle in the vicinity
of Dokolwayo was characterised by cool subsolidus conditions. |
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