Oxygen Fugacities of Super Mantle Rocks from Thermobarometry

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This project is to determine the extent of heterogeniety in upper-mantle oxygen fugacity using a newly calibrated thermobarometric method. This will involve measuring the compositions of coexisting olivine, orthopyroxene and spinel from spinel lherzolites by microprobe and Mossbauer methods. Spinel lherzolite xenoliths have been, and are being, collected from a number of different geologic areas and tectonic environments in both continental and oceanic plates - British Columbia, Japan, Massif Central of France, Southwestern Us., Eastern Australia and Hawaii. Abyssal peridotites dredged from fracture zones in the Atlantic and Indian Oceans will provide further coverage of oceanic lithosphere and facilitate distinction between subcontinental and suboceanic mantle with respect to oxygen fugacity. Analysis of samples from areas of recent subduction (Japan, British Columbia) will provide tests of the hypothesis that the mantle is becoming oxidized through the agency of subduction-derived fluids. Preliminary Mossbauer measurements of garnet and garnet-spinel lherzolites indicate that, at pressures above the spinel stability field, garnet contains substantial amounts of ferric iron. The thermobarometric method will be extended to the garnet field by determining Fe3-Al partitioning between coexisting spinel and garnet.