Upper Mantle Oxygen Fugacity and Its Relationship to Metasomatism

Abstract

We have calculated fO_2's and temperatures of various mantle environments worldwide using published analyses of coexisting olivine, orthopyroxene, clinopyroxene, and Fe^(3+)-bearing spinel from 280 peridotites. Most calculated fO_2's fall within ± 2 log units of the Fayalite-Magnetite-Quartz (FMQ) buffer at 15 kbar. Our data set defines a general trend in fO_2-T space that is not related to FMQ or to other Fe-bearing buffers. Variations in major-element, trace-element, and oxygen isotopic composition of xenoliths correlate with variations in calculated fO_2. Rare "fertile" xenoliths record fO_2's close to WM (Wüstite-Magnetite) buffer at 15 kbar and 900°C. Xenoliths with both cryptic and/or modal metasomatic overprinting are generally oxidized relative to xenoliths without evidence of such open system processing. Based on trace element and oxygen isotopic data, the best candidate for the metasomatic agent is a CO_2-H_2O-rich fluid. We suggest that metasomatic fluids are derived from oxidized, hydrated material subducted at convergent margins and that this process may have led to progressive oxidation of the earth's upper mantle through much of geologic time. This is consistent with the observation that xenoliths from Hawaii and Tahiti record fO_2's higher than mantle array's average, as do some xenoliths from the circumpacific region.