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
