Applications of olivine-orthopyroxene-spinel oxygen geobarometers to the redox state of the upper mantle

Oxygen fugacity has been calculated for upper-mantle xenoliths from eastern Australia (western Victoria and Queensland), eastern China, and southern Africa using several calibrations of the olivine-orthopyroxene-spinel oxygen geobarometer. The majority of calculated f_O2 values lie within a range of 3 log units between the quartz-fayalite-magnetite (QFM) and magnetite-wüstite (MW) buffers. Both the western Victoria and eastern China xenoliths show a systematic increase in f_o2 with increasing temperature; a similar but less pronounced trend is seen in the African xenoliths. Use of the O’Neill & Wall calibration suggests that subcratonic mantle contains a significant proportion of peridotites more reduced than MW, whereas such rocks are rare in xenolith suites from basalts. Metasomatic processes are responsible for cryptic and modal variations in the chemical and isotopic composition of the sub-continental lithosphere. Discrimination of the xenoliths from eastern Australia and eastern China into hydrous (modally metasomatized) and anhydrous (cryptically and/or unmetasomatized) assemblages shows no correlation with f_o2Variation of calculated f_o2 with indices of metasomatism (e.g.⁸⁷Sr/⁸⁶Sr, e_Nd) suggests that cryptically and modally metasomatized xenoliths from western Victoria equilibrated under similar f₀₂ conditions, and that oxygen fugacity decreased with increasing degree of metasomatism. The calculation of f_o2, combined with trace element and isotopic data, may allow distinction of different sources of metasomatic fluids.