It has recently been proposed that the Fe3O4 content of spinel coexisting with olivine and orthopyroxene may be used as an oxygen barometer for spinel peridotites (O'Neill and Wall, 1987; Mattioli and Wood, 1988). This barometer relies on the equilibrium: 6 Fe2SiO4 + O2 = 3 Fe2SiO6 + 2 Fe3O4. The two calibrations generally give results within 0.7 log fO2 units of each other for mantle compositions. These calibrations have been tested, together with a recent spinel model of Nell and Wood (1990) by reequilibrating mixtures of olivine orthopyroxene and spinel of varying composition at known T and fO2 at 1 atm pressure. The oxygen barometer was then tested by comparing oxygen fugacities calculated from the compositions of experimental products with the actual fO2 of the experiment. The Mattioli-Wood version of the barometer gives fO2 values which average close (-0.15 log units) to the true values but which show a dependence on (Cr/Cr+Al) in the spinel. The O'Neill-Wall version gives average values which are 0.65 log units low but which show no compositional bias. -from Author
