HIGH PRESSURE EXPERIMENTAL CALIBRATION OF THE OLIVINE-ORTHOPYROXENE-SPINEL OXYGEN GEOBAROMETER: IMPLICATIONS FOR THE OXIDATION STATE OF THE UPPER MANTLE

Abstract

Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under controlled oxygen fugacity (f O2). f O2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO2-C, Ni−NiO, and Fe3O4−Fe2O3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe3+/ΣFe in spinel is a linear function of f O2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog(f O2), but decreases slightly with increasing Cr content in spinel. The Fe3+/ΣFe ratio falls with increasing pressure at given Δlog(f O2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing f O2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from the fayalite-magnetite-quartz (FMQ) buffer: $$\Delta log (f_{O_2 } )^{FMQ} = 0.27 + 2505/T - 400P/T - 6 log(X_{Fe}^{olv} ) - 3200(1 - X_{Fe}^{olv} )^2 /T + 2 log(X_{Fe^{2 + } }^{sp} ) + 4 log(X_{Fe^{3 + } }^{sp} ) + 2630(X_{Al}^{sp} )^2 /T.$$ The oxygen barometer is applicable to the entire spectrum of spinel compositions occurring in mantle rocks and mantle-derived melts, and gives reasonable results to temperatures as low as 800° C.