OLIVINE-PYROXENE-PTFE ALLOY AS AN OXYGEN GEOBAROMETER

An equilibrium assemblage of olivine, Ca-free pyroxene, and PtFe alloy can be used to calculate oxygen fugacity at a known temperature and pressure. This oxygen geobarometer has been tested T = 1300-1450-degrees-C, P = 1 bar and 10 kbar. The calculated log fO2 may be compared with that determined by the CO2-H-2 mixture used to control the furnace atmosphere in the 1 bar experiments and, in the case of the 10 kbar experiments, the 109 fO2 determined from the graphite-fluid equilibrium. The average deviation of the calculated log fO2 and that determined by other means is 0.2; the maximum absolute difference is 0.4. The distribution coefficient (K(d)) for Fe and Mg between coexisting olivine and pyroxene was found to range from 1.14 at x(Fe2SiO4)ol = 0.164 to 1.54 at x(Fe2SiO4)ol = 0.655, and varies systematically with the Fe/(Fe + Mg) of the phases.