Oxygen fugacity calibration for (Fe)Au75Pd25 alloys in rock-melting experiments

Iron loss from silicate melt to gold-palladium capsules is strongly dependent on oxygen fugacity and, once calibrated, may be used as a tool to determine the fO2 of rock-melting experiments, provided that Fe partitioning between the melt and capsule is independent of melt composition. We performed nine rock-melting experiments in a gas-mixing furnace at 1 atm on three compositions of mafic rock: a MORB (9.5 wt% FeOT) and two subduction-related Mexican basaltic andesites (7.5 wt% and 7.0 wt% FeOT). The samples were melted for 48 hours in Au75Pd25 capsules at 1300°C and three different oxygen fugacities spanning -1 to +2 log units relative to the nickel-nickel oxide buffer. Electron microprobe analyses for Fe in both the melt and capsule material yield Fe-partition coefficients that vary linearly with log fO2 and are essentially constant and independent of melt composition at any given fO2. This relationship permits making post-run estimates of the prevailing fO2 in rock-melting experiments at 1300°C from the partitioning of Fe between the melt and capsule for bulk compositions within our calibrated limits. Work is now underway to extend this calibration to lower temperatures. Our plan is to use this calibration to estimate the fO2 in H2O-undersaturated piston-cylinder experiments in which fO2 cannot be buffered by conventional double capsule methods.