Thermodynamic properties of alloys of gold-74/palladium-26 with variable amounts of iron and the use of Au-Pd-Fe alloys as containers for experimental petrology

Iron oxide-alloy equilibration experiments were conducted in H(2)-CO(2) gas mixtures at 1 atm and 1125-1240 degrees C using strips of Au(74)Pd(26) (wt%) and produced Au-Pd-Fe alloys with 0.03-13 wt% iron. A thermodynamic calibration for the mixing of Au(74)Pd(26) with iron using an asymmetric regular solution leads to W(G-Fe) = -45.0 +/- 1.8 kJ/mol and W(G-AuPd) = +19.5 +/- 7.7 kJ/mol (1 sigma). Internal oxidation of iron was observed in a reversal experiment, suggesting that oxygen can be transferred across capsule boundaries during high-temperature experiments. This thermodynamic calibration is applicable to a wide range of oxygen fugacities and iron activities relevant to petrological and metallurgical applications at 1 atm and, as previous studies suggest excess volumes in this system are small, it can also be used to predict Fe activities in experiments at elevated pressure (up to 3 GPa). By pre-doping Au-Pd capsules to match Fe activities expected for the sample during an experiment, it is possible to maintain samples with little to no loss of iron. Pre-saturation of the capsule also provides a method for controlling the oxygen fugacity of samples if no formal oxygen buffer is available.