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 (sub 2) -CO (sub 2) gas mixtures at 1 atm and 1125-1240 degrees C using strips of Au (sub 74) Pd (sub 26) (wt%) and produced Au-Pd-Fe alloys with 0.03-13 wt% iron. A thermodynamic calibration for the mixing of Au (sub 74) Pd (sub 26) with iron using an asymmetric regular solution leads to W (sub G-Fe) =-45.0+ or -1.8 kJ/mol and W (sub G-AuPd) =+19.5+ or -7.7 kJ/mol (1sigma ). 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.