AuPdFe ternary solution model and applications to understanding the fO2 of hydrous, high-pressure experiments

This study provides an experimental calibration of the equilibrium constant for AuPdFe alloys with Fe-bearing silicate melts. The ideal metal capsules for H₂O-bearing experiments are pure Au, because of its slow hydrogen diffusivity. However, above the melting point of Au, other materials must be used. The solution to this problem is to use AuPd alloy capsules. However, under most relevant fO₂ conditions, this alloy absorbs Fe from the coexisting silicate melt, thus changing the bulk composition of the experimental charge. This study combines previous work on the Au–Pd, Pd–Fe, and Au–Fe binary systems to develop a ternary thermodynamic solution model for AuPdFe. This solution model is used with experiments to calculate an equilibrium reaction coefficient for the FeO_melt → Fe_alloy + 1/2O₂ exchange reaction. Using a non-ideal ternary solution model, the fO₂ conditions of hydrous, piston cylinder experiments can be estimated by analyzing the sample capsule alloy and the coexisting liquid composition.