THERMODYNAMIC DATA FOR FE-BEARING PHASES OBTAINED USING NOBLE METAL ALLOYS AS REDOX SENSORS

Abstract

The technique of using noble metal alloys as redox sensors has been adapted for use in hydrothermal experiments in the piston-cylinder apparatus at the reduced conditions of the iron-wustite (IW) buffer. The required high fH2 was maintained for several days using thick-walled (0.5 mm) Ag outer capsules. Such conditions permit the Gibbs free energy of Fe2+-bearing phases to be obtained from the composition of the co-existing alloy. The Ir-Fe system is more amenable than either Pt-Fe or Pd-Fe since only one phase (γ-fcc) is stable in this system over a large temperature range, and the solubility of Ag (capsule material) in Ir is negligible.Experiments with the assemblage fayalite-quartz-Fe (in alloy) (QFI) were used as a test of the method by comparison with the electrochemical measurements of O'Neill (1987a). The resulting agreement demonstrates the utility of the method between 700 and 1000°C at ~ 10 kbar. Further experiments with FeSiO3-bearing assemblages (ferrosilite-quartz-iron, PYQI, and ferrosilite-fayalite-iron, PYFAI) indicate the method is viable up to 20 kbar and between 700°C and 1000°C. However, small systematic deviations of about 1-2 kJ per mole of O2 in the quartz-saturated experiments are interpreted to indicate that the high solubility of SiO2 at high pressures may result in a lowering of the H2O activity in the co-existing supercritical fluid phase.