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

The technique of using noble metal alloys as redox sensors has been ad apted for use in hydrothermal experiments in the piston-cylinder appar atus at the reduced conditions of the iron-wustite (IW) buffer. The re quired high f(H2) Was maintained for several days using thick-walled ( 0.5 mm) Ag outer capsules. Such conditions permit the Gibbs free energ y 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 o r Pd-Fe since only one phase (gamma-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 w ith the electrochemical measurements of O'Neill (1987a). The resulting agreement demonstrates the utility of the method between 700 and 1000 degrees C at approximate to 10 kbar. Further experiments with FeSiO3- bearing assemblages (ferrosilite-quartz iron, PYQI, and ferrosilite-fa yalite-iron, PYFAI) indicate the method is viable up to 20 kbar and be tween 700 degrees C and 1000 degrees C. However, small systematic devi ations of about 1-2 kJ per mole of O-2 in the quartz-saturated experim ents are interpreted to indicate that the high solubility of SiO2 at h igh pressures may result in a lowering of the H2O activity in the co-e xisting supercritical fluid phase. Copyright (C) 1997 Elsevier Science Ltd.