Thermal reduction of molybdite and hematite in water and hydrogen peroxide-bearing solutions: Insights on redox conditions in Hydrothermal Diamond Anvil Cell (HDAC) experiments

Abstract

The oxygen fugacity in Hydrothermal Diamond Anvil Cell (HDAC) experiments was determined by means of direct observation of the thermal reduction of molybdite or hematite in water and water–H2O2 solutions. HDAC experiments were conducted with and without rhenium gasket in order to evaluate the effect of rhenium on oxygen fugacity of the contained sample. In experiments where pure water and no Re gasket were used, i.e., when the sample charge is loaded into a laser-drilled recess in the lower anvil, the Log(fO2) was shown to be −19.55±0.13 over a wide range of temperature and pressure conditions (335–500°C, 15.0–136.3MPa — pressure determined using EOS of water by Wagner and Pruss, 2002), i.e., HM+8 to HM−0.5, at the limit temperatures. In contrast, in experiments where a Re gasket was used to contain the sample, the Log(fO2) is slightly more reducing (−20.60±0.04), e.g., equals to HM+6 at 350°C, but is significantly higher than that predicted at a given temperature using the Re–ReO2 buffer. Experiments with hydrogen peroxide showed a non-linear increase of oxygen fugacity with increasing molarity of H2O2 from 0.10 to 0.25M. Results indicate that the oxygen fugacity in water dominated HDAC runs is imposed by the fluid medium, and that hydrogen peroxide may be used to increase oxygen fugacity of water-dominated fluid media in HDAC experiments.