Application of a solid-electrolyte oxygen fugacity sensor to high-pressure experiments

Abstract

To monitor the relative oxidation state, a calcia-stabilized zirconia solid-electrolyte oxygen fugacity sensor was used in high-pressure experiments in a multi-anvil type apparatus. The sensor was tested at high pressures using several combinations of common oxygen buffers, and was shown to work up to at least 7 GPa and 1350°C over a wide range of oxygen fugacities between the iron-wüstite (IW) and the magnetite-hematite (MH) oxygen buffers. Rapid responses of the e.m.f. of the sensor to the changes in P- T conditions allow us to carry out in situ oxygen fugacity measurement during high-pressure experiments. The sensor was also used to measure the relative oxygen fugacity prevailing within graphite sample capsules at conditions from 2 to 7 GPa and from 1000 to 1350°C. The redox condition within the graphite capsule is as follows: (1) with increasing temperature, relative to the wüstite-magnetite (WM) oxygen buffer, graphite becomes more reducing at constant pressure, and this causes the stable iron oxide phase in the capsule to change from magnetite to wüstite; (2) this temperature dependence is, however, affected by pressure—below 3 GPa, graphite becomes more oxidizing relative to WM with increasing pressure at constant temperature, whereas above 3 GPa, it becomes less oxidizing relative to WM with increasing pressure, and this results in an enlargement of the field of graphite-wüstite coexistence. Some other applications of the sensor to high-pressure research are briefly discussed, including the in situ detection of phase transformation and pressure calibration at elevated temperature.