Control and monitoring of oxygen fugacity in piston cylinder experiments

We present a newly developed capsule design that resolves some common problems associated with the monitoring and control of oxygen fugacity (fO₂) in high-pressure piston cylinder experiments. The new fO₂ control assembly consists of an AuPd outer capsule enclosing two inner capsules: one of AuPd capsule containing the experimental charge (including some water), and the other of Pt containing a solid oxygen buffer plus water. The inner capsules are separated by crushable alumina. The outer capsule is surrounded by a Pyrex sleeve to simultaneously minimise hydrogen loss from the cell and carbon infiltration from the graphite furnace. Controlled fO₂ experiments using this cell design were carried out at 1.0 GPa and 1,000 °C. We used NiPd, CoPd and (Ni, Mg)O fO₂ sensors, whose pressure sensitivity is well calibrated, to monitor the redox states achieved in experiments buffered by Re–ReO₂, Ni–NiO and Co–CoO, respectively. Results for the fO₂ sensors are in good agreement with the intended fO₂ established by the buffer, demonstrating excellent control for durations of 24–48 h, with uncertainties less than ± 0.3 log bar units of fO₂.