A filler-rod technique for controlling redox conditions in cold-seal pressure vessels

摘要

A new method has been developed to impose different redox conditions in high-temperature-pressure experiments in cold-seal pressure vessels, at 800 °C and 2000 bars. Experiments were conducted by loading a metallic filler rod into the autoclave together with H₂ sensor capsules, and pressuring the autoclave with H₂O. Rod materials tested successfully were Co, Ti, and C (graphite). The oxidation of these rods produces H₂, but because of diffusive H₂ loss through the walls of the autoclave, the system may not be buffered with respect to H₂. However, f_H2 quickly reaches a steady state value, and because f_H2 is easily measured by the hydrogen sensor method, the effect of the filler rods on the intrinsic f_O2 of the autoclave can be quantified. In order to produce oxidized conditions, Ar was used as the pressure medium and metal oxides, contained in Al₂O₃ tubes, were employed. By using either Ar or H₂O as a pressure medium, a log f_O2 range of NNO –3.9 to NNO +4.6 can be imposed by this method, where NNO is the log f_O2 value of the Ni-NiO buffer. The ability to conduct long-run-duration experiments at high temperature and high f_H2 conditions is not possible with the traditional double-capsule technique because the buffer assemblage is consumed too quickly. However, run durations of up to 4 weeks with constant f_H2 at reduced conditions have been conducted using the filler-rod technique. This technique has been shown to be an effective method in controlling redox conditions in cold-seal autoclaves, and thus can be applied to investigating redox-dependent reactions in a wide range of geochemical systems.