A new method to quantitatively control oxygen fugacity in externally heated pressure vessel experiments

Abstract

Abstract. Oxygen fugacity (fO2) is a fundamental variable affecting phase equilibrium in magmas, and in externally heated pressure vessel experiments it is typically controlled by using redox buffer assemblages. However, these do not allow fine enough resolution; for example, most arc magmas fall between the fO2 imposed by the neighboring Ni–NiO and Re–ReO2 buffers and so does the transition of S2− to S6+ in magmas. Here we propose a new method to quantitatively impose fO2 in hydrous high-P–T experiments in molybdenum hafnium carbide (MHC) pressure vessels by admixing small amounts of hydrogen into the Ar pressure medium. The thermodynamic calculation procedure used to determine the initial amount of hydrogen to be loaded to constrain desired fO2 values was verified by CoPd alloy redox sensor experiments to be accurate within ±0.3 log units for the pressure (P) – temperature (T) range of 940–2060 bar and 800–1100 ∘C. As hydrogen can be slowly lost from the pressure medium due to diffusion through the vessel walls at high T, we also determined the hydrogen permeability of the MHC alloy as a function of T. The such-obtained hydrogen permeability equation for the MHC alloy can be used to determine the rate of fO2 increase for any MHC pressure vessel configuration. As the rate of fO2 increase is slow (e.g., 0.36 log units per day in our setup at T= 1000 ∘C), we propose that H2 addition to the Ar pressure medium is an effective way to accurately impose fO2 in many types of experiments conducted in MHC vessels allowing experimentation up to T= 1200 ∘C and P= 300 MPa.

Alkaline magmas in shallow arc plutonic roots: a field and experimental investigation of hydrous cumulate melting in the southern Adamello batholith

“…The experiment started with vessel pressurisation at room temperature with half of the intended run pressure, and final pressures were reached via gas expansion upon heating. The limitations of this approach in precisely controlling the experimental fH 2 have been discussed by Marxer and Ulmer ( 2019 ) and Alex and Zajacz ( 2020 ). We applied a similar approach to monitor experimental fO 2 employing CoPd redox sensors.…”

In situ redox control and Raman spectroscopic characterisation of solutions below 300 °C

“…To simulate these processes in laboratories, several well established redox control techniques have been applied in hydrothermal experiments, including the double capsule (or oxygen buffer) (Eugster, 1957) and Shaw membrane techniques (Shaw, 1963). However, even after considerable refinements in the past six decades (Chou, 1987;Taylor et al, 1992;Berndt et al, 2002;Matthews et al, 2003;Alex and Zajacz, 2020), these techniques cannot be applied at low temperatures (T < 400 °C) because the precious metal hydrogen membranes that are commonly used in these techniques, such as Pt or Ag-Pd alloys, are not effective at these Ts (Chou, 1986).…”