High pressure effects on the iron–iron oxide and nickel–nickel oxide oxygen fugacity buffers

The chemical potential of oxygen in natural and experimental samples is commonly reported relative to a specific oxygen fugacity (fO2) buffer. These buffers are precisely known at 1 bar, but under high pressures corresponding to the conditions of the deep Earth, oxygen fugacity buffers are poorly calibrated. Reference (1 bar) fO2 buffers can be integrated to high pressure conditions by integrating the difference in volume between the solid phases, provided that their equations of state are known. In this work, the equations of state and volume difference between the metal–oxide pairs Fe–FeO and Ni–NiO were measured using synchrotron X-ray diffraction in a multi-anvil press and laser heated diamond anvil cells. The results were used to construct high pressure fO2 buffer curves for these systems. The difference between the Fe–FeO and Ni–NiO buffers is observed to decrease significantly, by several log units, over 80 GPa. The results can be used to improve interpretation of high pressure experiments, specifically Fe–Ni exchange between metallic and oxide phases.