Oxygen Fugacity at High Pressure: Equations of State of Metal-oxide Pairs. A

Introduction: Oxygen fugacity (fO 2) varies by orders of magnitude in nature, and can induce profound changes in the chemical state of a substance, and also in the chemical equilibrium of multicomponent systems. One prominent area in high pressure geochemistry, in which fO 2 is widely recognized as a principal controlling factor, is that of metal-silicate partitioning of siderophile trace elements (e.g., [1]). Numerous experiments have shown that high pressures and temperatures can significantly affect metal/silicate partitioning of siderophile and moderately siderophile elements. Parameterization of these experimental results over P, T, X, and fO 2 can allow the observed siderophile element composition of the mantle to be associated with particular thermodynamic conditions [2]. However, this is best done only if quantitative control exists over each thermodynamic variable relevant to the experiments. The fO 2 values for many of these partitioning experiments were determined relative to a particular metal-oxide buffer (e.g., Fe-FeO (IW), Ni-NiO (NNO), Co-CoO, Re-ReO 2 (RRO)), but the parameterization of all experimental results is weakened by the fact that the pressure-induced relative changes between these buffer systems are imprecisely known. For the general metal-oxide reaction M + (x/2) O 2 = MO x , the fO 2 is related to the Gibbs energies (G) by (x/2) RT ln fO 2 = G(MO x) – G(M). The pressure effect on this buffer is in the G terms. Along each isotherm, dG = VdP. Extension of the 1-bar fO 2 buffers to high pressure, high-temperature conditions thus depends on the equations of state of each phase, but some of the relevant equations of state are poorly known. Furthermore, comparing ∆V between two independently measured equations of state compounds the errors associated with them. In this study we measure metal-oxide buffer pairs simultaneously using synchrotron radiation at high pressure and temperature. This minimizes systematic biases that might appear between studies, improving the precision of the ∆V data used to determine the high pressure fO 2 buffers. Experimental: High purity (>99.9%, from Alfa Aesar) and fine grained (< 5 µm) mixtures of Ni-NiO, Fe-Fe 1-x O and Re-ReO 2 were mixed with NaCl, which was used as a pressure standard during the