Role of Ferric Iron and Protons in Mg-Fe Interdiffusion in (Mg,Fe)O

The knowledge of transport properties such as atomic diffusion, viscosity and electrical conductivity in the ferropericlase-magnesiowüstite solid solution, (Mg,Fe)O, is critical for understanding the dynamics of the lower mantle. Previous studies suggest that dominant positively-charged point defects in (Mg,Fe)O may change with pressure: ferric iron (Fe3+) dominates at low pressure while protons (H+) may dominate at high pressure. Consequently, the behavior of diffusion-related properties of (Mg,Fe)O likely changes with pressure from mechanisms sensitive to Fe3+ content (or oxygen fugacity) to those sensitive to H+ content (or water fugacity). However, the critical conditions above which water fugacity plays an important role are not well explored at present. In this study, we conducted high-pressure and high-temperature multianvil experiments on interdiffusion of Mg and Fe in (Mg,Fe)O at 5-15 GPa, 1673-1873 K with various oxygen fugacity buffers (Mo-MoO2, Ni-NiO, Re-ReO2) under nominally dry or water-saturated conditions. We also conducted experiments to constrain hydrogen solubility in MgO using various water fugacity buffers (periclase-brucite, periclase-clinohumite-forstelite). Our goals are a) to characterize the concentration of point defects to determine the conditions of solubility crossover between Fe3+ and H+ and b) to quantify the relative contributions of oxygen fugacity and water fugacity on Mg-Fe interdiffusivity. We measured the iron oxidation state of (Mg,Fe)O diffusion pairs by the flank method and Mössbauer spectroscopy, and hydrogen concentration by infrared spectroscopy. We found that Fe3+ concentration and Mg-Fe interdiffusivity are positively dependent on oxygen fugacity but not on water fugacity under the conditions we explored. Strong oxygen fugacity dependence is consistent with a diffusion model based on defect mechanisms since cation vacancies charge-balanced by Fe3+ are significantly more abundant than those charge-balanced by H+ under these conditions.