Equation of State of Iron-Rich (Mg,Fe)O

Recent seismic observations of the core-mantle boundary (CMB) have provided increasing evidence for the presence of a chemically heterogeneous boundary layer, whose lateral variations in seismic velocities and densities may be explained by iron enrichment in lower mantle minerals like (Mg,Fe)O. Relatively little study has been directed toward iron-rich members of the (Mg,Fe)O solid solution despite the possibility for even low levels of iron enrichment to have significant impact on elastic properties. To that end, we present results from a powder synchrotron x-ray diffraction study on (Mg_0.06Fe_0.94)O up to 90 GPa at 300 K using helium as a pressure-transmitting medium. The diffraction data are used to determine the equations of state for the material’s B1 cubic and rhombohedral phases and constrain the transition pressure at ambient temperature. We combine our results with pressure-temperature-volume measurements on an identical composition [1] to produce a well-constrained thermal equation of state. Using these results, we report a thermal elasticity model for iron-rich (Mg,Fe)O at CMB conditions for use in dynamic modeling and comparison against seismic observations. [1] Wicks et al., PEPI, 249, 28 (2015).