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 boundary layer rich in chemical diversity with lateral variations in seismic velocities and densities. Exploring causes of observed anomalies such as ultralow-velocity zones (ULVZs) in this region can lead to a deeper understanding of phenomena like hotspot volcanism and heat flow from the core. One potential explanation for the presence of these lateral heterogeneities may be iron enrichment in lower mantle minerals such as magnesiowüstite, possibly resulting from melting events or interactions with the iron-dominant outer core. 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 x-ray diffraction study on (Mg0.06Fe0.94)O up to 90 GPa at 300 K using helium as a pressure-transmitting medium. The measurements were conducted at beamline 12.2.2 of the Advanced Light Source of Lawrence Berkeley National Laboratory. The diffraction data were 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 (Wicks et al. 2015) to produce a well-constrained thermal equation of state. Using these results, we report a thermal elasticity model for magnesiowüstite at CMB conditions for use in dynamic modeling and comparison against seismic observations.