Equation of State and Spin Transition of (Mg,Fe)O at High Pressures

Abstract:

Earth’s lower mantle occupies more than half of Earth’s volume, and is composed primarily of bridgmanites and (Mg,Fe)O “ferropericlase.” Knowledge of the behavior of lower mantle minerals is essential for interpreting complexity in the deep Earth. Although it is suggested that (Mg,Fe)O represents a major volume fraction of Earth’s interior, the iron concentration of (Mg,Fe)O is not very well constrained at all depths of the lower mantle. Near the base of the lower mantle, an enhanced iron content may be found due to melting events in Earth’s history and/or reactions with the iron-dominant liquid outer core. Here we examine the high-pressure behavior of polycrystalline (Mg,Fe)O containing 48 mol% FeO (“FP48”), loaded hydrostatically with neon as a pressure medium. Using x-ray diffraction and synchrotron Mössbauer spectroscopy we measure the equation of state of FP48 to about 80 GPa at 300 K and hyperfine parameters to 100 GPa at 300 K, respectively. A gradual volume drop is observed between ∼55 and 75 GPa. To confirm that the observed volume drop is due to a spin crossover, the quadrupole splitting (QS) and isomer shift (IS) of Fe²⁺ are determined as a function of pressure. At low pressures, our spectra are fitted with two Fe²⁺-like sites. At pressures between 43 and 81 GPa, an additional Fe²⁺-like site with a QS of 0 is required, indicative of low-spin iron. Above 93 GPa, two low-spin Fe²⁺-like sites explain the data well, signifying the completion of the spin crossover. Using a spin crossover equation of state, we have determined the volume drop at the calculated transition pressure and the relevant elastic properties of FP48. Our results are compared to previous measurements of (Mg,Fe)O with varying concentrations of iron at high pressures.