Elastic Properties of (Mg0.9Fe0.1)O in Earth’s Lower Mantle

Ferropericlase is the second most abundant mineral in Earth’s lower mantle and knowledge of its elastic properties at relevant conditions is needed to adequately interpret seismic observations in terms of mineralogy and thermal state of the deep Earth. Here, we report the complete elastic tensor of (Mg0.9Fe0.1)O ferropericlase to 81.2 GPa at room temperature measured by Brillouin Spectroscopy and X-ray diffraction in the diamond-anvil cell. Single-crystal ferropericlase was synthesised from a powder in a multi-anvil apparatus at P-T-conditions corresponding to Earth’s lower mantle (24 GPa, 1800°C). This growing procedure ensured a low ferric iron content and produced slightly greenish crystals with a diameter of up to 100 µm. Our data indicate that the spin transition of iron between 45 and 63 GPa dramatically affects the longitudinal and off-diagonal elements of the elastic stiffness tensor c11 and c12, whereas it leaves the shear constant c44 almost unaffected. Based on our results, the spin transition markedly changes the pressure (and temperature) dependence of the compressional and bulk velocities but has no comparable effect on average shear velocities. The different pressure dependence of compressional (and bulk) and shear velocities across the HS-LS transition suggest that the spin transition might be best observed in the vp/vs ratio and its pressure (and temperature) derivative. We also point out the possibility that the spin transition leads to an anti-correlated temperature dependence of shear and compressional wave speeds in certain pressure-temperature regimes in Earth’s lower mantle. These findings must be taken into account in any attempt to match seismic velocity models with mineral physics observations.