Single-crystal elasticity of (Mg0.9Fe0.1)O to 81 GPa

Ferropericlase is the second most abundant mineral in Earthʹs lower mantle and knowledge of its elastic properties at relevant conditions is needed to adequately interpret seismic observations in terms of the 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. 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 sound velocities and must be taken into account in any attempt to match average radial seismic velocity profiles with mineral physics observations. The different pressure dependence of compressional (and bulk) and shear sound velocities across the high-spin to low-spin (HS–LS) transition implies 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 Earthʹs lower mantle.