Elastic anisotropy in textured hcp-iron to 112 GPa from sound wave propagation measurements

Abstract

We report the first experimental determination of elastic anisotropy in highly textured polycrystalline hexagonal-closed-packed (hcp)-iron obtained at room temperature and 112 GPa by inelastic X-ray scattering (IXS) in a diamond anvil cell (DAC). The compressional sound velocity V P at 50° and 90° from the c-axis was determined to be V P{50°}=9900±200 m/s and V P{90°}=9450±150 m/s, respectively. The difference of 4–5% between the two velocities is of the same order of the seismic observations of the anisotropy in Earth's inner core. Combining our new results with the equation of state of hcp-iron, we derive the pressure dependence of the shear velocity V S. The V S-values, extrapolated to inner core densities, are 30% higher than the PREM results. The extrapolated V P-values are instead in fair agreement with the PREM seismic model, though differences in the slope remain. Furthermore, assuming a strong texture characterized by cylindrical symmetry, we provide an estimate for the single crystal elastic modulus C 11 as a function of pressure. While our results are in general agreement with the predominant existence of hcp-iron in Earth's inner core, the remaining differences might be assigned to the presence of another solid iron phase, light elements, or liquid inclusions as well as anharmonic high-temperature effects.