Single-crystal elasticity and sound velocities of (Mg0.94Fe0.06)O ferropericlase to 20 GPa

[ 1] The single-crystal elastic properties of high-spin (Mg0.94Fe0.06)O ferropericlase were measured by Brillouin spectroscopy on a sample compressed to 20 GPa with diamond anvil cells using methanol-ethanol-water as a pressure-transmitting medium. At room pressure, the adiabatic bulk (K-0S) and shear (mu(0S)) moduli are K-0S = 163 +/- 3 GPa and mu(0S) = 121 +/- 2 GPa, in excellent agreement with ultrasonic results from the same bulk sample (Jacobsen et al., 2002). A fit to all our high-pressure Brillouin data using a third-order finite-strain equation of state yields the following pressure derivatives of the adiabatic bulk and shear moduli: K'(0S) = 3.9 +/- 0.2 and mu'(0S) = 2.1 +/- 0.1. Within the uncertainties, we find that K-0S and K'(0S) of (Mg0.94Fe0.06) O are unchanged from MgO. However, mu(0S) and mu'(0S) of (Mg0.94Fe0.06) O are reduced by 8% and 11%, respectively. The aggregate compressional (V-P) and shear (V-S) wave velocities are reduced by 4% and 6%, respectively, as compared to MgO. The pressure dependence of the single-crystal elastic moduli and aggregate sound velocities is linear within the investigated pressure range. The elastic anisotropy of (Mg0.94Fe0.06) O is about 10% greater than that of MgO at ambient conditions. At the highest pressure obtained here, the elastic anisotropy of (Mg0.94Fe0.06) O is close to zero. On the basis of our measurements and earlier ultrasonic measurements, we find that the pressure derivatives of shear moduli obtained at room pressure for low iron concentrations (< 20 mol% FeO) of high-spin ferropericlase are inconsistent with those inferred from the lower mantle PREM model.