Sound velocity and elasticity of single-crystal forsterite to 16 GPa

As an abundant component of Earth's upper mantle, forsterite (Mg₂SiO₄) has been the subject of extensive investigation for many decades. The pressure dependences of sound velocity and elasticity are of great importance for modeling the composition and evolution of Earth. For this reason, room temperature Brillouin scattering experiments were performed on single-crystal forsterite at pressures up to 16 GPa in a diamond anvil cell. The nine elastic moduli increase linearly over the pressure range studied. Together with previous ambient pressure data, the pressure variation of the bulk modulus is K_S(GPa) = 128.8(5) + 4.2(2)P, and the shear modulus is G(GPa) = 81.6(2) + 1.4(1)P, where the numbers in parentheses are one standard deviation uncertainties in the last digit. Combining with previous ultrasonic data for polycrystalline β-Mg₂SiO₄ to 3 GPa, the sound velocity contrasts between α- and β-Mg₂SiO₄ were found to be largely pressure independent at room temperature. If, on the other hand, the pressure dependences of the bulk and shear modulus of the β phase of Mg₂SiO₄ are assumed to be identical to those for the α phase, the room temperature velocity contrasts between the two phases decrease by 20% and 12% from 1 bar to 13.8 GPa for compressional and shear waves, respectively.