Sound velocities and elastic properties of γ-Mg₂SiO₄ to 873 K by Brillouin spectroscopy

The sound velocities and single-crystal elastic moduli of spinel-structured γ-Mg₂SiO₄ were measured to 873 K by Brillouin spectroscopy using a new high-temperature cell designed for single-crystal measurements. These are the first reported acoustic measurements of γ-Mg₂SiO₄ elasticity at high temperatures. A linear decrease of elastic moduli and sound velocities with temperature adequately describes the data. The adiabatic bulk modulus, K(S), shear modulus, μ, and respective temperature derivatives for γ-Mg₂SiO₄ are: K(S) = 185(3) GPa, μ = 120.4(2.0) GPa, (partial differentialK(S)/partial differentialT)(P) = -0.024(3) GPa/K and (partial differentialμ/partial differentialT)(P) = -0.015(2) GPa/K. Extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance contrasts associated with β- → γ-(Mg,Fe)₂SiO₄ transition are sufficient to produce an observable discontinuity at 520 km depth, even with a moderate (30-50%) amount of olivine.