Sound velocities and elastic properties of γ-Mg ₂ SiO ₄ to 873 K by Brillouin spectroscopy

The sound velocities and single-crystal elastic moduli of spinel-structured g-Mg 2 SiO 4 were measured to 873 K by Brillouin spectroscopy using a new high-temperature cell designed for singlecrystal measurements. These are the first reported acoustic measurements of g-Mg 2 SiO 4 elasticity at high temperatures. A linear decrease of elastic moduli and sound velocities with temperature adequately describes the data. The adiabatic bulk modulus, KS, shear modulus, m, and respective temperature derivatives for γ-Mg 2 SiO 4 are: KS = 185(3) GPa, μ = 120.4(2.0) GPa, (∂KS/∂T) P = -0.024(3) GPa/K and (∂m/∂T) 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- → ∂-(Mg,Fe) 2 SiO 4 transition are sufficient to produce an observable discontinuity at 520 km depth, even with a moderate (30-50%) amount of olivine