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

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Abstract
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, KS, shear modulus, μ, and respective temperature derivatives for γ-Mg2SiO4 are: KS = 185(3) GPa, μ = 120.4(2.0) GPa, (∂KS/∂T)P = −0.024(3) GPa/K and (∂μ/∂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- →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.

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