Elasticity of magnesium silicate perovskite at high pressure and temperature by ultrasonic interferometry

The elastic properties of hot-pressed polycrystalline MgSiO₃ perovskite were measured by ultrasonic interferometry at high pressure and temperature. The experimental values of the shear modulus pressure and temperature derivatives were obtained by in-situ ultrasonic experiments in DIA-type, cubic anvil apparatus installed on the superconducting wiggler beamline at National Synchrotron Light Source of the Brookhaven National Laboratory. The measured (∂G/∂P)_T = 1.8(4) and (∂G/∂T)_P = −2.9(3) × 10−2 GPa/K agree well with previous estimates from lower mantle modeling and elasticity systematics. The technique to measure acoustic wave velocities up to 12 GPa at room pressure in multi-anvil 1000-ton Kennedy-Getting type apparatus using San Carlos olivine as a pressure marker was developed. The measured compressional velocity and longitudinal moduli of silicate perovskite specimens were consistently lower than that from Brillouin scattering studies. The mean values of the pressure derivatives are 79(5) m/sec/GPa and 9.1(5) for compressional velocity and longitudinal modulus respectively. Calculated values of the adiabatic bulk modulus and its pressure derivative are K = 235(9) GPa and K′ = 5.5(3) in good agreement with some of the early studies; however this bulk modulus is significantly lower than that generally accepted in static compression studies and measured by Brillouin scattering. Dual mode acoustic measurements were conducted at high pressure using a high-quality polycrystalline $Al2O3$ specimen and two synthesized MgSiO₃ perovskite specimens. The calculated average velocities and pressure derivatives of alumina in this study are V_P(0) = 10.83(3) km/sec and (∂V_P/∂P) _P = 53(2) m/sec/GPa, and V_s(0) = 6.39(2) km/sec and (∂V _S/∂P)_P = 22(2) m/sec/GPa, in excellent agreement with previous studies.; With our new measurements of (∂G/∂T)_P, and ambient adiabatic bulk modulus of MgSiO₃ perovskite we performed more direct comparisons with K_s and G values derived from seismic models of the lower mantle. Combining measured (∂G/∂T)_P of MgSiO ₃ perovskite with those measured for its bulk modulus and thermal expansion in static compression studies, these data suggest lower mantle compositions intermediate between pure enstatite and Cl chondrite and a lower mantle potential temperature of 1700 ± 200 K.