P-V-T relations of MgSiO₃ perovskite determined by in situ X-ray diffraction using a large-volume high-pressure apparatus

The volume of MgSiO₃ perovskite has been precisely measured at pressures of 19 to 53 GPa and temperatures of 300 to 2300 K by means of in situ X-ray diffraction in a multi-anvil apparatus. The present results indicate the isothermal bulk modulus K_T0 = 256(2) GPa and its pressure derivative K′_T0 = 3.8(2). The fixed Debye temperature θ₀ = 1030 K gives a Grüneisen parameter at ambient pressure γ_o = 2.6(1) and its logarithmic volume dependence q = 1.7(1). The pressure derivative of the isothermal bulk modulus, Anderson-Grüneisen parameter and thermal expansion coefficient at ambient pressure are found to be (∂K_T/∂T)_P = -0.035(2) GPa/K, δ_T = 6.5(5), α₀ = 2.6(1) × 10^-5 + 1.0(1) × 10^-8(T-300)/K. Thus the thermal expansion coefficient largely becomes smaller with increasing pressure. The adiabatic geotherm would be fairly large, such as 0.41 K/km at a 660 km depth, and becoming smaller with increasing depth. The temperature and adiabatic geothermal gradient at the bottom of the D′ layer would be 2400 K and 0.14 K/km. The buoyancydriven mantle convection could be very small in the lower part of the lower mantle.