Thermal expansion of forsterite at high pressures determined by in situ X-ray diffraction: The adiabatic geotherm in the upper mantle

Volumes of forsterite were measured by in situ X-ray diffraction at simultaneously high pressures and high temperatures up to 14 GPa and 1900 K in a Kawai-type multi-anvil apparatus. By fitting the present data to the Mie-Grüneisen-Debye thermal equation of state with the results of the previous reliable studies, we obtained γ0 = 1.31(1) and q = 3.2(1). The logarithmic volume dependence of the thermal expansion coefficient was found to be δT = 8.4(2). The average temperature derivative of the isothermal bulk modulus in the investigated pressure–temperature ranges was found to be (∂KT/∂T)P = −0.028(1) GPa/K. The isobaric heat capacity and thermal pressure at high pressure and high temperature were estimated using the present volume data. The adiabatic geotherm at the top and base of the uppermost mantle are 0.60(2) and 0.34(2) K/km. The temperature estimated from the 410-km discontinuity depth and the olivine-wadsleyite transition pressure suggests a mantle potential temperature of 1520 ± 40 K.