Thermal equation of state of CaFe2O4-type MgAl2O4

In situ X-ray diffraction measurements of CaFe2O4-type MgAl2O4 have been conducted at pressures up to 42 GPa and temperatures to 2400 K using Kawai-type multianvil apparatus with sintered diamond anvils. Additional measurements have also been conducted at pressures to 12 GPa using diamond anvil cell with helium as a pressure medium at room temperature, and at temperatures to 836 K at the ambient pressure using a high-temperature X-ray diffractometer. The analysis of room-temperature data yielded V0 = 240.1(2) Å3, K0 = 205(6) GPa, and K ′ 0 = 4.1 ( 3 ) . A fit of the present data to high-temperature Birch–Murnaghan equation of state (EOS) yielded (∂K0/∂T)P = −0.030(2) GPa/K and α0 = a0 + b0T with values of a0 = 1.96(13) × 10−5 K−1 and b0 = 1.64(24) × 10−8 K−2. The present data set was also fitted to Mie–Grüneisen–Debye (MGD) EOS and we obtained γ0 = 1.73(7), q = 2.03(37), and θ0 = 1546(104) K. Density changes of MORB have been estimated using the newly obtained thermoelastic parameters, assuming that the Al-rich phase in this composition possesses the CaFe2O4-type structure under the lower mantle P, T conditions. The calculated densities along geotherms for the normal mantle and subducting cold slabs are both significantly higher than those of typical seismological models, confirming the conclusion of some recent results on MORB by laser-heated diamond anvil cell experiments.