Enthalpy, heat capacity and thermal expansivity measurements of MgSiO3 akimotoite: Reassessment of its self-consistent thermodynamic data set

Abstract

Enthalpy, isobaric heat capacity (CP) and thermal expansivity (α) of MgSiO3 akimotoite were re-determined by drop-solution calorimetry, differential scanning calorimetry and high-temperature powder X-ray diffraction measurement, respectively. From the obtained α, thermal Grüneisen parameter of MgSiO3 akimotoite was constrained to be 1.3. Using isochoric heat capacity by lattice vibrational model calculation and the Grüneisen relation equation, α and CP above maximum measurement temperature and the temperature derivative of isothermal bulk modulus were optimized by the least squares fitting to an existing P–V–T dataset which makes these parameters self-consistent. The obtained thermodynamic data set for MgSiO3 akimotoite was used to calculate the akimotoite–bridgmanite phase equilibrium boundary in MgSiO3 and high-pressure phase relations in Mg2SiO4. The result shows the stability field of the phase assemblage of MgSiO3 akimotoite + MgO below about 1400 K and around 23 GPa, suggesting the phase transition from ringwoodite to bridgmanite + ferropericlase via akimotoite + ferropericlase in cold subducted slabs.