Ab initio thermal expansion and thermoelastic properties of ringwoodite (<i>γ</i>-Mg<sub>2</sub>SiO<sub>4</sub>) at mantle transition zone conditions

Abstract. Thermal convection in the Earth's mantle is driven by lateral variations temperature and density, which are substantially controlled local volume thermal expansion of constituent mineral phases. Ringwoodite a major component lower transition zone, but its expansivity thermoelastic properties still affected large uncertainties. Ambient coefficient (αV0​​​​​​​), for instance, can vary as much 100 % according to different experimental investigations available from literature. In this work, we perform ab initio density functional theory calculations vibrational spinel-structured Mg2SiO4 ringwoodite order provide reliable thermophysical data up zone conditions. Temperature- pressure-dependent has been obtained phonon dispersion framework quasi-harmonic approximation (QHA) 25 GPa 2000 K. Theoretical analysis spectra reveals that accurate prediction IR silent modes, along with their relative mode Grüneisen parameters, crucial define expansivity. A six-parameter analytical function able reproduce values fairly well whole investigated P–T range, i.e., αV(P,T)=(1.6033×10-5+8.839×10-9T+11.586×10-3T-1-6.055T-2+804.31T-3) ×exp⁡(-2.52×10-2P), kelvin pressure gigapascal. Ab static isothermal bulk moduli have derived P, T cross derivatives, K0 = 184.3 GPa, KT,300 K 176.6 K0′ 4.13, KT,300K′ 4.16, ∂KT∂TP −0.0233 K−1 ∂2KT∂P∂T0=1.0×10-4 K−1. Computed support evidence QHA performs remarkably temperatures. Since strongly dependence becomes more marked increasing temperature, internally consistent assessments empirical extrapolation deep conditions should be taken care avoid inaccurate or spurious predictions phase equilibrium numerical modeling.