Lattice Thermal Conductivity of MgSiO₃ Postperovskite Under the Lowermost Mantle Conditions From Ab Initio Anharmonic Lattice Dynamics

Abstract

The lattice thermal conductivity (κlat) of MgSiO3 postperovskite (Mg‐PPv) under the lowermost mantle pressure (P) and temperature (T) conditions was calculated using density functional theory combining anharmonic lattice dynamics theory. The κlat of Mg‐PPv was found to be ~50% higher than that of MgSiO3 bridgmanite (Mg‐Brg) owing to the larger phonon group velocity and lifetime. The lateral variation in the core‐mantle boundary heat flux (qCMB) can be enhanced by the Brg‐PPv phase transition. We also found that the κlat along the c axis of Mg‐PPv is close to the conductivity of isotropic aggregate. This suggests that the effect of anisotropy in the κlat on qCMB would be minor if the transversely isotropic aggregate with the c axis vertical alignment, a potential source of seismic anisotropy in the D″ layer, is developed.

Thermal Conductivity of Silicate Liquid Determined by Machine Learning Potentials

“…Based on our analysis, we estimate the thermal conductivity of silicate liquid at the CMB to be 5.3 W m −1 K −1 . This is about half that of the solid mantle at the same pressure-temperature conditions: recent studies show that the total thermal conductivity of a bridgmanite/ferropericlase assemblage with a pyrolytic bulk composition is 9.4 W m −1 K −1 , and that of post-perovskite/ferropericlase is ∼1.3 times higher (Ammann et al, 2014;Dekura & Tsuchiya, 2019;Okuda et al, 2020).…”

Ab initio lattice thermal conductivity of MgSiO3 across the perovskite-postperovskite phase transition

“…Besides, by computing anharmonic phonon dispersion, the obtained phonon velocities are also temperature-dependent. Such features are not present in the conventional perturbative methods to evaluate 𝜅 "#$ [10,20,21].…”

“…Ab initio calculations provide an alternative to investigate 𝜅 "#$ of crystalline minerals at such extreme conditions. Dekura and Tsuchiya [10] calculated 𝜅 "#$ of MgPPv up to the CMB conditions by solving the linearized phonon Boltzmann transport equation. However, the perturbative approach adopted by Dekura and Tsuchiya [10] only accounts for up to third-order phonon-phonon scattering processes.…”

“…Dekura and Tsuchiya [10] calculated 𝜅 "#$ of MgPPv up to the CMB conditions by solving the linearized phonon Boltzmann transport equation. However, the perturbative approach adopted by Dekura and Tsuchiya [10] only accounts for up to third-order phonon-phonon scattering processes. Thus, the obtained phonon lifetimes may be overestimated at high temperatures.…”

“…In principle, the phonon quasiparticle approach captures anharmonic interactions to infinite orders. Therefore, compared to the conventional perturbative methods [10,20,21] accounting for up to three-phonon scatterings, the present approach should yield more reliable phonon lifetimes at high temperatures where higher-order phonon-phonon scatterings arise. Besides, by computing anharmonic phonon dispersion, the obtained phonon velocities are also temperature-dependent.…”