Thermal conductivity of MgO periclase at high pressure: Implications for the D″ region

Abstract

The thermal conductivity of the lowermost mantle determines the rate at which heat flows across the core–mantle boundary, and consequently plays a central role in the thermal evolution of both the mantle as well as the core. In an effort to test and improve model estimates of thermal conductivity in the deep mantle, we perform first-principles computations of the lattice conductivity for MgO periclase. The method uses the Peierls–Boltzmann transport equation via a combination of first-principles molecular dynamics and first-principles lattice dynamics. Phonon lifetimes are found to be inversely proportional to temperature, and increases 3-fold as the density is increased from 3.37 to 5.49 g/cm 3. Thermal conductivities increase nearly 6-fold over the same density interval, and show excellent agreement with the available experimental measurements. We use these results to assess how conductivity measurements of mantle minerals can be reliably extrapolated to conditions characteristic of the lowermost mantle, and find Debye theory to be superior for this task. We then apply this insight to estimate the conductivity of MgSiO 3 perovskite in the lowermost mantle, and combine this with our computational results for MgO periclase to construct a model estimate for a representative lower mantle assemblage. We find the mantle conductivity to be 5.9 ± 0.6 W/m K at the top of the thermal boundary layer and 4.0 ± 0.5 W/m K at its base, implying core–mantle boundary heat flux values at the lower end of geophysical estimates.