Thermal and compositional evolution of the Earth’s core

Recent ab initio calculations and experiments, including the recent one using an internally-heated DAC (Inoue et al., 2020 EPSL), demonstrate the high thermal conductivity of the Earth’s core, although it still remains controversial. The high thermal conductivity suggests that core convection has been driven by chemical rather than thermal buoyancy. The distributions of siderophile elements between core and mantle suggest that core-forming metals segregated from silicate at high pressure and high temperature, leading to the incorporation of large amounts of Si, O, and possibly Mg into the core. Our thermodynamic modeling (Helffrich et al. 2020 GRL) based on metal-silicate partitioning experiments reported to date show that with <1.8 wt% Mg, the liquid core exsolves SiO2 only. The rate of SiO2 crystallization required to sustain geodynamo is as low as 1 wt% per 109 years, which corresponds to a cooling rate of 100–200 K/Gyr (Hirose et al., 2017). Above 1.8 wt% Mg, (Mg, Fe)-silicate melts also exsolve from the core and transfer core-hosted elements to the mantle. The core-derived silicate melts may have evolved toward FeO-rich compositions and now represent ultra-low velocity zone above the CMB.