Thermodynamical Modeling of Liquid Fe‐Si‐Mg‐O:Molten Magnesium Silicate Release From the Core.

We developed a thermodynamic model to explore the joint solubility of Mg, Si, and O in liquid Fe on the basis of high‐pressure metal‐silicate partitioning data in the literature, with more Mg kept in the metal when Si and O are present. With <1.7 ± 0.5 wt% Mg, the metal in the young Earth's core retains Mg as the core evolves and crystallizes SiO2. Higher Mg concentrations require either the late addition of metal that equilibrated with silicate in a super‐liquidus magma ocean or the assimilation of silicate into the core at the time of a giant impact. Above 1.7 wt% Mg, (Mg,Fe)‐silicate melts also exsolve from the core and transfer core‐hosted elements to the mantle. Fractional crystallization of the core‐derived silicate melts in the core or at the core‐mantle boundary could, additionally, yield a persistent molten silicate layer that could also contribute to ultralow velocity zone formation in the lowermost mantle. Plain Language Summary: Some think that crystallization of oxide in the liquid core, either MgO or SiO2, could have been responsible for driving core convection and geodynamo since early Earth times, at least until iron started crystallizing the inner core. However, the joint solubility of Mg, Si, and O in liquid Fe has not been explored yet. Here we developed a thermodynamic model to examine the exsolution of MgO and SiO2 from liquid Fe at core‐mantle boundary (CMB) pressure based on earlier experimental data on metal‐silicate partitioning. Our main findings are that (1) more Mg is retained in the presence of Si and O in metal; (2) with <1.7 wt% Mg, the core expels solid SiO2 but not MgO; and (3) with higher Mg, magnesium silicate melt also exolves and transfers core‐hosted elements to the mantle. The seismologically observed molten silicate layer above the CMB may have been originally derived from the core. Key Points: Mg uptake in liquid iron is controlled by the liquid's Si content; silicate Mg is reduced by Si in the metalIf the core later expelled Mg ingested while accreting as (Mg,Fe)‐silicate, it could transfer siderophile trace elements back to the mantleULVZs at the base of the mantle could be related to fractional crystallization of (Mg,Fe) silicate expelled by the core [ABSTRACT FROM AUTHOR]