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MgO exsolution has been proposed to drive an early geodynamo. Experimental studies, however, have drawn different conclusions regarding the applicability of MgO exsolution. While many studies suggest that significant Mg can dissolve into the Earth's core, the amount of MgO exsolved out of the core, which hinges on the temperature dependence of MgO solubility, remains unclear. Here we present new high-temperature experiments to better constrain the temperature and compositional dependence of Mg partitioning between Fe alloys and silicate liquids. Our experiments show that Mg partitioning is weakly dependent on temperature, while confirming its strong dependence on oxygen content in Fe alloys. This implies that MgO exolution is limited as the core cools but can help drive an early geodyanamo if the core heat loss is slightly subadiabatic. If an exosolution-driven geodynamo did occur, it was likely over a limited time span that depends on the core thermal history and conductivity.

Plain Language Summary The existence of Earth's magnetic field has been dated back to at least 3.5 billion years ago. Yet its origin is under intense debates. A recent hypothesis is that abundant MgO, a major component of the Earth's mantle, may exsolve out of the Earth's core, providing energy to generate long-lasting magnetic field. However, how much MgO can exsolve out of the core is still poorly constrained. Here we conduct new high-pressure experiments to study the geochemical behavior of MgO under the Earth's core conditions. We find that the solubility of MgO in the Earth's core is weakly dependent on temperature. This suggests that the exsolution of MgO as the core cools is limited and can only power the geodynamo over a relatively limited time. Thus, the continuous operation of the Earth's ancient magnetic field remains a mystery.