Magnesium partitioning between metal and silicate was experimentally investigated between 34 and 138GPa, 3,500 and 5,450K using laser-heated diamond anvil cells. The 22 measurements are combined with previously published data (total of 49 measurements) to model magnesium metal-silicate partitioning using a thermodynamically consistent framework based on the interaction parameter formalism. The observations support the mechanism of MgO dissolution in the metal, ruling out other mechanisms. The magnesium partition coefficient depends on temperature and metal composition, but not on pressure or silicate composition. The equilibrium concentration and the exsolution rate of MgO in Earth's core can therefore be calculated for any P, T, and composition. Using a core thermal evolution model, the buoyancy flux converts to a magnetic field at Earth's surface, with dipole intensities between 40 and 70 mu T prior to inner core growth, consistent with the paleomagnetic record going back to the Archean.

Plain Language Summary We measure the incorporation of magnesium oxide (one of the main components of Earth's mantle) into iron (the main constituent Earth's core), using extremely high pressure and temperature experiments that mimic the conditions of Earth's mantle and core. We find that magnesium oxide dissolution depends on temperature but not on pressure, and on metal (i.e., core) composition but not silicate (i.e., mantle) composition. Our findings support the idea that magnesium oxide dissolved in the core during its formation will precipitate out during subsequent core cooling. The precipitation should stir the entire core to produce a magnetic field in Earth's distant past, at least as intense as the present-day field.