Abstract
The standard model of Earth’s core evolution has the bulk composition set at formation, with slow cooling beneath a solid mantle providing power for geomagnetic field generation. However, controversy surrounding the incorporation of oxygen, a critical light element, and the rapid cooling rates needed to maintain the early dynamo have called this model into question. The predicted cooling rates imply early core temperatures that far exceed estimates of the lower mantle solidus, suggesting that early core evolution was governed by interaction with a molten lower mantle. Here we develop ab initio techniques to compute the chemical potentials of arbitrary solutes in solution and use them to calculate oxygen partitioning between liquid Fe-O metal and silicate melts at the pressure-temperature (P-T) conditions expected for the early core-mantle system. Our distribution coefficients are compatible with those obtained by extrapolating experimental data at lower P-T values and reveal that oxygen strongly partitions into metal at core conditions via an exothermic reaction. Our results suggest that the bulk of Earth’s core was undersaturated in oxygen compared to the FeO content of the magma ocean during the latter stages of its formation, implying the early creation of a stably stratified oxygen-enriched layer below the core-mantle boundary (CMB). FeO partitioning is accompanied by heat release due to the exothermic reaction. If the reaction occurred at the CMB, this heat sink could have significantly reduced the heat flow driving the core convection and magnetic field generation.
Highlights
It is generally believed that after its formation 4.54 billion years ago, Earth quickly differentiated with a heavy ironbased core sinking to its center during the first 100 million years [1]
The methods are based on density-functional theory (DFT) [36,37], and we present three independent statistical mechanics techniques to compute free-energy differences, and from those, the chemical potentials of solutes in solution
The tests of the previous section lend confidence to both method 1 and method 2, and we are ready to extend the calculations to the magma ocean
Summary
It is generally believed that after its formation 4.54 billion years ago, Earth quickly differentiated with a heavy ironbased core sinking to its center during the first 100 million years [1]. As Earth cooled, a solid inner core started to form at the center of the planet, and in the process, it began to expel the primordial—solid incompatible—elements into the liquid. Of particular importance is the expulsion of light elements because Earth uses their gravitational potential energy to drive convection in the liquid outer core, which is presently the main power source for the geodynamo process that generates Earth’s magnetic field [2,3]. The dynamics and evolution of Earth’s core depend strongly on the nature of its light element inventory
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