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 () conditions expected for the early core-mantle system. Our distribution coefficients are compatible with those obtained by extrapolating experimental data at lower 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.
- Received 1 May 2019
- Revised 19 July 2019
DOI:https://doi.org/10.1103/PhysRevX.9.041018
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Earth’s core is the engine of our planet, heating the mantle and driving plate tectonics, earthquakes, and volcanic eruptions. It is also where Earth’s magnetic field is generated, a feature that our planet has had for most of its history. Crucial to our understanding of the magnetic field is the composition of the core, and in particular the inventory of light elements dissolved in its iron-nickel mixture. Here, we calculate how and when the light elements entered the core, a fundamental insight for understanding Earth’s formation and evolution.
We focus on oxygen, a crucial light element, and present first-principles calculations of oxygen separating between liquid iron-oxygen metal and silicate melts. After careful validation of our methods by comparison with experiments, we extend the calculations to the pressure and temperature conditions at Earth’s core, where little experimental evidence exists. We show that a significant amount of oxygen can transfer into the core following its formation, providing the main fuel for driving the magnetic field at later stages in the history of the planet.
Future work could look at the partitioning and interdependences of magnesium and silicon and establish how much of the total inventory of light elements in the core was set after formation—all of which could help researchers better understand the history of Earth’s magnetic field and how it might evolve in the future.
