Recent palaeomagnetic observations1 report the existence of a magnetic field on Earth that is at least 3.45 billion years old. Compositional buoyancy caused by inner-core growth2 is the primary driver of Earth’s present-day geodynamo3,4,5, but the inner core is too young6 to explain the existence of a magnetic field before about one billion years ago. Theoretical models7 propose that the exsolution of magnesium oxide—the major constituent of Earth’s mantle—from the core provided a major source of the energy required to drive an early dynamo, but experimental evidence for the incorporation of mantle components into the core has been lacking. Indeed, terrestrial core formation occurred in the early molten Earth by gravitational segregation of immiscible metal and silicate melts, transporting iron-loving (siderophile) elements from the silicate mantle to the metallic core8,9,10 and leaving rock-loving (lithophile) mantle components behind. Here we present experiments showing that magnesium oxide dissolves in core-forming iron melt at very high temperatures. Using core-formation models11, we show that extreme events during Earth’s accretion (such as the Moon-forming giant impact12) could have contributed large amounts of magnesium to the early core. As the core subsequently cooled, exsolution7 of buoyant magnesium oxide would have taken place at the core–mantle boundary, generating a substantial amount of gravitational energy as a result of compositional buoyancy. This amount of energy is comparable to, if not more than, that produced by inner-core growth, resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core.
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The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 207467. Parts of this work were supported by the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02), IPGP multidisciplinary programme PARI, and by Paris–IdF region SESAME grant no. 12015908. J.S. acknowledges support from the French National Research Agency (ANR project VolTerre, grant no. ANR-14-CE33-0017-01). We thank J. Aubert, S. Stewart and P. Asimow for discussions. We thank R. Ryerson for comments on the manuscript.
Reviewer Information Nature thanks B. Buffett and Y. Fei for their contribution to the peer review of this work.
Extended data figures
This file contains EPMA analyses of experimental runs. The full electron probe analyses of the metal and silicate phases in all 6 experiments reported in the paper.
About this article
Geophysical & Astrophysical Fluid Dynamics (2019)