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Powering Earth’s dynamo with magnesium precipitation from the core

Nature volume 529, pages 387–389 (2016)Cite this article

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Abstract

Earth’s global magnetic field arises from vigorous convection within the liquid outer core. Palaeomagnetic evidence reveals that the geodynamo has operated for at least 3.4 billion years1, which places constraints on Earth’s formation and evolution. Available power sources in standard models include compositional convection (driven by the solidifying inner core’s expulsion of light elements), thermal convection (from slow cooling), and perhaps heat from the decay of radioactive isotopes. However, recent first-principles calculations2,3 and diamond-anvil cell experiments4,5 indicate that the thermal conductivity of iron is two or three times larger than typically assumed in these models. This presents a problem: a large increase in the conductive heat flux along the adiabat (due to the higher conductivity of iron) implies that the inner core is young (less than one billion years old4), but thermal convection and radiogenic heating alone may not have been able to sustain the geodynamo during earlier epochs. Here we show that the precipitation of magnesium-bearing minerals from the core could have served as an alternative power source. Equilibration at high temperatures in the aftermath of giant impacts allows a small amount of magnesium (one or two weight per cent) to partition into the core while still producing the observed abundances of siderophile elements in the mantle and avoiding an excess of silicon and oxygen in the core. The transport of magnesium as oxide or silicate from the cooling core to underneath the mantle is an order of magnitude more efficient per unit mass as a source of buoyancy than inner-core growth. We therefore conclude that Earth’s dynamo would survive throughout geologic time (from at least 3.4 billion years ago to the present) even if core radiogenic heating were minimal and core cooling were slow.

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Figure 1: Composition of Earth’s core and mantle immediately after accretion from models of silicate–metal equilibrium.
Figure 2: Estimates of mass precipitated from the cooling core.
Figure 3: Thermochemical evolution of the core for various rates of precipitation and entropy production associated with ohmic dissipation.

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Acknowledgements

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant number DGE-1144469 (J.G.O’R.).

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Authors and Affiliations

  1. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, 91125, California, USA

    Joseph G. O’Rourke & David J. Stevenson

Authors
  1. Joseph G. O’Rourke
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  2. David J. Stevenson
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Contributions

J.G.O’R. performed the calculations and wrote the manuscript. D.J.S. designed the project, discussed the results, and commented on the manuscript.

Corresponding author

Correspondence to Joseph G. O’Rourke.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 One example of the evolution of the composition of the precipitate.

Here the core initially contains 2 wt% Mg, 3 wt% Si and 6 wt% O. Additionally, the constants aO, bO and cO are each reduced by 0.25σ from their estimated mean values. The actual mineralogy of the precipitate (for example, the amount and composition of perovskite) is not modelled in detail.

Extended Data Figure 2 Additional results from models of Earth’s core–mantle differentiation.

Normalized distributions of chi-squared values, p(χ2), for both models of core formation (a), along with posterior probability densities for the coefficients ai, bi, ci, and for various elements i and j used in the two-stage model to calculate partitioning behaviour (b–e) and elemental abundances in bulk Earth (f).

Extended Data Figure 3 Posterior probability densities for parameters in the two-stage model of Earth’s core–mantle differentiation.

Extended Data Table 1 Parameters with uncertainties used in models of Earth’s differentiation
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O’Rourke, J., Stevenson, D. Powering Earth’s dynamo with magnesium precipitation from the core. Nature 529, 387–389 (2016). https://doi.org/10.1038/nature16495

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