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Powering Earth’s dynamo with magnesium precipitation from the core

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.

References

  1. Tarduno, J. A. et al. Geodynamo, solar wind, and magnetopause 3.4 to 3.45 billion years ago. Science 327, 1238–1240 (2010)

    CAS  ADS  Article  Google Scholar 

  2. de Koker, N., Steinle-Neumann, G. & Vlcek, V. Electrical resistivity and thermal conductivity of liquid Fe alloys at high P and T, and heat flux in Earth’s core. Proc. Natl Acad. Sci. USA 109, 4070–4073 (2012)

    CAS  ADS  Article  Google Scholar 

  3. Pozzo, M., Davies, C., Gubbins, D. & Alfe, D. Thermal and electrical conductivity of iron at Earth’s core conditions. Nature 485, 355–358 (2012)

    CAS  ADS  Article  Google Scholar 

  4. Gomi, H. et al. The high conductivity of iron and the thermal evolution of the Earth’s core. Phys. Earth Planet. Inter. 224, 88–103 (2013)

    CAS  ADS  Article  Google Scholar 

  5. Seagle, C. T., Cottrell, E., Fei, Y., Hummer, D. R. & Prakapenka, V. B. Electrical and thermal transport properties of iron and iron-silicon alloy at high pressure. Geophys. Res. Lett. 40, 5377–5381 (2013)

    CAS  ADS  Article  Google Scholar 

  6. Chambers, J. E. Planetary accretion in the inner Solar System. Earth Planet. Sci. Lett. 223, 241–252 (2004)

    CAS  ADS  Article  Google Scholar 

  7. Ogihara, M., Ida, S. & Morbidelli, A. Accretion of terrestrial planets from oligarchs in a turbulent disk. Icarus 188, 522–534 (2007)

    ADS  Article  Google Scholar 

  8. Wade, J. & Wood, B. J. Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, 78–95 (2005)

    CAS  ADS  Article  Google Scholar 

  9. Wood, B. J., Walter, M. J. & Wade, J. Accretion of the Earth and segregation of its core. Nature 441, 825–833 (2006)

    CAS  ADS  Article  Google Scholar 

  10. Rubie, D. C. et al. Heterogeneous accretion, composition and core-mantle differentiation of the Earth. Earth Planet. Sci. Lett. 301, 31–42 (2011)

    CAS  ADS  Article  Google Scholar 

  11. Siebert, J., Badro, J., Antonangeli, D. & Ryerson, F. J. Terrestrial accretion under oxidizing conditions. Science 339, 1194–1197 (2013)

    CAS  ADS  Article  Google Scholar 

  12. Rubie, D. C. et al. Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water. Icarus 248, 89–108 (2015)

    CAS  ADS  Article  Google Scholar 

  13. Shi, C. Y. et al. Formation of an interconnected network of iron melt at Earth’s lower mantle conditions. Nature Geosci. 6, 971–975 (2013)

    CAS  ADS  Article  Google Scholar 

  14. Helffrich, G. Outer core compositional layering and constraints on core liquid transport properties. Earth Planet. Sci. Lett. 391, 256–262 (2014)

    CAS  ADS  Article  Google Scholar 

  15. Canup, R. M. Accretion of the Earth. Phil. Trans. R. Soc. A 366, 4061–4075 (2008)

    ADS  Article  Google Scholar 

  16. Canup, R. M. Forming a Moon with an Earth-like composition via a giant impact. Science 338, 1052–1055 (2012)

    CAS  ADS  Article  Google Scholar 

  17. Cuk, M. & Stewart, S. T. Making the Moon from a fast-spinning Earth: a giant impact followed by resonant despinning. Science 338, 1047–1052 (2012)

    CAS  ADS  Article  Google Scholar 

  18. Dahl, T. & Stevenson, D. J. Turbulent mixing of metal and silicate during planet accretion—and interpretation of the Hf-W chronometer. Earth Planet. Sci. Lett. 295, 177–186 (2010)

    CAS  ADS  Article  Google Scholar 

  19. Poirier, J. Light elements in the Earth’s outer core: a critical review. Phys. Earth Planet. Inter. 85, 319–337 (1994)

    CAS  ADS  Article  Google Scholar 

  20. Wahl, S. M. & Militzer, B. High-temperature miscibility of iron and rock in terrestrial planet formation. Earth Planet. Sci. Lett. 410, 25–33 (2015)

    CAS  ADS  Article  Google Scholar 

  21. Takafuji, N., Hirose, K., Mitome, M. & Bando, Y. Solubilities of O and Si in liquid iron in equilibrium with (Mg,Fe)SiO3 perovskite and the light elements in the core. Geophys. Res. Lett. 32, L06313 (2005)

    ADS  Article  Google Scholar 

  22. Fischer, R. A. et al. High pressure metal-silicate partitioning of Ni, Co, V, Cr, Si, and O. Geochim. Cosmochim. Acta 167, 177–194 (2015)

    CAS  ADS  Article  Google Scholar 

  23. Palme, H. & O’Neill, H. in Treatise on Geochemistry 2nd edn (eds Holland, H. & Turekian, K. ) 1–39 (Elsevier, 2013)

  24. Badro, J., Cote, A. S. & Brodholt, J. P. A seismologically consistent compositional model of Earth’s core. Proc. Natl Acad. Sci. USA 111, 7542–7545 (2014)

    CAS  ADS  Article  Google Scholar 

  25. Stevenson, D. J. Planetary magnetic fields. Earth Planet. Sci. Lett. 208, 1–11 (2003)

    CAS  ADS  Article  Google Scholar 

  26. Nimmo, F. in Treatise on Geophysics 2nd edn (ed. Schubert, G. ) 31–65 (Elsevier, 2015)

  27. Zhang, P., Cohen, R. E. & Haule, K. Effects of electron correlations on transport properties of iron at Earth’s core conditions. Nature 517, 605–607 (2015)

    CAS  ADS  Article  Google Scholar 

  28. Corgne, A., Shantanu, K., Fei, Y. & McDonough, W. F. How much potassium is in the Earth’s core? New insights from partitioning experiments. Earth Planet. Sci. Lett. 256, 567–576 (2007)

    CAS  ADS  Article  Google Scholar 

  29. Labrosse, S., Hernlund, J. W. & Coltice, N. A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature 450, 866–869 (2007)

    CAS  ADS  Article  Google Scholar 

  30. Biggin, A. J. et al. Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation. Nature 526, 245–248 (2015)

    CAS  ADS  Article  Google Scholar 

  31. Nomura, R. et al. Partitioning of potassium into the Earth’s core and implications for thermal history of the Earth. AGU Fall Meet. Abstr. DI33A–2411 (2012)

  32. McDonough, W. F. in Earthquake Thermodynamics and Phase Transformation in the Earth’s Interior (eds Teisseyre, R. & Majewski, E. ) 3–23 (Academic Press, 2001)

  33. Chib, S. & Greenberg, E. Understanding the Metropolis-Hastings algorithm. J. Am. Stat. Assoc. 49, 327–335 (1995)

    Google Scholar 

  34. Labrosse, S. Thermal evolution of the core with a high thermal conductivity. Phys. Earth Planet. Inter. 247, 36–55 (2015)

    ADS  Article  Google Scholar 

Download references

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

Authors

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 (be) 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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