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Crystallization of silicon dioxide and compositional evolution of the Earth’s core

Abstract

The Earth’s core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40–60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean1,2,3,4,5 predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core6,7,8,9. However, only the binary systems Fe–Si and Fe–O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe–Si–O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe–Si–O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO2) is unexpectedly wide at the iron-rich portion of the Fe–Si–O ternary, such that an initial Fe–Si–O core crystallizes SiO2 as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity10,11, from as early on as the Hadean eon12. SiO2 saturation also sets limits on silicon and oxygen concentrations in the present-day outer core.

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Figure 1: Melting experiment (run 1) at 142 GPa.
Figure 2: Crystallization of SiO2 from liquid Fe–Si–O.
Figure 3: Core cooling rate with SiO2 crystallization.

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Acknowledgements

We thank H. Ozawa, Y. Kidokoro and K. Yonemitsu for their assistance in high high-pressure, high-temperature experiments. Discussions with J. Badro and R. Deguen helped develop the model for core energetics.

Author information

Authors and Affiliations

Authors

Contributions

K.H. and G.M. designed the project. K.H., G.M. and R.S. performed experiments, K.U. carried out ab initio calculations, G.H. developed the thermodynamic model, and J.H. and S.L. did the dynamical modelling. The manuscript was written by all authors.

Corresponding author

Correspondence to Kei Hirose.

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Competing interests

The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks A. Jephcoat and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 TEM bright-field image of the Fe–Si–O starting material.

It consists of fine-grained (<5 nm) metal and oxide.

Extended Data Figure 2 Melting experiment at 133 GPa in run 4.

Radial temperature distribution (as a function of distance from the centre of the laser-heated hot spot) and the composite X-ray maps for Fe, Si, O and Al for a cross-section of the recovered sample; quenched liquid Fe–5.8 wt% Si–0.5 wt% O (green) at the centre, surrounded by solid SiO2 (purple) and unmelted portion (green). They were sandwiched by Al2O3 insulation layers (red). Note that the central part included Al2O3 blocks, which is the evidence that it was molten at high pressure and high temperature.

Extended Data Figure 3 Solubility of O and Si in liquid Fe.

Solid and dashed lines represent and as parameterized by ref. 9. Points represent experimental data analysed here plotted in various ways; those in red are from this study’s experiments. shows an essentially linear 1/T dependence, similar to that found in ref. 35. More thermodynamically correct representation of the solubility K is shown by the dependence, whose points spread below the line. This suggests that is largely controlled by , but with additional, minor dependence on . When these are accounted for by the modelling procedure described in the text, the result—an effective corrected for pressure and non-ideal Si and O interactions in the metal—is shown as . Grey dotted lines show our model’s , for comparison.

Source data

Extended Data Figure 4 Solubility limits for SiO2 in liquid Fe superimposed on the present experimental results and earlier metal–silicate partitioning data.

Labelled points show experimental data obtained in this study. Unlabelled points are values used in the model fits5,7,9. No data at pressures <16 GPa are used in the fit, so values below this limit are not well represented by the model. Black curves indicate temperature contours of SiO2 saturation at 135 GPa. Green, blue and red curves are for 2,500 K at 20 GPa, 60 GPa and 100 GPa, respectively.

Extended Data Figure 5 Pressure dependence of SiO2 solubility.

For adiabats initiated at the CMB for a range of present-day core temperatures 3,500 K ≤ TCMB ≤ 4,500 K, we show the combined pressure and temperature dependence of the solubility assuming saturation at the CMB. Each labelled solid line is bounded by the ±2σ confidence interval, showing that within uncertainty, solubility is constant with depth in the core. SiO2 crystallization at the top of the core is a likely scenario, because otherwise crystallization creates compositional stratification, which is inconsistent with seismological observations.

Extended Data Table 1 Model parameters

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Hirose, K., Morard, G., Sinmyo, R. et al. Crystallization of silicon dioxide and compositional evolution of the Earth’s core. Nature 543, 99–102 (2017). https://doi.org/10.1038/nature21367

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