The Earth’s core as a reservoir of water

Abstract

Current estimates of the budget and distribution of water in the Earth have large uncertainties, most of which are due to the lack of information about the deep Earth. Recent studies suggest that the Earth could have gained a considerable amount of water during the early stages of its evolution from the hydrogen-rich solar nebula, and that a large amount of the water in the Earth may have partitioned into the core. Here we calculate the partitioning of water between iron and silicate melts at 20–135 GPa and 2,800–5,000 K, using ab initio molecular dynamics and thermodynamic integration techniques. Our results indicate a siderophile nature of water at core–mantle differentiation and core–mantle boundary conditions, which weakens with increasing temperature; nevertheless, we found that water always partitions strongly into the iron liquid under core-formation conditions for both reducing and oxidizing scenarios. The siderophile nature of water was also verified by an empirical-counting method that calculates the distribution of hydrogen in an equilibrated iron and silicate melt. We therefore conclude that the Earth’s core may act as a large reservoir that contains most of the Earth’s water. In addition to constraining the accretion models of volatile delivery, the findings may partially account for the low density of the Earth’s core implied by measured seismic velocities.

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Fig. 1: Calculated free energies.
Fig. 2: Partition coefficient.
Fig. 3: Temperature dependence of H partitioning.
Fig. 4: Empirical counting of H partitioning.
Fig. 5: Comparison of partition coefficients with the literature.

Data availability

The raw outputs can be accessed in the UK National Geoscience Data Centre (NGDC) (https://doi.org/10.5285/d0677edf-c987-497d-aae8-23bf22ef774d). Any additional data can be requested by e-mailing the corresponding author.

Code availability

The Vienna Ab Initio Simulation Package is a proprietary software available for purchase at https://www.vasp.at/.

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Acknowledgements

This work was supported by NERC grant NE/M015181/1 and NE/S01134X/1. We acknowledge the use of the NEXCS system, a collaborative facility supplied under the Joint Weather and Climate Research Program, a strategic partnership between the Met Office and the Natural Environment Research Council. This work also used the ARCHER UK National Supercomputing Service. T.S. acknowledges the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant no. XDB18000000).

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Contributions

Y.L. carried out the simulations and analysis. L.V. and J.P.B. supervised the project. All the authors contributed to the data analysis and writing the paper.

Corresponding author

Correspondence to Yunguo Li.

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The authors declare no competing interests.

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Peer review information Primary Handling Editor: Tamara Goldin.

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Extended data

Extended Data Fig. 1 Calculated free energies.

Calculated volumes and Gibbs free energies \(\bar G\left( {p,T,x} \right)\) of iron and silicate melts with H and H2O at 20, 50, 90, and 135 GPa, corresponding to temperatures of 2800, 3500, 3900 and 4200 K, respectively.

Extended Data Fig. 2 Temperature dependence of free energies.

Calculated volumes, enthalpies and Gibbs free energies \(\bar G\left( {p,T,x} \right)\) of Fe64, Fe64H4, (MgSiO3)32 and (MgSiO3)32H8 at temperatures from 4200 to 5000 K under 135 GPa.

Supplementary information

41561_2020_578_MOESM2_ESM.mpg

Two-phase AIMD Trajectory Video. The video shows that most hydrogen atoms (white balls) enter the liquid Fe (golden balls), but few go into the silicate melt (green, cyan and red balls represent Si, Mg and O, respectively) at ~50 GPa and 3,500 K. This simulation clearly demonstrates the siderophile (‘iron-loving’) nature of hydrogen, and implies that the Earth’s core can be a reservoir of hydrogen.

Supplementary Information

Supplementary Figs. 1–5, and discussion.

Supplementary Video 1

Two-phase AIMD Trajectory Video. The video shows that most hydrogen atoms (white balls) enter the liquid Fe (golden balls), but few go into the silicate melt (green, cyan and red balls represent Si, Mg and O, respectively) at ~50 GPa and 3,500 K. This simulation clearly demonstrates the siderophile (‘iron-loving’) nature of hydrogen, and implies that the Earth’s core can be a reservoir of hydrogen.

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Li, Y., Vočadlo, L., Sun, T. et al. The Earth’s core as a reservoir of water. Nat. Geosci. (2020). https://doi.org/10.1038/s41561-020-0578-1

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