Helium in Earth’s early core

Abstract

The observed escape of the primordial helium isotope, 3He, from the Earth’s interior indicates that primordial helium survived the energetic process of planetary accretion and has been trapped within the Earth to the present day. Two distinct reservoirs in the Earth’s interior have been invoked to account for variations in the 3He/4He ratio observed at the surface in ocean basalts: a conventional depleted mantle source and a deep, still enigmatic, source that must have been isolated from processing throughout Earth history. The Earth’s iron-based core has not been considered a potential helium source because partitioning of helium into metal liquid has been assumed to be negligible. Here we determine helium partitioning in experiments between molten silicates and iron-rich metal liquids at conditions up to 16 GPa and 3,000 K. Analyses of the samples by ultraviolet laser ablation mass spectrometry yield metal–silicate helium partition coefficients that range between 4.7×10−3 and 1.7×10−2 and suggest that significant quantities of helium may reside in the core. Based on estimated concentrations of primordial helium, we conclude that the early core could have incorporated enough helium to supply deep-rooted plumes enriched in 3He throughout the age of the Earth.

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Figure 1: Back-scattered electron images of samples that were recovered from high pressure and high temperature, and analysed for He concentrations by ultraviolet laser ablation microprobe.
Figure 2: Depth profiles of He concentrations in the silicate and metal phases.
Figure 3: Metal–silicate partition coefficients of He, U and Th.
Figure 4: He concentrations in CI-chondrite melt at high pressures.
Figure 5: He concentrations in molten metal alloys at high pressures.

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Acknowledgements

We thank D. Porcelli for discussions and comments, S. Sherlock and J. Schwanethal for their help with ultraviolet laser ablation microprobe analyses. We thank S. Mukhopadhyay and S. Parman for comments on the original manuscript. M.A.B. acknowledges the support of a NERC fellowship and the LabEx-ClerVolc programme at Clermont-Ferrand. A.P.J. acknowledges a NERC Senior Research Fellowship and NERC research grants.

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M.A.B. and A.P.J. designed the project and wrote the paper with contributions from S.P.K. All authors contributed to the data interpretation. Trials of the helium analytical procedure on DAC samples were carried out by V.S.H. and the rest by M.A.B. at the Open University at S.P.K. laboratory.

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Correspondence to M. A. Bouhifd or Andrew P. Jephcoat.

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Bouhifd, M., Jephcoat, A., Heber, V. et al. Helium in Earth’s early core. Nature Geosci 6, 982–986 (2013). https://doi.org/10.1038/ngeo1959

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