Abstract
Helium is considered a key tracer for processes in Earth’s deep interior as the core is thought to be a reservoir for the primordial isotope 3He. High 3He/4He ratios in ocean island basalts fed by mantle plumes are indicative of relatively undegassed reservoirs preserved in the deep mantle. Notably, ocean island basalts with tungsten isotopic and 3He/4He anomalies point to possible material contributions from the core. However, it remains unclear how helium is transported from the core to the mantle. Here we use first-principles calculations to show that helium strongly favours entering magnesium oxide at core–mantle boundary conditions. This suggests that magnesium oxide exsolved from the core can deliver appreciable amounts of helium back into the mantle. We also modelled the expected helium flux due to magnesium oxide exsolution since core formation, showing that magnesium oxide exsolved from the core may have continuously supplied helium from the core into the mantle throughout much of Earth’s history, imprinting its primordial helium signature into the mantle materials. Moreover, magnesium oxide exsolution may inherit other distinct geochemical signatures from the core, thereby offering a pathway for probing core processes.
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Data availability
Authors can confirm that all relevant data are included in the paper and Supplementary Information files. The trajectory of exsolution simulation has been uploaded to the Open Science Framework (https://osf.io/ufxd8/)56. Data of MgO exsolution rates are taken from refs. 33,43.
Code availability
The Vienna Ab Initio Simulation Package (VASP) is a proprietary software available for purchase at https://www.vasp.at/.
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Acknowledgements
We thank L. Stxirude, B. Karki, C. Jackson and M. Jackson for discussions. Z.D. expresses thanks for the funding from National Natural Science Foundation of China (no. 42150102) and from the Strategic Priority Research Program (B) of the Chinese Academy of Science (grant no. XDB18000000). The simulations presented in this article are performed on computational resources managed and supported by Princeton Research Computing, a consortium of groups, including the Princeton Institute for Computational Science and Engineering (PICSciE) and the Office of Information Technology’s High Performance Computing Center and Visualization Laboratory at Princeton University. High-performance computing resources are also made available partially from Beijing PARATERA Technology Co., LTD.
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J.D. and Z.D. conceived the project. J.D. performed ab initio computations. Both authors discussed the results, performed the modelling and co-wrote the paper.
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Extended data
Extended Data Fig. 1 Two-phase simulation of He partitioning at core-mantle boundary conditions.
(a) A snapshot at 47520 fs of two-phase coexistence simulations of Mg64O64He4 solid and Fe96He4 liquid at 4000 K and ~135 GPa. (b) The corresponding instantaneous coarse-grained density profile (blue dots) along the z-axis of the simulation box and the best fitting curve using Eq. 2. The two dashed lines (z0 and z1 in Eq. 2) are the locations of Gibbs dividing surfaces. The red shaded regions are interfaces.
Extended Data Fig. 2 Evolution of helium distribution in two-phase simulations.
Variations of numbers of He atoms in MgO (red) and Fe (blue) are derived from Gibbs dividing surface method. a, Mg64O64He8Fe96 with solid MgO. b, Mg48O48He12Fe96 with liquid MgO.
Extended Data Fig. 3
Partial radial distribution functions of the two-phase coexistence simulations. The system consists of liquid Mg48O48 and liquid Fe96He12. The simulation is performed at ~135 GPa and 4000 K.
Extended Data Fig. 4 Trajectories of He atoms of simulations with different initial configurations shown in the Fig. 1.
(a) a slab of Mg48O48 liquid in contact with a slab of Fe96He12 liquid. (b) a slab of Mg64O64 crystal containing four He atoms in contact with a slab of Fe96He4 liquid. (c) exsolution simulation starting with a nearly homogenous Mg-Fe-O-He liquid (Mg48O48He8Fe96). Trajectories are colored with simulation time. The equilibrium snapshots at 7555 fs, 47520 fs, and 42333 fs are also shown underneath the trajectories.
Extended Data Fig. 5 Molecular dynamic simulation of spontaneous MgO exsolution from a Mg-He-Fe-O mixture.
Evolution of (a) the internal energy and (b) the number of He atoms in the MgO (red) and Fe (blue) with time. Here, the number of He atoms is estimated based on its local environment without differentiating the interface, solid, and liquid regimes (that is, the empirical counting method described in Methods). Four snapshots at 1 fs (c), 6662 fs (d), 12204 fs (e), and 42333 fs (f) are also shown with the corresponding energies arrow-indicated in (a). (c) Initial configuration. (d) The whole system is molten with MgO liquid and Fe liquid demixed. (e) MgO patch to the left is crystallized with an ordered atomic arrangement while the MgO patch close to the center is still molten and disordered. (f) Both MgO patches are crystalized. The energy drops between (c) and (d), and between (d) and (e) are due to the crystallization of MgO. The partition coefficient of He estimated from the trajectory after both patches are crystalized is 0.14 based on the empirical counting method and is corrected to 0.04 (Methods and Supplementary Table 1). The simulation cell is 7.8 Å × 7.8 Å × 28.9 Å.
Extended Data Fig. 6 Mean square displacements of helium.
of (a) Mg48O48He liquid (dashed lines) and solid (solid lines), and (b) Fe64He liquid at 3000 K (blue), 4000 K (green), and 6000 K (red). The duration of simulations is 10000–20000 fs. The self-diffusivities of He are calculated by fitting the MSDs beyond 100 fs to half of the simulation duration (Supplementary Table 3).
Extended Data Fig. 7 Schematic of a model setup for He flux across the core-mantle boundary (CMB).
We assume the core is fully mixed with a uniform He concentration Cc in black line. He concentration just above CMB in the mantle is denoted with Cm in red line with Cm/Cc = DFe/MgO. MgO is exsolved just beneath CMB, advected to and accumulated at z = 0. Thick red arrows represent He flux by MgO at z = 0. We note that z = 0 is a moving boundary, defined as the bottom of the last formed MgO layer.
Extended Data Fig. 8 Mixing model showing correlations between 3He/4He ratios and µ182W of OIB samples.
Mixing between (1) ambient mantle, (2) primitive reservoir, and (3) MgO exsolution. W concentrations in MgO are determined assuming DFe/MgO,solid (W) = 0.1 (a) and 1 (b), respectively (Supplementary Table 5). (c) Enlarged red dashed box in (a). Percentages given are the proportion of MgO exsolution mixed with the primitive reservoir. OIB samples compiled by ref. 14 are also shown in filled circles with 2 SD uncertainties. Refer to ref. 14 and references therein for detailed discussion on the uncertainties of isotopic compositions of OIBs.
Extended Data Fig. 9 Fitted pressure-volume results.
(a) Mg48O48 liquid (solid curves), Mg48O48He liquid (dashed curves), and (b) Fe64 liquid (solid curves), Fe64He (dashed curves) at 4000 K(blue), 5000 K (green), and 6000 K (red).
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Supplementary Discussions 1–3 and Tables 1–5.
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Deng, J., Du, Z. Primordial helium extracted from the Earth’s core through magnesium oxide exsolution. Nat. Geosci. 16, 541–545 (2023). https://doi.org/10.1038/s41561-023-01182-7
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DOI: https://doi.org/10.1038/s41561-023-01182-7
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