Abstract
Seismic studies have found fine-scale anomalies at the core–mantle boundary (CMB), such as ultralow velocity zones (ULVZs)1,2 and the core rigidity zone3,4. ULVZs have been attributed to mantle-related processes5,6,7,8,9,10, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone3, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core11, the melting behaviour of Fe–Si–H at relevant pressure–temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe–9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies3,4. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs10. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts12, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.
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Data availability
Datasets supporting the findings of this research have been deposited online at https://doi.org/10.5281/zenodo.7413050. Source data are also provided with this paper as Extended Data Tables. Any additional data are available upon request.
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Acknowledgements
We thank E. J. Garnero and M. Li for their discussion. This work is supported by the NSF-Astronomical Science (AST200567) and the NSF-Earth Science (EAR1921298). We acknowledge the support of GeoSoilEnviroCARS (University of Chicago, Sector 13) for the synchrotron experiments. GeoSoilEnviroCARS was supported by the National Science Foundation—Earth Sciences (EAR-1634415). This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by the Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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S.F. and S.-H.S. conceived the project, S.F., S.C., V.B.P. and S.-H.S. conducted the synchrotron experiments, S.F. analysed the results, and S.F. and S.-H.S. wrote the manuscript. All authors reviewed the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Schematic diagrams of the experimental setup used in this study.
Top and bottom figures show side and top views, respectively, of a DAC loaded with the Fe-9wt%Si alloy foil in the presence of H as a medium. Beveled diamonds with a 120-µm inner culet and 300-µm outer bevel were used for the experiments over 100 GPa. The initial thickness of the compressed foil is less than 7 µm. In this setup, the loaded H can penetrate the compressed sample foil and exist between the grain boundaries.
Extended Data Fig. 2 X-ray diffraction patterns measured before and during heating Fe-9wt%Si in a H-bearing medium at 72.2 GPa.
At temperatures below 1491 K, no melting was observed. At 2150 K, the Si-rich (a molar ratio of Si:Fe≈1) B2 phase appears. The vertical ticks indicate peak positions of the identified phases. The wavelength of incident X-ray beam is 0.3344 Å.
Extended Data Fig. 3 Pressure–temperature conditions of our experiments.
The solid and open circles represent pulsed and continuous heating runs, respectively. Melting curves of the related alloy phases are shown: Fe metal (black)30, Fe-Si alloys containing 7-22 wt% Si (olive)32, FeSi (shaded orange area)29, and FeH (red)31. The error bar is the representative uncertainty (1σ) for the experimental melting curves.
Extended Data Fig. 4 Representative X-ray diffraction patterns of Au loaded for a pressure standard.
We provided patterns measured before and after laser-heating the Fe-9wt%Si alloy in a H-bearing medium at 121.6 GPa. The Miller indices of Au were provided, and “Re” denotes weak peaks from the gasket. The gold was intentionally loaded near the gasket inside the sample chamber to avoid direct contact with the sample foil. Because of the setup, the grain is close to the gasket, which can result in detection of weak diffraction lines from the Re gasket. Au was not heated in our experiments. The Au diffraction peaks do not show any changes before and after heating the sample and we did not find any sign of hydrogenation of gold. The wavelength of the incident X-ray beam is 0.3344 Å.
Extended Data Fig. 5
Pressure-volume relationship of a cubic FeH3 and b the hcp phase observed after laser heating at 300 K. The red, black, and olive circles are from DACs 1, 2, and 3, respectively. The solid and open circles are measured volumes during compression and decompression, respectively. The FeH3 appeared only after heating above 100 GPa. Previous experimental data on pure FeH3 (P14)17 and H-free Fe-9wt%Si (T15)50 were plotted as solid lines for comparison. The dashed line in b is the best fit for the compression data of the hcp phase in DAC 3, which has a 4–5% higher volume than H-free Fe-9wt%Si (see Extended Data Table 2). Uncertainty is the 1σ standard deviation. Some errors are smaller than the size of the symbols.
Extended Data Fig. 6 Si content of the eutectic liquid in an Fe-FeSi binary.
The figure is revised from ref. 27. Data are from refs. 27,32,50,51,52. The large red arrow highlights that the Si content in the eutectic liquid composition of a H-free Fe-Si system decreases with pressure. The grey band shows the expected Si content for the Earth’s outer core. Errors represent 1σ standard deviations and are not shown when smaller than the symbols.
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Fu, S., Chariton, S., Prakapenka, V.B. et al. Core origin of seismic velocity anomalies at Earth’s core–mantle boundary. Nature 615, 646–651 (2023). https://doi.org/10.1038/s41586-023-05713-5
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DOI: https://doi.org/10.1038/s41586-023-05713-5
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