The Earth’s core–mantle boundary presents a dramatic change in materials, from silicate to metal. While little is known about chemical interactions between them, a thin layer with a lower velocity has been proposed at the topmost outer core (Eʹ layer) that is difficult to explain with a change in concentration of a single light element. Here we perform high-temperature and -pressure laser-heated diamond-anvil cell experiments and report the formation of SiO2 and FeHx from a reaction between water from hydrous minerals and Fe–Si alloys at the pressure–temperature conditions relevant to the Earth’s core–mantle boundary. We suggest that, if water has been delivered to the core–mantle boundary by subduction, this reaction could enable exchange of hydrogen and silicon between the mantle and the core. The resulting H-rich, Si-deficient layer formed at the topmost core would have a lower density, stabilizing chemical stratification at the top of the core, and a lower velocity. We suggest that such chemical exchange between the core and mantle over gigayears of deep transport of water may have contributed to the formation of the putative Eʹ layer.
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All data supporting this study are available at https://zenodo.org/record/8404634 or by contacting the corresponding authors.
Codes to reproduce the results are available at https://zenodo.org/record/6383505#.Yj1acE3P1D8.
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Y.L., T.K. and J.L. were supported by the Leader Researcher programme (NRF-2018R1A3B1052042) of the Korean Ministry of Science and ICT (MSICT). S.-H.S. and T.K. were supported by NSF grants EAR1338810, EAR2019565 and AST2108129. They were also supported by National Aeronautics and Space Administration (NASA) grant 80NSSC18K0353. S.-H.S. also benefited from collaborations and information exchange within the Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate. 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 Argonne National Laboratory under contract no. DE-AC02-06CH11357. We acknowledge the support of GSECARS (Sector 13), which is supported by the National Science Foundation—Earth Sciences (EAR-1634415), and the Department of Energy, Geosciences (DE-FG02-94ER14466). Parts of this research were carried out at the P02.2 beamline at PETRA III, and we acknowledge Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. We also acknowledge the scientific exchange and support of the Center for Molecular Water Science (CMWS) at DESY. We acknowledge resources and support from the Goldwater Environmental Laboratory, part of the Chemical and Environmental Core Facilities at Arizona State University (ASU). We also thank A. Chizmeshya at ASU for providing the synthetic lizardite.
The authors declare no competing interests.
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Extended Data Fig. 1 X-ray diffraction patterns showing the reaction products from a, FeSi in a H2O medium (setup 1), b, Fe-9wt%Si sandwiched between hydrous silicate layers (setup 5), and c, Fe-9wt%Si sandwiched between hydrous aluminous silicate layers (setup 6).
X-ray diffraction patterns were collected (a) at 32 GPa and 1282 K (A30A_14), and (b) after temperature quenching from 3040 K at 62 GPa (A15H_18), and (c) at 111 GPa and 3550 K (A15I_29). The X-ray energy was 30 keV for (a) and 37 keV for (b) and (c). The vertical colour bars indicate the positions of the diffraction peaks from the identified phases. The background feature indicated by a double-sided grey arrow in low two-theta region in patterns (b) and (c) is from laser carbon mirrors.
Extended Data Fig. 2 In situ X-ray diffraction patterns measured during laser heating (a, 60 GPa and 3601 K (A30B_36), b, 137 GPa and 3700 K (A15G_28), and c, 68 GPa and 4100 K (A15H_26)) and after temperature quenching (44 GPa and 300 K (a), 123 GPa and 300 K (b), and 50 GPa and 300 K (c)).
In the high-temperature patterns, the broad diffuse X-ray scattering mainly from FeHx melt is highlighted by grey-shaded area. The diffraction peaks of fcc FeHx (with Miller indices) were observed only after temperature quench to 300 K.
Solid and open symbols indicate temperature (T) conditions were higher than melting T of FeHx or lower than melting T of FeHx, respectively. The volume of bcc Fe at 1 bar, measured from FeSi + H2O setup after heating and pressure released 1 bar, is shown as a red cross. Previous results are shown as separate curves or points (FeH and FeH2 (300 K)35; FeH (300 K)36 for higher pressure; bcc Fe77 (black cross at 1 bar); hcp Fe78 (black crosses at high pressures)). ΔT(m) is a temperature difference between the measured T during heating and the melting temperature of fcc FeH37.
Extended Data Fig. 4 The volumes of B2 FeSi at high pressure and 300 K from this study and previous studies79,80,81.
Solid and open symbols indicate temperature conditions were higher than melting T of FeH37 or lower than melting T of FeH, respectively.
Extended Data Fig. 5 XRD pattern and SEM analysis of the recovered sample from laser heating of Fe-9wt%Si sandwiched between two Mg(OH)2 layers.
a, In situ X-ray diffraction pattern, and b, c, chemical analysis obtained from experiments at 65 GPa and 300 K after heated at 1800 K at 74 GPa (P30A_18). The experimental setup was a Fe-9wt%Si foil sandwiched between Mg(OH)2 layers. The scale bars are 7 µm.
Extended Data Fig. 6 SEM analysis of the recovered sample from laser heating of FeSi sandwiched between two Mg(OH)2 layers.
a, A cross-sectional view of the recovered sample after heating at 2810 K and 129 GPa (A15G_18). The two-dimensional elemental distribution maps of b, Fe (red) and Si (cyan), and c, Fe (red) and Mg (green).
Extended Data Fig. 7 SEM-EDS analysis of the recovered sample from laser heating to 4100 K at 68 GPa of Fe-9wt%Si sandwiched between two Lizardite+MgSiO3 layers (setup 5).
a, An SEM image of the cross-section of the recovered hot spot (A15H_26). b, The porous texture at the centre of metallic melt (the black box in a) indicates hydrogen escaped from the metallic melt upon decompression to 1 bar. c, An SEM image of the boundary between the metallic melt and silicate melt. The bottom row shows elemental distribution maps for Fe, Si and Mg obtained for the yellow box in (a). The SiO2 grain was produced by the reaction between Si from the Fe-Si alloy and water from silicate at the boundary between metallic melt and silicate melt.
a, The centre of the heated spot after quenching from 1282 K at 32 GPa (A30A_14). b, The centre of the heated spot after quenching from 1202 K at 28 GPa (A30A_09). Elemental distributions of c, Fe, and d, Si of the area in (b). When H2O medium was used, the recovered samples showed complete conversion of FeSi alloy to SiO2 and FeH, which also agrees with in situ X-ray diffraction result (Extended Data Fig. 1a). The cracks in (a) are caused by decompression. The scale bars are 1 µm.
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Kim, T., O’Rourke, J.G., Lee, J. et al. A hydrogen-enriched layer in the topmost outer core sourced from deeply subducted water. Nat. Geosci. (2023). https://doi.org/10.1038/s41561-023-01324-x