Possible formation of hydrogen-rich layer in the topmost outer core by deeply subducted water

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Possible formation of hydrogen-rich layer in the topmost outer core by deeply subducted water

https://doi.org/10.21203/rs.3.rs-1425697/v1

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A thin layer with a lower velocity has been documented at the topmost outer core (Eʹ layer). However, the observation has been difficult to explain with a light element concentration change. Here, performing laser-heated diamond anvil cell experiment, we report the formation of FeHx and SiO2 from a reaction between water and FeSi alloy at the conditions relevant to the Earth’s core-mantle boundary (CMB). If water has been delivered to the CMB, this reaction could enable the exchange of hydrogen and silicon between the mantle and the core. As a result, a H-rich, Si-deficient layer formed at the topmost core would have a lower density, stabilizing the stratification, and a lower velocity, consistent with seismic studies. Therefore, the Eʹ layer may result from the deep transport of water to the CMB.

Seismic studies have shown that the density of the Earth’s outer core is ~10% lower than that of pure iron-nickel alloy¹. In order to explain such a density deficit, significant amounts of light elements are required in the core. Although the exact make-up of the light elements in the Earth’s core is still debate², silicon has been proposed as a potential candidate^3,4. Nonetheless, the density deficit in the Earth’s core cannot be explained by Si alone^2,3,5.

Hydrogen has also been considered as a potential light element in the core because it becomes increasingly siderophile with pressure⁶. Hydrogen also strongly favors metallic liquid over silicate melt, suggesting large partitioning of H into the iron alloy during core formation⁷. For H in the core, it is also important to consider possible deep transport of water (or hydrogen) to the CMB region. Since the onset of plate tectonics, volatiles (e.g., water) could have been delivered to the deep mantle through subducting slabs⁸. Studies suggested that low-temperature subduction (i.e., cold subduction) might have started between 2.0 and 2.5 Gya^9,10 and the amount may have not changed much since then¹¹.

Most water in the subducting slab is cycled back to the surface via the breakdown of hydrous (i.e., water-containing) minerals at shallow depths, by ca. 300 km¹². However, some recently discovered dense hydrous minerals (e.g., FeO₂H_x, δ-AlOOH and hydrous stishovite) are stable at the pressure-temperature (P-T) conditions of the lower mantle and therefore may play an important role in deep transport^13,14. Seismological and geodynamical studies have shown that slabs could have accumulated at the bottom of the lower mantle^15,16. Due to a rapid increase in temperature with depth in the D” region, the temperature may reach a sufficient level for the dehydration of the hydrous minerals^14,17. The released water can then induce reactions between mantle and core materials at the CMB.

At the mantle side of the CMB, strong lateral variations in seismic properties have been identified¹⁸, suggesting chemical heterogeneities in the region. While subducted materials may be responsible for some of the structures¹⁹, in situ formation of chemical heterogeneities at the CMB has also been proposed^20,21. For the outer core side, a thin layer with a low velocity has been documented in seismology, Eʹ layer²². While the observation has been debated²³, the most recent results favor the existence of the layer²⁴.

Given the fact that the Eʹ layer exists near the contacting interface with the lowermost mantle, a gradient in light element concentration has been considered as an explanation for its origin. For example, if FeO-rich materials from the early magma ocean could be piled up at the lowermost mantle region (e.g., ref.²⁵), oxygen could be supplied to the core²⁶. However, ref.²⁷ found that the seismic observation and dynamic stability of the Eʹ layer are difficult to be satisfied simultaneously by increasing concentration of a single light element as this would decrease the density while increasing the velocity. Although they suggest that reducing the concentration of one light element while increasing another as a plausible explanation, a process which can cause this type of exchange is currently unknown to our knowledge.

H and Si exist in both the mantle and the core, and they are important components of deep subducted materials. Therefore, processes involving these elements at CMB could shed some light into the origin of the Eʹ layer. In this article, we report chemical reaction between Fe-Si alloys (Fe_0.7Si_0.3 or Fe_0.91Si_0.09) and water (or Mg(OH)₂) at the P-T conditions of the deep mantle. The reaction can potentially lead to an exchange of Si and H between the mantle and the core. We also explore how such chemical exchange could lead to in situ generation of chemical heterogeneities at both sides of the CMB.

Fe_0.7Si_0.3 alloy was heated in a water medium to temperatures between 1202 and 2650 K at 20–32 GPa. In the heating runs, the Fe-Si alloy was completely converted to stishovite (SiO₂) and a face-centered cubic (fcc) form of FeH_x (fcc FeH_x) (Fig. 1a). When a limited amount of H₂O was used for higher P-T conditions up to 137 GPa and 3700 K using Fe_0.7Si_0.3 or Fe_0.91Si_0.09 with 2 or 8 wt% Mg(OH)₂ (see Method), we observed the breakdown of Mg(OH)₂ into MgO and H₂O, and subsequent reaction between H₂O and Fe-Si alloys to form: fcc FeH_x, high-pressure polymorphs of SiO₂, and the lower-mantle magnesium silicate phases (Figs. 1b, 1c). For example, after heating to 3601 K at 60 GPa, bridgmanite (MgSiO₃) and periclase (MgO) formed with fcc FeH_x and stishovite (Fig. 1b). Between 2810 and 3700 K in the 129–137 GPa range, we found changes in the reaction products from stishovite to CaCl₂-type and then to seifertite (α-PbO₂)-type. We also found bridgmanite coexisting with post-perovskite at the highest P-T conditions of this study (Fig. 1c)^28,29. The fcc FeH_x phase existed as the major iron phase at the entire P-T range we covered (Table S1). We occasionally observed weak X-ray diffraction spots from dhcp FeH, FeH₃ or FeO₂H_x phases in diffraction patterns.

The unit-cell volume of the fcc FeH_x phase is systematically larger than that of pure fcc Fe metal over the entire P-T range of our experiments (Fig. 2). The magnitude of the volume expansion was, in most cases, consistent with x = 1 for FeH_x^33,34, regardless of whether it is formed from reaction with H₂O or Mg(OH)₂ (Fig. 2b). It should, however, be noted that when quenched from temperatures at least 1000 K above the expected melting of fcc FeH_x the volume of fcc FeH_x is larger than those quenched from temperature below the melting temperature (Fig. 2b). It is likely because of higher H solubility in molten Fe metal (Fig. S1, see Text S1). H content in FeH_x does not depend either on the H₂O or Si content in the starting samples; we observed the same chemical reactions and products under different silicon (9 mol% vs. 30 mol%) or water (2 or 8 wt% vs. saturated) contents (Table S2).

We have probed selected cross-sections of the recovered samples using focused-ion beam and electron microscopy (Figs. 3, S2-S4). Figs. 3a-3c shows reaction products from the homogeneous mixture of Fe_0.7Si_0.3 and Mg(OH)₂ (as 2 wt% H₂O) recovered from 2632 K and 61 GPa. As this P-T condition is above the melting temperature of FeH_x, we found a metallic melt pocket at the heating center. Si content increased toward its boundary with Mg-silicate/oxide phases (Fig. 3b). Within the metallic melt pocket, sub-micron sized pores are distributed with their sizes decreasing towards the margin (Fig. 3c). This is interpreted to be the result of hydrogen escape from FeH_x (or its higher H concentration in the hottest central region) during decompression^36,37. We observed a similar chemical separation into a metallic melt and silicate region from the samples recovered from P-T conditions relevant to the CMB (Figs. 3e, 3f). Consistent with the XRD observations, the metallic melt (or fcc FeH_x at 300 K), Mg-silicate/oxide, and SiO₂ phases were found (Figs. 3f, 1c, S1). The morphology of the metallic melt in the heating spot, however, differs from the case at 61 GPa because a different sample configuration was adopted where a separate FeSi layer was in contact with a Mg(OH)₂ layer (see Method).

Because our starting sample materials did not contain any silica, the silicate regions found in the recovered samples must have formed by oxidizing silicon from the Fe-Si alloy. For the samples heated with Mg(OH)₂, a homogeneous mixture of low-Fe bridgmanite [(Mg_0.99,Fe_0.01)SiO₃] and ferropericlase [(Mg_0.99,Fe_0.01)O] was found at the heated spot (Figs. 3b, 3f, and Table S3). A local equilibrium was likely established because both the Mg-silicate/oxide and metallic regions were in contact at (partially) molten conditions during heating. The sample also enabled us to infer the redox conditions of the heated spots (Text S2). The oxygen fugacity (fO₂) of the two different heated spots shown in Figs. 3a, 3e were 4 log unit below iron-wüstite (IW) buffer (Table S3). Studies have estimated at least 2 log unit below IW buffer for the fO₂ of the deep mantle³⁸. We found that sufficient heating at the high-temperature is the key to achieve local equilibrium (Text S3).

Our experiments show that water reacts with Fe-Si alloy at the pressure, temperature, and redox conditions relevant for the Earth’s deep interior. The reaction hydrogenates the Fe liquid while oxidizing Si in the Fe-Si liquid to form silica (Fig. 1). In this reaction, the amount of H₂O required for oxidizing Si can be constrained through a redox reaction: e.g., Si⁰ (metal) + 2H₂O → 4H⁰ (metal) + SiO₂. Furthermore, the amount of H alloyed with Fe metal can be estimated.

Studies have suggested that the bottom of early Earth’s magma ocean could have reached pressures up to 60 GPa³⁹. Our experiments up to 60 GPa and 3650 K could therefore be related to the bottom of the deep magma ocean where iron metal diapirs are expected to pond before sinking through the solidified silicate part to merge with the growing core. If water existed in the magma ocean of the early Earth (Text S4), our experiments show that it would have reacted with liquid metallic Fe and oxidized Si to silica. The amount of water estimated to be in the early magma ocean ranges from a few hundred ppm to 1.8 wt% H₂O^40,41. If 1 wt% H₂O existed in the magma ocean and most of the water reacted with Fe-Si liquid, the amount of hydrogen that could be incorporated into the core would be ca. 6.5 x 10²¹ kg, which is equivalent to ca. 0.34 wt% of hydrogen in the present-day Earth’s core. This value is consistent with an estimation based on partitioning of H between Fe metal and silicate liquid at high pressure⁷.

Although hydrogen could have been incorporated into the core during early processes, it is also feasible that the subducting slabs have supplied a significant amount of hydrogen (or water) to the CMB region over an extended period (Fig. 4a). Recent high-pressure studies have found some hydrous minerals to be stable in the subducting slabs in the lower mantle, although some water loss at the 660-km discontinuity seems to be inevitable due to storage capacity changes in mineral assemblage^42-44. While there are many factors to be considered for the estimation of the amount of water delivered to the CMB (and participated into the reaction), the fraction of the total subducted surface water transported to the CMB and its duration would be important (Fig. 4b). Although these two parameters are uncertain, if 1% of the total subducted water i.e., an annual input of 10¹² kg^ref.12 can be delivered to the core, approximately 10¹⁸ kg of hydrogen could have been supplied over the period of modern-style subduction since 2.5 Gya. For a lower efficiency of 0.1% transport to the CMB, the total mass of delivered hydrogen would be 10¹⁷ kg. If the water delivered to the CMB is released by dehydration and induces the reaction we observed here, H could be incorporated into the outer core while Si in the outer core could be oxidized (i.e., SiO₂) and released to the mantle (Fig. 4a). Silica from the core may not exist as a free phase at the CMB region. Instead, it can react with ferropericlase to form bridgmanite or post-perovskite as observed in our experiments (Fig. 1c).

Because hydrogen has a relatively large effect on the density of iron alloys, the above process could create chemical stratification at the top of the core (Fig. 4a). Flows associated with thermal (or thermochemical) convection in the rest of the outer core are not strong enough to erode such a stable layer, even over geologic time⁴⁵. While mixing hydrogen throughout the entire core would require a huge amount of energy to counteract its buoyancy, segregating hydrogen in a stable layer does not challenge the bulk energetics of the geodynamo. However, a stable layer could affect the structure of the magnetic field observed at the surface and appear in seismic data. Indeed, several studies support the existence of such a stable layer based on anomalously slow seismic wave speeds at the top of the core^22,46 and various geomagnetic observations (e.g., refs.^47-49). However, these studies predict different properties for the stable layer (e.g., thicknesses between 60 and 400 km with varying degrees of stability). Other works argue that a stable layer is not required to explain the available observations, but also cannot be excluded (e.g., refs.^23,50).

To assess the range of possibilities, we computed the Brunt-Väisälä period (T_BV) of a layer with an assumed thickness and extra mass of hydrogen to assess its stability (see Method). Lower values of T_BV represent stronger stratification. Although Si is removed from the core, the resulting metallic liquid is still buoyant with respect to the bulk core because H is added⁵¹. Transferring ca. 2.7 x 10¹⁸ kg of H into the core (and removing the associated amount of Si) could produce a stable layer with a thickness of 400 km and T_BV ~ 48 hours, in agreement with ref.⁴⁹. Even less H (ca. 1.2 x 10¹⁸ kg) is required to produce a stable layer with a thickness of ca. 130 km and T_BV ~ 24 hours as estimated by ref.⁵² (Fig. 4c). The amount of hydrogen required to explain these existing models is well within the range we estimated above between 0.4 to 1% of the subducted water (Fig. 4b). Producing the most stable and thick layer (i.e., ca. 300 km thick, T_BV ~ 1.6–3.4 hours) compatible with ref.⁴⁶ with this process alone, however, would need at least ca. 4.0 x 10²⁰ kg of H, which would require a deep water cycle with an unrealistic efficiency of more than ~90%.

Over a few decades, attempts have been made to explain the Eʹ layer with light element enrichment. However, as pointed out by ref.²⁷, no single light elements can explain the low density and low velocity required for the observed stable Eʹ layer at the topmost core simultaneously. While some combination of light element addition and subtraction can provide explanation, it has been unknown what process can drive such an exchange of light elements.

In the case we present here, the core-mantle exchange process would add H while subtracting Si at the topmost outer core. According to ref.⁵³, an increase in H by ca. 0.04 wt% in the Eʹ layer (i.e., a top region of outer core) would increase the velocity by ca. 0.2% at 150 GPa and 4000 K. Using parameters in ref.⁵, if ca. 0.28 wt% of Si has been removed by 0.04 wt% H addition (see Method, Text S5), the velocity would decrease by ca. 0.72%. Combining these two effects, ca. 0.52% decrease in velocity is expected from the core-mantle chemical exchange we found in our experiments. Such a velocity decrease is within the observed values from seismology, i.e., 0.2–0.7% decrease^22,46,54,55. In addition to the exchange process we used in our model, there could be contributions from other processes, such as chemical exchange between oxygen and silicon (e.g., refs.^26,56), sub-adiabatic heat flow across the CMB (e.g., ref.⁵⁷), and lateral variations in the heat flux (e.g., ref.⁵⁸). Furthermore, input of H from the mantle may induce stratification by immiscible liquids such as Fe-H and Fe-S⁵⁹. If more than a single factor can contribute, the required amount of H₂O for explaining the E’ layer by the chemical exchange could further decrease.

As shown above, the chemical exchange process promoted by subducted water at the CMB could explain the seismic properties and at the same time satisfy the lower density requirement for the dynamic stability of the Eʹ layer. While more studies are desirable, our study reveals the intriguing possibility that the Eʹ layer is the consequence of deep transport of subducted surface water over giga years of deep hydrogen cycle. In addition, our model presented here shows that the mantle and the core systems are not chemically separated over geological time scale, but they may have exchanged some elements even after their differentiations since the early processes.

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Acknowledgements

This work was supported by the Leader Researcher programme (NRF-2018R1A3B1052042) of the Korean Ministry of Science and ICT (MSIT). We also acknowledge the support by grants NRF-2019K1A3A7A09033395 and NRF-2016K1A4A3914691 of the MSIT.

S.-H.S. was supported by NSF grant EAR1338810 and 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.

Author contributions

T.K. and J.L. performed the experiments and T.K. analyzed the XRD, SEM, TEM and EDS data. S.C., V.P., R.J.H., N.G., and H.-P.L. supported synchrotron beamline setups and operations. J.G.O’R. performed the calculations. S.-H.S. and Y.L. supervised the research, discussed the results with T.K., and worked on the manuscript with all authors.

Competing interests

Authors declare that they have no competing interests.

Data availability

All data supporting this study are available at https://zenodo.org/record/6385064#.Yj3ooudBwuV or by contacting the corresponding authors ([email protected] and [email protected]).

Code availability

Codes to reproduce the results are available at https://zenodo.org/record/6383505#.Yj1acE3P1D8.

There is NO Competing Interest.

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published 13 Nov, 2023

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Possible formation of hydrogen-rich layer in the topmost outer core by deeply subducted water

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