Extensive iron–water exchange at Earth’s core–mantle boundary can explain seismic anomalies

Domain: earth_science
Published: 2024-10-15
Editorial outcome: minor_revision

Abstract
Seismological observations indicate the presence of chemical heterogeneities at the lowermost mantle, just above the core–mantle boundary (CMB), sparking debate over their origins. A plausible explanation for the enigmatic seismic wave velocities observed in ultra-low-velocity zones (ULVZs) is the process of iron enrichment from the core to the silicate mantle. However, traditional models based on diffusion of atoms and penetration of molten iron fail to account for the significant iron enrichment observed in ULVZs. Here, we show that the chemical reaction between silicate bridgmanite and iron under hydrous conditions leads to profound iron enrichment within silicate, a process not seen in anhydrous conditions. Our findings suggest that the interaction between the core and mantle facilitates deep iron enrichment over a few kilometres at the bottom of the mantle when water is present. We propose that the seismic signatures observed in ULVZs indicate whole mantle convection, accompanied by deep water cycles from the crust to the core through Earth’s history.

Introduction
At the Earth’s core–mantle boundary (CMB), an environment characterised by extreme pressure and temperature, dynamic interactions between the liquid metallic core and mantle minerals occur. Seismological observations have revealed the presence of heterogeneous structures known as ultra-low-velocity zones (ULVZs) in the lowermost mantle, extending tens of kilometres in thickness. These zones exhibit a significant reduction in seismic wave velocities1–3.

Many studies have shown that iron enrichment in mantle minerals is a significant factor contributing to the properties of ULVZs. For instance, it has been demonstrated that iron-rich mantle minerals exhibit low sound velocities4–7. The diminished seismic wave velocities observed in FeO-rich mantle minerals quantitatively align with those of ULVZs8,9. Several hypotheses have been proposed to explain the origin of iron-rich ULVZs, including a basal magma ocean10 and the subduction of banded iron formations11, both of which involve the increase in iron content. In addition to the hypotheses above, iron enrichment from the core to the mantle also emerges as a compelling explanation for the unique seismic characteristics observed within ULVZs12–14. The gradual decrease in seismic velocities toward the CMB within ULVZs provides additional evidence supporting the notion of core–mantle interactions15.

However, the process of iron enrichment from the core to the mantle encounters significant obstacles due to reaction kinetics. For instance, the low diffusion coefficient of iron in silicate minerals impedes iron enrichment to a degree comparable with the thickness of ULVZs, even when considering the age of the Earth16–18. Additionally, the concept of molten iron penetration into mantle minerals via morphological instability, as observed in the reaction between (Mg,Fe)O ferropericlase and molten iron, has been proposed13,14. Yet, recent experimental studies have shown no evidence of iron penetration occurring within silicate bridgmanite and post-spinel phases, which are the predominant mineral aggregates in the lower mantle19. Thus, both diffusion and penetration mechanisms within silicate mantle minerals fail to account for iron enrichment at a significant distance from the CMB.

Water emerges as a crucial component facilitating active chemical reactions between the core and mantle owing to its siderophile nature and exceptionally high mobility through minerals. Experimental studies suggest that surface water is transported to the deep lower mantle via hydrous phases and nominally anhydrous minerals through plate subduction20–23. FeO H domains resulting from iron–water reactions at the CMB can effectively explain the density and seismic wave velocities of both P- and S-waves in ULVZs24. However, a recent study considering realistic water concentration, the unlimited availability of iron in the core and the limited water supply resulting from mantle downflow revealed that the FeO H phase becomes unstable, leading to the local accumulation of FeO-rich layers at the bottom of the mantle25. Strong partitioning of hydrogen into liquid iron within ULVZs, as revealed in recent studies, also suggests instability of FeO H owing to hydrogen incorporation into the core25,26.

Here, we conducted reaction experiments between bridgmanite and iron using a multi-anvil apparatus. To investigate the impact of water on iron enrichment from the core to the mantle, we compared bridgmanite polycrystals synthesised under hydrous and anhydrous conditions.

Results
Deep iron incorporation via iron–water exchange
The experimental conditions and results are summarised in Supplementary Table 1. Back-scattered electron (BSE) images and elemental mappings of the samples recovered after the experiments are shown in Fig. 1 and Supplementary Fig. 3. In contrast to previous experimental results indicating no reaction layer between molten iron alloy and polycrystalline silicate bridgmanite19, our findings revealed the formation of an FeO-rich layer under hydrous conditions, comprising ferropericlase, iron-rich bridgmanite, and metallic iron (Fig. 1a). Similar reaction layers were observed at lower temperatures, even when iron was solid under hydrous conditions (Fig. 1b–e). The thickness of these layers, several tens of micrometres, exceeds the Fe–Mg diffusion length in bridgmanite at comparable pressure and temperature conditions by several orders of magnitude16,17, highlighting the essential role of water in facilitating the reaction between iron and bridgmanite.

Fourier-transform infrared spectroscopy (FTIR) spectra of recovered polycrystalline bridgmanite before and after the iron–water exchange revealed the reduction of broad peaks in the range of 2800–3500 cm−1, attributed to OH stretching vibrations (Supplementary Fig. 1), thus indicating the likely movement of water components into the metallic iron. In situ X-ray and neutron diffraction studies suggest that water induces iron oxidation and iron hydrogenation to form FeO and FeH under high pressure, following the reaction25,27,28:
3Fe + H O = FeO + 2FeH (1)

Compositional analysis of recovered samples (Supplementary Table 2) demonstrates that FeO generation leads to further partitioning reactions, forming Fe-bearing bridgmanite and ferropericlase, described by the equation:
FeO + MgSiO = (Mg,Fe)O + (Mg,Fe)SiO (2)

A negligible quantity of Al for convenience was ignored in the equation. Thus, the growth of the FeO-rich layer is initiated by water-induced iron oxidation and subsequent excess FeO partitioning among iron and minerals (referred to as iron–water exchange hereafter).

Our experimental findings indicate that iron–water exchange occurred regardless of whether the iron was in liquid or solid form in the presence of water (Fig. 1). The presence of small grains of metallic iron within the FeO-rich layers may be explained by the penetration of liquid iron into the polycrystalline bridgmanite. On the other hand, metallic iron was observed even after solid–solid reactions at lower temperatures (Fig. 1e), suggesting its separation from bridgmanite through a charge disproportionation reaction of ferrous iron (3Fe2+ to 2Fe3+ and Fe0), as Fe3+ is stabilised in bridgmanites with a high Fe3+/Fe2+ ratio29,30. As confirmed through elemental maps (Fig. 1), neither FeSi phase nor hydrous FeOOH phases were observed.

Mechanisms and kinetics of the reaction
According to the iron–water exchange mechanism outlined earlier, the growth of the FeO-rich layer occurs through chemical reactions between water and iron. Consequently, the ratio between the quantity of water and the contact surface area with iron (referred to as the water/interface ratio) is expected to control the thickness of the FeO-rich layer. Figure 2a illustrates the change in rim thickness of the FeO-rich layer as a function of the water/interface ratio. At temperatures where iron is solid, the rim thickness increases with the water/interface ratio. Our experimental results indicate that neither temperature nor annealing time significantly affected the thickness of the FeO-rich layer within the timescales of our experiments when the iron is solid (Supplementary Table 1).

Figure 2b presents the expected diffusion length of iron within bridgmanite and ferropericlase based on their Fe–Mg interdiffusion coefficients at 24 GPa16,31. The calculated diffusion length within bridgmanite at the experimental temperatures and timescales (i.e., within a few tens of hours) is ∼10−4−10−2 μm, rendering it too slow to observe via BSE imaging. Considering such slow diffusivity, the diffusion-controlled iron enrichment within bridgmanite at the bottom of the lower mantle is calculated to be only a few meters16. However, our experimental findings revealed iron enrichment (FeO-rich) layers with thicknesses of ∼101−102 μm (as indicated by the three symbols in Fig. 2b), several orders of magnitude larger than the Fe diffusion length within bridgmanite at comparable pressure and temperature conditions16. Such substantial iron enrichment could be feasible only if Fe–Mg interdiffusion in ferropericlase controlled the reaction (red lines in Fig. 2b). Additionally, grain boundary diffusion may support the progress of the reaction, as suggested by microtextures observed in Fig. 1.

Despite the limited solubility of oxygen in solid metallic iron, liquid iron can absorb a significant quantity of FeO under pressure32,33. Consequently, the thickness of the FeO-rich layer is expected to decrease when iron melts. However, contrary to this simple estimation, the thickness of the layer significantly increased upon iron melting (Fig. 2a). Furthermore, when we utilised the Fe–Ni alloy as the starting material and melted it, its particles migrated into the FeO-rich layer while retaining their original composition (Fig. 1f). In contrast, the flux of Ni from the solid metal to the FeO-rich layer via diffusion was limited due to the higher siderophility of Ni relative to Fe (Fig. 1g)34. These observations suggest the occurrence of liquid metal penetration into the FeO-rich layer, a phenomenon not observed within bridgmanite-rich domains19. Although direct comparison with the previous study13 is difficult due to different experimental conditions and the limited ferropericlase fraction in our study, we observed iron enrichment with a thickness of approximately 100 μm, which is comparable to the previous findings. Thus, the increase in the ferropericlase fraction through the iron–water exchange facilitated the penetration of liquid iron, thereby significantly influencing the reaction mechanism and its kinetics.

Effects of iron–water exchange on seismic properties
The iron–water exchange mechanism involves a combination of two distinct processes (Fig. 3). The first process entails the chemical reaction between H O and Fe, resulting in the formation of FeO at their interface. Previous experiments have demonstrated the occurrence of this reaction under a wide range of pressures, including conditions representative of the CMB25,27. The second process involves the common partitioning of FeO between mantle minerals and the metallic core after the initial reaction32. Thus, both processes are applicable to the interaction between the mantle and core.

Recent investigations suggest that water exhibited a higher affinity for partitioning into molten iron than silicate melt during the early stages of Earth’s evolution26,35. This characteristic of water is likely accentuated at the present CMB owing to its pressure dependence26 and mantle solidification. Consequently, deep water cycles facilitated by mantle convection through hydrous or nominally anhydrous phases cause continuous iron–water exchange throughout Earth’s history. Seismological observations imply that some ULVZs are related to plate subducted to the deep lower mantle36,37. This supports our hypothesis that iron-rich layers can form through the transport of water by subducted slabs and their reaction with the outer core.

Figure 4 illustrates the density, P-wave velocity (V ), and S-wave velocity (V) of FeO-enriched ULVZs at CMB conditions (136 GPa and 4000 K) as a function of water content (See “Methods”). This estimation considered a pyrolytic lowermost mantle with a volume fraction of 80% (Mg,Fe)SiO post-perovskite (or bridgmanite) and 20% (Mg,Fe)O ferropericlase as dominant minerals. Mass balance calculations with a fixed ULVZ volume (3.65×108 km3) predicted changes in mineral proportions and compositions within ULVZs. The supply of H O to the core increased the FeO component and augmented the volume fraction of ferropericlase, leading to the formation of FeO-enriched ULVZs (Fig. 4a, b). Consequently, density increased while V and V decreased as a function of input water content (Fig. 4c−f), which explains the seismic structure of the ULVZs.

The significant reduction in V relative to V in some ULVZs suggests the occurrence of partial melting, a phenomenon commonly known in ULVZs38,39. Iron enrichment via iron–water exchange may trigger partial melting in ULVZs, as increased FeO content substantially lowers the melting temperature of ferropericlase14. Our results indicate that a total water mass of approximately 3×1020 kg, combined with 5 vol.% partial melting of ferropericlase, adequately explains the seismic structure of ULVZs, including the reduction in V and V and the increase in density. This water mass corresponds to ~1/5 of the mass of water in the oceans. Further experimental studies on sound velocity measurements of ferropericlase with different Fe components, along with seismological studies related to ULVZ volume fractions including the discovery of new ULVZs, will contribute to better water quantification.

As discussed, the high Fe diffusivity in ferropericlase relative to silicate minerals drives the growth of FeO-rich regions. However, even considering the rapid diffusivity in ferropericlase-rich domains, achieving deep iron enrichment over a few kilometres requires extremely high temperatures (>3000−4000 K) at CMB pressure31,40,41. Consequently, the detectability of FeO-rich layers, typically >5 km in thickness, may be localised, only becoming apparent after a sharp increase in subducted unit temperature (Fig. 3). Hence, both water content and temperature gradients around the CMB are crucial factors in ULVZ formation, contributing to their complex distribution. ULVZs may contain some metallic iron due to molten iron penetration and charge disproportionation reactions, as observed experimentally. This iron may form FeH and FeO through additional water from the mantle, further aiding the growth of FeO-rich layers.

Through iron–water exchange at the CMB, hydrogen and oxygen are simultaneously incorporated into the outer core alongside FeO enrichment in the mantle. Mass balance calculation suggests that the iron–water exchange that forms the volume of ULVZs does not significantly affect the composition of the whole outer core. However, if we assume a thin, stable layer separated by composition at the topmost outer core, an increase in oxygen and hydrogen by a few percent in the layer can be expected. As highlighted by a recent study42, the incorporation of hydrogen into the core offers a plausible explanation for the formation of a low-velocity layer of outermost core43,44. This alteration in the composition of the uppermost outer core becomes reasonable when considering the dynamic reaction mechanism involved in the iron–water exchange. Thus, the formation of ULVZs via iron–water exchange can be linked to the enigmatic seismic structures observed in the outermost core. We conclude that whole mantle convection coupled with deep water cycling20–24 offers reasonable explanations for the seismic structure observed at the core−mantle boundary.

Methods
Synthesis of bridgmanite under anhydrous and hydrous conditions
Two types of polycrystalline bridgmanite were synthesised as starting materials under hydrous and anhydrous conditions in gold capsules utilising a 3000-tonne multi-anvil apparatus (ORANGE-3000) at Ehime University. The sample assembly was similar to that reported previously45. The hydrous sample was synthesised at 27 GPa and 1900 K directly from a mixture of MgO, SiO , Al O , and Mg(OH) brucite, aiming for a target composition of MgSi Al H O . Conversely, the anhydrous sample was synthesised from MgSiO glass at 25 GPa and 1700 K. The glass was derived from a mixture of MgO and SiO , melted in a high-temperature furnace. XRD spectra confirmed a single phase of bridgmanite in both samples synthesised under anhydrous and hydrous conditions.

FTIR spectra of the polycrystalline bridgmanite synthesised under hydrous conditions exhibited broad peaks in the range of 2800–3500 cm−1, attributed to OH stretching vibrations (Supplementary Fig. 1). Using the method described by Paterson46, the water content of the sample was estimated to be 0.51(11) wt.%, consistent with the quantity of Mg(OH) brucite mixed into the starting oxides. Apart from potential hydrogen substitution mechanisms in bridgmanite47–49, such as Al3++H+=Si4+ and 2H+=Mg2+, trace quantities of superhydrous phase B (indiscernible from XRD) may also contain water in the polycrystalline structure, as indicated by similar FTIR peaks observed with this phase50. Utilising this hydrous starting sample, we conducted reaction experiments between bridgmanite and iron under hydrous conditions.

Reaction experiments
The cell assembly employed for the reaction experiments is depicted in Supplementary Fig. 2. An Re sleeve served as the heater, surrounded by a LaCrO thermal insulator, with gold or MgO used as sample capsules. Temperature monitoring was facilitated using W3%Re–W25%Re thermocouples. WC cubes with a truncated edge length of 4 mm served as second-stage anvils. Pure Fe or Fe–Ni alloy foils, with a thickness of 100 μm, were brought into contact with the synthesised bridgmanite within the capsules. Water content was approximately controlled by the space volume of the sample chambers. Accounting for the surface roughness of the polycrystalline bridgmanite, the volume of bridgmanite was estimated to be half the volume of the sample chamber in calculations. The samples were compressed to 25 GPa at room temperature and then heated to temperatures ranging from 1473 to 2473 K. In runs OT2915 and OT2933, temperatures were estimated using a power-temperature relation due to instability in monitored temperatures above 1200–1400 K.

Quenched recovered samples were polished perpendicular to the sample interface. The microtexture of the reaction rim, elemental maps for Fe, Mg, Si, Ni, and O, and the chemical compositions of the present phases were acquired using a field-emission scanning electron microscope (FE-SEM, JSM-IT500HR) with an energy-dispersive spectrometer (EDS).

Estimation of the seismic structure of the FeO-rich layer
The iron–water exchange initiates through a chemical reaction between H O and Fe, yielding FeO and FeH . The reaction can be expressed as follows:
H O + 3Fe = XFeO [mantle] + (1−X)FeO [core] + 2FeH [core] (3)
where XFeO [mantle] represents the FeO component partitioned into the mantle, and (1−X)FeO [core] and FeHx [core] denote the FeO and FeH components dissolved into the core, respectively. For convenience, we fixed the X value at 0.5 in our calculations, as per previous studies32,51,52. From Eq. (3), the FeO component within the ULVZs increased and the corresponding amount of ferropericlase and post-perovskite in the ULVZs were subtracted according to their respective volume ratios.

We assumed a simplified pyrolytic lowermost mantle comprising 80% volume fraction of (Mg,Fe)SiO 3 post-perovskite (or bridgmanite) and 20% (Mg,Fe)O ferropericlase as dominant minerals53, which approximately reproduces the density and sound velocities of the Preliminary Reference Earth Model (PREM). To assess the effects of the increased FeO on the mineral proportions and compositions of minerals in ULVZs (Fig. 4a, b), we employed mass balance calculations:
V ULVZ = W fp / ρ fp + W ppv / ρ ppv (4)
ρ fp = (1−Fe# fp) ρ MgO + Fe# fp ρ FeO (5)
ρ ppv = (1−Fe# ppv) ρ MgSiO3 + Fe# ppv ρ FeSiO3 (6)
where V ULVZ represents the fixed volume of ULVZs (3.65×108 km3), based on a thickness of approximately 20 km over about 12% of the surface at the CMB3,11. The partition coefficient D of FeO between post-perovskite and ferropericlase ((Fe/Mg) / (Fe/Mg) ) is fixed at 0.2 in the calculation. In addition, we performed calculations using D values of 0.05 and 0.5 to explore the sensitivity of our results to variations in FeO partitioning54–56 (Supplementary Fig. 4). W fp and W ppv are the weight of ferropericlase and post-perovskite in the system, respectively. Fe# fp and Fe# ppv denote the Fe/(Fe+Mg) values in ferropericlase and post-perovskite, respectively, with an initial Fe# fp value of 0.18 used in our calculation. ρ MgO, ρ FeO, ρ MgSiO3 and ρ FeSiO3 represent the densities of minerals with hypothetical compositions at 135 GPa and 4000 K.

The iron–water exchange increases the FeO contents of (Mg,Fe)O ferropericlase and (Mg,Fe)SiO post-perovskite or bridgmanite (Fig. 4b). It is known that an increase in Fe/(Mg+Fe) in ferropericlase from 0.2 to 0.4 decreases its melting temperature by at least a few hundred kelvin14, potentially crossing the CMB temperature threshold and triggering partial melting in ULVZs.

Figure 4c–f and Supplementary Figs. 4, 5 summarise ρ, V , and V of FeO-enriched ULVZs as a function of input water, based on changes in the mineral fraction and composition at 135 GPa and 4000 K. The references used for the sound velocities of each phase are detailed in Supplementary Table 3. We have used the reference values taken from the theoretical calculations in the previous study6,7. Linear compositional dependence on the elastic parameters of given phases was assumed. This estimate roughly reproduces the V p and V s obtained from experiments with a composition of Fe Mg O (Supplementary Fig. 6), which is the closest available experimental data to the required composition range8,9,57. Velocities of bulk rock are made by Voigt-Reuss-Hill average. The effect of 5% partial melting on FeO-enriched ULVZs (dashed lines in Fig. 4a, b) was calculated based on the equilibrium geometry model58, assuming a dihedral angle of 20°. The melt composition ((Fe ,Mg )O) and density (7.5 g/cm3) were fixed59.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of the manuscript by Kawano et al.\nKawano et al. performed the high P-T experiments at 25-27 GPa and 1473-2473K under hydrous and\nanhydrous conditions to try explaining ULVZs at CMB. I like the idea, but their implication strongly replies\non overmuch estimation, assumption and incorrect citation, e.g., extrapolating the experimental data at\nand Vs of Brg (or PPv). I cannot recommend this manuscript for publication in Nature\nCommunications.\nMy comments/suggestions see the below:\nOne of main assumptions is that the reactions of 3Fe + H2O = FeO + 2FeH and FeO + MgSiO3 = (Mg,Fe)O\n+ (Mg,Fe)SiO3 must happen when water meets Earth’s core. But, the exsolved ferrous (Fe2+) could\nundergo disproportionation (Fe2+  Fe0 + Fe3+) at CMB conditions. If that, how does Fe3+ affect your\nassumed reaction? Can it still efficiently work on ‘Fe diffusion’ to Brg or Fp?\nNishi et al. (2017) experimentally showed pyrite-type FeOOH stabilizing at the CMB conditions, which is\nconfusing to me why the authors claimed Fe + H2O -> FeO + FeH, rather than FeO2Hx in this study.\nActually, the reaction of FeO + H2O -> FeO2Hx + FeHx should be the result, which has been studied and\nused to explain ULVZs (e.g., Liu et al., 2017 Nature). Therefore, ‘the idea’ is not new anymore. Again, the\nexperimental conditions in this study (25-27 GPa) are far away from the stable field of FeO2Hx ( >~90\nGPa), where the authors tried to make the implication. How is possible to tell a story happening at CMB\nwhereas using the data from >2000 km shallower?\nTheir estimations of seismic velocities of Brg (or PPv) could be totally wrong in Fig. 4c,d and Table S3.\nTheir velocity result fully depends on the extrapolation of sound velocities data of olivine at the\nextremely low P-T conditions compared to the CMB’s (Zhang et al., 2016). I cannot imagine how big error\nat their estimations could be because of the different mineral phases, different P-T conditions. It would\nbe much more convincing if the authors could provide the experimentally measured velocity data of\nBrg+Fp at their targeted P-T conditions.\nLines 299-300: “D is the partition coefficient of FeO between post-perovskite and ferropericlase\n((Fe/Mg)ppv / (Fe/Mg)fp), fixed at 0.4 in the calculation.” In general, D ranges at ~0.4-0.9 at lower mantle\nP-T conditions (Irifune et al., 2010; Prescher et al., 2014; Sinmyo and Hirose, 2013), but a mantle heavily\nenriched in Fe2+ would have a D of <0.1 (Nakajma et al., 2012). The implication of this ms actually fits\nbetter with the second case – D should be much lower than 0.4. What will the results be if the authors\napply D of 0.1 or 0.9? Still matching the volume of ULVZs?\nLines 109-111: What is the effect of pressure on the rim thickness of the FeO-rich layer? Will it become\nthinner or thicker at the CMB conditions than your estimation from the experimental conditions at the\ntopmost lower mantle?\nLines 112-114: If I understand Table S1 correctly, T does significantly affect the thickness of the FeO-rich\nlayer, which is different with the description from the authors.", "reviewer_2": "Kawano and co-authors present a series of experiments on bridgmanites interacting with iron(-nickel)\nmetal. By using identical experiments that directly compare the effect of water in bridgmanite, as well as\nthe effect of solid or liquid iron metal, the authors are able to make inferences on how water may affect\nreactions at Earth’s core-mantle boundary. In particular, they show evidence that solid anhydrous\nbridgmanite will not react with iron, while the presence of water seems to induce the formation of FeO\nand FeH through reaction of water with iron. The production of FeO in the form of iron-rich (Mg,Fe)O\nseems to further promote diffusion of iron metal that otherwise would not occur. By varying the amount\nof water (or rather, volume of the hydrous bridgmanite sample), the authors demonstrate that the\nextent of FeO formation and iron penetration into the silicate is controlled by the amount of water\navailable for reaction. Thus, the presence of water may promote iron enrichment of the mantle base,\nproviding a plausible origin scenario for observed seismic structures like ultralow velocity zones that\ncould be explained by iron-rich rock.\nI believe this article would be well suited for publication in Nature Communications after several aspects\nare addressed and revised. Previous experiments have considered some similar reactions, especially\nlooking at anhydrous silicates, anhydrous oxides, and less realistic water-saturated scenarios. This work is\nnovel in its direct comparison of hydrous with anhydrous silicate material, which demonstrates a clear\ndifference in rock-metal interaction and does so for a relatively realistic water concentration. Repetition\nof experiments and reproducibility of the findings is assuring to see. The scientific topic is an active hot\ntopic in Earth science, with many open questions and ongoing work on the nature of the mantle-core\ninterface and the role of water, especially in relation to heterogeneous mantle base structures that are\nactively being discovered by seismic studies. This work is a timely and valuable contribution to these\ndiscussions that I expect will motivate further experiments, especially at higher pressures. I identify a\nfew key areas that need revision, particularly on showing more data, including some missing literature\nreferences, and reworking the interpretation for sound velocities, as well as some questions that need to be\nclarified by the authors before the work can be acceptable for publication.\n1. Chemical analysis\n(a) I am very interested to see more elemental maps reported. I do not see any maps for Mg or O. This\nmakes it difficult to assess the claim that one single phase of iron-rich (Mg,Fe)O is being produced along\nwith Fe metal. This is very important because this claim is the central point of the paper. How can the\nauthors be sure that the bright grains in Fig. 1 are iron metal alloy and not some other iron-rich phase?\nCan the authors provide the full set of elemental maps that were collected for the heating runs that are\nshown in Fig. 1? What about for other experiments, such as the one shown in Table S2? It would\nbe helpful to see all the elemental maps for that experiment (2829). See more on this below (comment 1d).\n(b) In addition, I am curious to hear discussion on hydrous Fe-O-H phases that have been\npreviously\nreported (e.g., ref 14 – Liu et al. 2017 Nature) and to have the authors comment on whether they think\nthe production of such phases is possible in these samples. In other words, without the use of XRD, how\ncan the authors be sure that the new phases formed are iron metal alloy and iron-rich (Mg,Fe)O and not\nsome other phases? How can the authors be sure that phases like exotic Fe-O-H phases and FeSi (e.g.,\nKnittle and Jeanloz 1991 Science, Fu et al. 2023 Nature) are not being formed?\n(c) My understanding is that EPMA should have some finite activation volume in the sample usually at\nleast ~1 um? This would mean that the probe is sensitive to a volume of sample that likely includes\ngrains deeper inside the sample below the surface. Are the uncertainties on compositions reported in\nTable S2 realistic?\n(d) Is there a reason that compositional data is provided for only one sample? It would be very helpful to\nsee compositions for multiple samples, especially those shown in Fig. 1. This would clarify how\nconsistent the resulting compositions of (Mg,Fe)O and bridgmanite are. In particular, does the iron\ncontent of the (Mg,Fe)O phase that is produced depend on the extent of the reaction / amount of\nwater? Further, can the authors report an Fe partitioning coefficient between bridgmanite and\nmagnesiowüstite across the range of samples for which compositions are measured? How do the\ncalculated coefficients compare to previous work by Dorfman et al. (2021) Minerals?\n2. Missing references\n(a) The authors motivate the paper by stating that iron enrichment of mantle minerals is a strong\ncandidate for ULVZs (lines 44-47). However, there are no references included for this in the introduction\nor anywhere else. There is a whole body of recent work about iron enrichment in ULVZs that is\ncompletely absent from the manuscript. This should be corrected in the introduction, which I will\ncomment on here, and then plays into the analysis of sound velocities at the end of the paper, which I\nwill comment on below.\nPapers that should be included at minimum: Wicks et al (2010) GRL; Wicks et al. (2017) GRL; Lai et al\n(2022) G3. These reported low sound velocities for iron-rich (Mg,Fe)O (Wicks) and demonstrated that\nsuch FeO-rich solid rocks can explain ULVZ velocities (Lai). Other papers to be considered are\nDobrosavljevic et al. (2019) Minerals (effect of iron content and methods of sound velocity modeling)\nand Jackson and Thomas (2021) AGU Monograph (broad range of ULVZ locations).\n(b) Other mechanisms have been proposed for production of iron enrichment at the mantle base.\nThese\ninclude in particular a basal magma ocean (Labrosse et al. 2007 Nature) and subduction of banded iron\nformations (Dobson and Brodholt 2005 Nature). These should at least be mentioned in the introduction\nas another possible way to further enrich the mantle base in iron, beyond the mechanism proposed in\nthis study. Authors should also include Knittle and Jeanloz (1991) Science as an earliest experiment\nproposing iron enrichment at the CMB due to reaction of silicates with the core.\n3. Sound velocity calculations\n(a) The choices made for sound velocity calculations are not very clear to me. I understand the point\nabout mass balance due to differing amounts of water. However, the elastic properties used for (Mg,Fe)O\nare listed only as those calculated computationally for pure MgO and for (Mg0.75Fe0.25)O. This is a\nmuch lower iron content that what the authors themselves found experimentally, where the\nexperimental reaction produced an (Mg,Fe)O composition with 65% Fe. This experimental value is much\ncloser to the iron-rich (Mg,Fe)O compositions proposed previously to explain ULVZ signatures (see\nreference list in my comment 2), such as iron levels of 75%-90%. In addition, those\ncompositions have\nexperimentally determined sound velocities and densities. So it is not clear to me why those values are\nnot being used for example calculations. It’s further not clear what the compositions of (Mg,Fe)O are in\nthe calculations for high amounts of water content. Do these exceed the Fe=25% level? It seems they\nmust given how low the calculated bulk sound velocities are. In that case, it is likely that a linear\nextrapolation from MgO and (Mg0.75Fe0.25)O is no longer valid at higher Fe contents. Our community\ndoes not have a good understanding, if any, of iron content effect on sound velocities at intermediate\ncompositions between Fe~25% and Fe~75%. If the authors want to model iron-rich (Mg,Fe)O sound\nvelocities above the Fe=25% compositions, then I believe they should in some way use the\nexperimentally reported sound velocities for very iron-rich (Mg,Fe)O as in the references above. See a\nsuggestion below in comment 3c.\n(b) Further, it is not clear from the methods how exactly the sound velocities are calculated. The authors\nstate in line 308 that “Linear compositional dependence on the elastic parameters of given phases was\nassumed.” There is not enough detail provided. Is this being done for elastic moduli? What assumptions\nare being made for stress and strain partitioning? Calculating velocities of bulk rock requires decisions on\nhow to average elastic moduli, as in the common Voigt and Reuss bounds or the Voigt-Reuss-Hill average.\nAuthors may refer to calculations detailed in e.g., Dobrosavljevic et al., (2019) Minerals and Lai et al.\n(2022) G3. This point is especially important for rocks comprised of minerals with very different elastic\nmoduli, as in bridgmanite and iron-rich (Mg,Fe)O. In such a case, the difference between Voigt and Reuss\nbounds (or uncertainties on V-R-H averages) can be incredibly large (again, refer to e.g., Lai et al. (2022)\nG3). These details need to be explicitly stated at least in the supplementary material if not in the\nMethods.\n(c) It seems to me that the goal of the authors is to show that reasonable amounts of subducted water\ncan produce a high level of iron enrichment through reaction with the core and explain ULVZ properties.\nIn light of my comments above, I think a stronger approach (or at least a strong complement to the\ncurrent approach) would be to graphically show how much iron enrichment in (Mg,Fe)O is produced due\nto such a reaction. If the authors can plot (Mg,Fe)O composition and concentration of (Mg,Fe)O in the\nbulk rock as a function of reacted water, then the authors could simply refer to existing literature on FeO-\nrich rocks as an explanation for ULVZs (see reference list in my comment 2 above). This would be a much\ncleaner message in the discussion and in Figure 4 without needing to go into the many details of sound\nvelocity modeling done in previous work. Authors can also consider plotting the x-axis as units of fraction\nof ocean mass instead of in units of kg (as mentioned in line 168), which are hard to interpret.\n4. Discussion around CMB temperature and melting in the context of ULVZs: recent seismic studies are\nincreasingly finding evidence for ULVZ existence in areas of active subduction where temperatures are\npresumed to be relatively low (e.g., Su et al. 2024 Nature Geoscience, Wolf et al. 2024 Nature\nGeoscience). I think these studies actually strengthen the findings of this paper. Instead of focusing the\ndiscussion so much on high temperature and partial melting, the authors could consider discussing how\ntheir findings actually provide a compelling mechanism to create iron enrichment in regions of\nsubducted slab debris by reaction of hydrous silicate with core fluid. My question to the authors here: do\nthe experimental findings actually require higher temperatures to promote enough reaction for iron\nenrichment? Or can the authors apply the results even to colder regions similarly well to give a viable\nmechanism for CMB iron enrichment and production of iron-rich ULVZs in slab regions? It seems to me\nthe latter can be true. This then is actually basically a novel idea, that water reaction with the core could\nproduce iron-rich ULVZ material in cold subduction regions, perhaps even in the absence of partial\nmelting. I believe such a discussion would improve the impact and significance of these findings quite\nstrongly.\n5. The authors cite the work of Otsuka and Karato (2012) (ref. 8). However, in discussion of the results\n(e.g., line 133) where the authors find penetration of molten iron into FeO-rich (Mg,Fe)O, the authors do\nnot compare their results to the earlier publication. There should be some comment on whether the\nfindings agree with the previous work, which also observed molten iron penetration into (Mg,Fe)O.\nIn\nparticular, is the length-scale of penetration similar or greater than in Otsuka and Karato? If it is higher\nhere due to water, that point should be made. If it is similar, then the point should be clear that a\nmechanism for molten iron penetration has previously been demonstrated. The key difference here is\nthat such iron enrichment can be promoted by the presence of water even in situations where (Mg,Fe)O\ngrains are likely to represent only a small fraction of the bulk rock and thus may be unlikely to\ninterconnect and promote iron penetration / enrichment at the mantle base without the presence of\nwater.\n6. Possible iron disproportionation and the role of nickel: The authors mention that the presence of iron\nmetal in the silicate matrix for experiments where the iron was solid and not molten is evidence for\ndisproportionation of iron in the silicate (Fe2+ -> Fe3+ + Fe0). However, in Fig. 1, there is a faint signature\nof nickel in the silicate matrix in identical locations as where iron enrichment (perhaps as metal, though\nthis is not obvious with oxygen and magnesium maps). The amount of nickel visually seems to be lower\nthan in the case of molten iron, but it is not really clear given that the color scheme of the composition\nmaps can be somewhat arbitrary. Can the authors comment on the presence of nickel in the silicate in\nFig. 1e? And could Ni be implicated in the reaction? What about the production of NiO and NiH and\npossible solid solution of FeO with NiO? Have the authors looked into any possible literature on such an\nidea? This could be an interesting point to speculate on.", "reviewer_author_response": "Response to review of the manuscript “Extensive iron–water exchange\nat Earth’s core–mantle boundary explains seismic anomalies”\nThe reviewer’s comments are shown in black, and our responses are in red.\nResponse to Referee #1\n> Kawano et al. performed the high P-T experiments at 25-27 GPa and 1473-2473K under\nhydrous and anhydrous conditions to try explaining ULVZs at CMB. I like the idea, but\ntheir implication strongly replies on overmuch estimation, assumption and incorrect\ncitation, e.g., extrapolating the experimental data at 25 GPa to CMB (~135 GPa),\npartitioning coefficient of Fe between post-perovskite and ferropericlase, Vp and Vs of\nBrg (or PPv). I cannot recommend this manuscript for publication in Nature\nCommunications. My comments/suggestions see the below:\nReply: We sincerely appreciate the valuable comments concerning this manuscript. The\nmanuscript has been revised following the referee’s comments to convince all readers of\nour conclusions. We apologize for the confusion caused by the incorrect citation of Zhang\net al., 2016 GRL, instead of the correct reference Zhang et al., 2016 EPSL. The correct\nreference, which we have now included in the reference list, uses data directly relevant to\nCMB conditions. Our original calculations were also based on accurate values for CMB\npressures (~136 GPa), and we did not rely on extrapolation from 25 GPa. This issue arose\nsimply due to a citation error in the reference list.\n> One of main assumptions is that the reactions of 3Fe + H2O = FeO + 2FeH and FeO +\nMgSiO3 = (Mg,Fe)O + (Mg,Fe)SiO3 must happen when water meets Earth’s core. But,\nthe exsolved ferrous (Fe2+) could undergo disproportionation (Fe2+ Fe0 + Fe3+) at\nCMB conditions. If that, how does Fe3+ affect your assumed reaction? Can it still\nefficiently work on ‘Fe diffusion’ to Brg or Fp?\nReply: Disproportionation reactions occurred partially after the diffusion of FeO when\nMgSiO reacted with the FeO component. Therefore, Fe3+ does not dominate the iron\n3\ndiffusion itself in the system. Moreover, as shown in additional supplementary Table 2,\nwe observed (Mg,Fe)O in the recovered sample, which should contain ferrous iron even\nafter diffusion. Although we do not deny the possibility of some influence on the kinetics,\nwe consider it certain that partial disproportionation reactions do not strongly prevent Fe\ndiffusion, as evidenced by the rapid iron movement (Fig. 1,2b).\n> Nishi et al. (2017) experimentally showed pyrite-type FeOOH stabilizing at the CMB\nconditions, which is confusing to me why the authors claimed Fe + H2O -> FeO + FeH,\nrather than FeO2Hx in this study. Actually, the reaction of FeO + H2O -> FeO2Hx + FeHx\nshould be the result, which has been studied and used to explain ULVZs (e.g., Liu et al.,\nconditions in this study (25-27 GPa) are far away from the stable field of FeO2Hx ( >~90\nGPa), where the authors tried to make the implication. How is possible to tell a story\nhappening at CMB whereas using the data from >2000 km shallower?\nReply: We carefully considered the reviewer’s comments and realized that we should\nhave explained and discussed the problems of FeO H formation at ULVZs more\n2 x\nthoroughly in the original manuscript. Many previous studies on this phase can confuse\nthe actual chemical reactions at the CMB. To avoid confusion related to FeO H,\nwe\n2\nprovided details of Nishi et al. (2020) in lines 7176 of the revised manuscript.\nFirst, in contrast to Liu et al. (2017), Nishi et al. (2017) did not show the stability of pyrite-\ntype FeOOH (or FeO H ) at the CMB due to the release of H O at high temperatures.\n2 x 2\nInstead, they demonstrated the incorporation of hydrogen into the outer core via the\nreaction FeO + H2O -> FeO + FeH, as stated in their Abstract (Nishi et al., 2017).\nAdditionally, experimental results at CMB pressure conditions using a diamond anvil cell\n(DAC) (Nishi et al., 2020, GRL) clearly indicated the formation of FeO instead of FeO H\n2 x\nunder realistic water concentrations, even within the stability field of FeO H above 70\n2 x\nGPa. As far as we know, FeO H can only appear under H O-saturated conditions and\n2 x 2\nnever coexist with metallic pure iron (not FeH ) (see Fig. 3 in Nishi et al., 2020). Thus,\nx\nwe consider that the reaction FeO + H O -> FeO + FeH should be applicable at the CMB,\n2\nwhere an unlimited availability of iron and a limited amount of water are expected.\nIn the present study, we have uncovered a novel reaction mechanism that facilitates the\nactivation of chemical interactions between minerals and iron under pressure, aided by\nthe presence of minute quantities of water. As written in lines 157163 and 133139 of\nthe revised manuscript, this mechanism includes not only FeO formation but also\nchemical partitioning, rapid diffusion paths of Fe, and liquid Fe penetration. This\nmechanism addresses the longstanding issue of the reaction kinetics within ULVZs.\n> Their estimations of seismic velocities of Brg (or PPv) could be totally wrong in Fig.\n4c,d and Table S3. Their velocity result fully depends on the extrapolation of sound\nvelocities data of olivine at the extremely low P-T conditions compared to the CMB’s\n(Zhang et al., 2016). I cannot imagine how big error at their estimations could be because\nof the different mineral phases, different P-T conditions. It would be much more\nconvincing if the authors could provide the experimentally measured velocity data of\nBrg+Fp at their targeted P-T conditions.\nReply: We apologize for the misleading citation of Zhang (2016, GRL) instead of the\ncorrect reference Zhang et al. (2016, EPSL). In our manuscript, we calculated the sound\nvelocities using data for bridgmanite and post-perovskite at CMB conditions. We did not\nuse data from olivine at lower pressures.\n> Lines 299-300: “D is the partition coefficient of FeO between post-perovskite and\nferropericlase ((Fe/Mg)ppv / (Fe/Mg)fp), fixed at 0.4 in the calculation.” In general, D\nranges at ~0.4-0.9 at lower mantle P-T conditions (Irifune et al., 2010; Prescher et al.,\n2014; Sinmyo and Hirose, 2013), but a mantle heavily enriched in Fe2+ would have a D\nof <0.1 (Nakajma et al., 2012). The implication of this ms actually fits better with the\nsecond case – D should be much lower than 0.4. What will the results be if the authors\napply D of 0.1 or 0.9? Still matching the volume of ULVZs?\nReply: Based on the reviewer’s suggestion and further consideration, we recalculated our\nresults using a D value of 0.2, which we now recognize as more appropriate for our study.\nConsequently, we have revised the manuscript (lines 279282) and Figure 4 to reflect\nthis change. In addition, we performed calculations using D values of 0.05 and 0.5 to\nexplore the sensitivity of our results to variations in FeO partitioning. The results of these\nadditional calculations have been included in Supplementary Fig. 5. As shown in these\nfigures, variations in FeO partitioning within this range do not significantly impact our\nconclusions. This is because the FeO component reduces the seismic velocities of both\npostperovskite (or bridgmanite) and ferropericlase.\n> Lines 109-111: What is the effect of pressure on the rim thickness of the FeO-rich layer?\nWill it become thinner or thicker at the CMB conditions than your estimation from the\nexperimental conditions at the topmost lower mantle?\nReply: As the formation of FeO requires the reaction between Fe and H O in our model,\n2\nthe thickness of the FeO-rich layer in our experiments is controlled by the amount of\nwater supplied at the interface rather than by the pressure, as shown in lines 118126 and\nin Fig. 2a of the revised manuscript. Since the volume of ULVZ can be roughly\ndetermined from seismological observations (lines 277278), we can estimate the water\ncontent assuming a fixed volume of ULVZs (Fig. 4 and supplementary Figs. 5 and 6).\nThrough this estimation, the V , V , and ρ of minerals at the CMB condition were used.\np s\nThe extremely active reaction mechanisms (spanning over a few kilometers as estimated\nfrom the diffusion coefficient at the CMB in the previous studies) found in this study\nmake the above estimation possible (see discussion in lines 127137). This mechanism\nrepresents a significant advancement from previous studies, which only showed the phase\nrelation (e.g., Nishi et al., 2022 GRL; Liu et al., 2017 Nature; Nishi et al., 2017 Nature).\nLines 112-114: If I understand Table S1 correctly, T does significantly affect the thickness\nof the FeO-rich layer, which is different with the description from the authors.\nReply: Thank you for pointing this out. We added the sentence “when iron is solid” in\nline 123 in the revised manuscript. The description was applicable at temperatures where\niron is solid. The discussion is not reasonable at higher temperatures where iron is molten\nbecause iron penetration adittionary occurs.\nResponse to referee#2\n>Kawano and co-authors present a series of experiments on bridgmanites interacting with\niron(-nickel) metal. By using identical experiments that directly compare the effect of\nwater in bridgmanite, as well as the effect of solid or liquid iron metal, the authors are\nable to make inferences on how water may affect reactions at Earth’s core-mantle\nboundary. In particular, they show evidence that solid anhydrous bridgmanite will not\nreact with iron, while the presence of water seems to induce the formation of FeO and\nFeH through reaction of water with iron. The production of FeO in the form of iron-rich\n(Mg,Fe)O seems to further promote diffusion of iron metal that otherwise would not occur.\nBy varying the amount of water (or rather, volume of the hydrous bridgmanite sample),\nthe authors demonstrate that the extent of FeO formation and iron penetration into the\nsilicate is controlled by the amount of water available for reaction. Thus, the presence of\nwater may promote iron enrichment of the mantle base, providing a plausible origin\nscenario for observed seismic structures like ultralow velocity zones that could be\nexplained by iron-rich rock.\n>I believe this article would be well suited for publication in Nature Communications\nafter several aspects are addressed and revised. Previous experiments have considered\nsome similar reactions, especially looking at anhydrous silicates, anhydrous oxides, and\nless realistic water-saturated scenarios. This work is novel in its direct comparison of\nhydrous with anhydrous silicate material, which demonstrates a clear difference in rock-\nmetal interaction and does so for a relatively realistic water concentration. Repetition of\nexperiments and reproducibility of the findings is assuring to see. The scientific topic is\nan active hot topic in Earth science, with many open questions and ongoing work on the\nnature of the mantle-core interface and the role of water, especially in relation to\nheterogeneous mantle base structures that are actively being discovered by seismic\nstudies. This work is a timely and valuable contribution to these discussions that I expect\nwill motivate further experiments, especially at higher pressures. I identify a few key\nareas that need revision, particularly on showing more data, including some missing\nliterature references, and reworking the interpretation for sound velocities, as well as\nsome questions that need to be clarified by the authors before the work can be acceptable\nfor publication.\nWe thank the referee for highlighting the important points of our paper. Moreover, we\nthank you for the valuable comments and careful check of this manuscript. We found the\ncomments below very helpful and have revised the manuscript accordingly.\n1. Chemical analysis\n(a) I am very interested to see more elemental maps reported. I do not see any maps for\nMg or O. This makes it difficult to assess the claim that one single phase of iron-rich\n(Mg,Fe)O is being produced along with Fe metal. This is very important because this\nclaim is the central point of the paper. How can the authors be sure that the bright grains\nin Fig. 1 are iron metal alloy and not some other iron-rich phase? Can the authors provide\nthe full set of elemental maps that were collected for the heating runs that are shown in\nFig. 1? What about for other experiments, such as the one shown in Table S2? It would\nbe helpful to see all the elemental maps for that experiment (2829). See more on this\nbelow (comment 1d).\nReply: We agree with the referee’s suggestion that EDS maps for Mg and O are useful\nfor identifying the phases for readers. We have revised Fig. 1 to increase the resolution\nand added maps for Mg and O in the revised manuscript. Additionally, we have included\nthe maps in the supplementary Figs. 3 (OT2842) and 4 (OT2829). From supplementary\nFig. 4, the presence of Fe metal, (Mg,Fe)SiO , and (Mg,Fe)O phases can be easily\n3\nrecognized. Through this revision, we have replaced the EPMA maps with the EDS maps\nobtained using FE-SEM (as written in lines 253256 of the revised manuscript), as the\nlatter had provided better data.\n(b) In addition, I am curious to hear discussion on hydrous Fe-O-H phases that have been\npreviously reported (e.g., ref 14 – Liu et al. 2017 Nature) and to have the authors comment\non whether they think the production of such phases is possible in these samples. In other\nwords, without the use of XRD, how can the authors be sure that the new phases formed\nare iron metal alloy and iron-rich (Mg,Fe)O and not some other phases? How can the\nauthors be sure that phases like exotic Fe-O-H phases and FeSi (e.g., Knittle and Jeanloz\nReply: We consider that the Fe-O-H phase, reported in the previous studies, is unrealistic\nto appear at CMB because this phase is easily reduced by metallic Fe. Experimental\nresults at CMB pressure condition using diamond anvil cell (DAC) (Nishi et al., 2020\nGRL) clearly indicated the formation of FeO instead of FeOOH (or FeO H ) under\n2 x\nrealistic water concertation even in the stability field of pyrite-type FeOOH above 70 GPa.\nAt lower pressures where εFeOOH (a low-pressure polymorph of pyrite-type FeOOH) is\nstable, FeOOH is reduced by Fe to form FeO (Figure S4 in Nish et al., 2020 GRL).\nIn our samples, the elemental maps are useful to detect FeOOH because this phase does\nnot contain Mg. As shown in Supplementary Figure 4 and Supplementary Table 2 (the\nmost likely condition to appear FeOOH because of the low temperature), the FeO-rich\nphase contains Mg (this phase should be ferropericlase). Additionally, all elemental maps\n(Figs. 1e, 1d, supplementary figs. 3, 4) provided in the revised manuscript suggest that\nthere is no FeSi phase in the FeO-rich layer.\n(c) My understanding is that EPMA should have some finite activation volume in the\nsample usually at least ~1 um? This would mean that the probe is sensitive to a volume\nof sample that likely includes grains deeper inside the sample below the surface. Are the\nuncertainties on compositions reported in Table S2 realistic?\nReply: We have reanalyzed and added the compositions of bridgmanite and\nferropericlase from runs OT 2829, 2933b, and OT 2834b in Supplementary Table 2. For\ncompositional analysis, we used EDS by FE-SEM instead of WDS by EPMA to reduce\nthe overlap in compositional analysis. The uncertainties in the composition of\nbridgmanite are realistic because its grain size (>3 m) is large enough to analyze\nquantitatively. However, despite our efforts, a small overlap with other grains was\nobserved in ferropericlase. We added the sentence “Note that the minute amount of SiO\n2\nand Al O components appearing in ferropericlase are likely due to overlap with\n2 3\nbridgmanite” in the caption of Supplementary Table 2.\n(d) Is there a reason that compositional data is provided for only one sample? It would be\nvery helpful to see compositions for multiple samples, especially those shown in Fig. 1.\nThis would clarify how consistent the resulting compositions of (Mg,Fe)O and\nbridgmanite are. In particular, does the iron content of the (Mg,Fe)O phase that is\nproduced depend on the extent of the reaction / amount of water? Further, can the authors\nreport an Fe partitioning coefficient between bridgmanite and magnesiowüstite across the\nrange of samples for which compositions are measured? How do the calculated\ncoefficients compare to previous work by Dorfman et al. (2021) Minerals?\nReply: We have reanalyzed and added the compositions of bridgmanite and\nferropericlase from runs OT 2829, 2933b, and OT 2834b in Supplementary Table 2.\nAlthough we made an effort to conduct further chemical analysis, it was difficult to obtain\nmeaningful data from the other runs because of the small grain size of ferropericlase. The\nobtained partition coefficients (D = 0.050.22, supplementary Table 2) are roughly\nconsistent with those previously reported by Dorfman et al. (2021). We recalculated our\nresults using a D value of 0.2 (we used 0.4 previously), which we now recognize as more\nappropriate for our study. Also, we performed additional calculations using D values of\n0.05 and 0.5 to explore the sensitivity of our results to variations in FeO partitioning\n(Supplementary Fig. 5 and lines 280282 of the revised manuscript). These different D\nvalues do not impact our conclusions, and our conclusion remains the same: extensive\niron-water interaction can account for the formation of ULVZs.\n2. Missing references\n(a) The authors motivate the paper by stating that iron enrichment of mantle minerals is\na strong candidate for ULVZs (lines 44-47). However, there are no references included\nfor this in the introduction or anywhere else. There is a whole body of recent work about\niron enrichment in ULVZs that is completely absent from the manuscript. This should be\ncorrected in the introduction, which I will comment on here, and then plays into the\nanalysis of sound velocities at the end of the paper, which I will comment on below.\nPapers that should be included at minimum: Wicks et al (2010) GRL; Wicks et al. (2017)\nGRL; Lai et al (2022) G3. These reported low sound velocities for iron-rich (Mg,Fe)O\n(Wicks) and demonstrated that such FeO-rich solid rocks can explain ULVZ velocities\n(Lai). Other papers to be considered are Dobrosavljevic et al. (2019) Minerals (effect of\niron content and methods of sound velocity modeling) and Jackson and Thomas (2021)\nAGU Monograph (broad range of ULVZ locations).\nReply: We agree with the referees’ comments regarding the missing references on iron\nenrichment in mantle minerals as a strong candidate for ULVZs. These papers clearly\nsupport our research motivation, demonstrating that iron enrichment in mantle minerals\nis a strong candidate for ULVZs. We have included key references (Wicks et al., 2010\nGRL; Wicks et al., 2017 GRL; Lai et al., 2022 G3; Dobrosavljevic et al., 2019) in lines\n4548 in the revised introduction as follows:\nMany studies have shown that iron enrichment in mantle minerals is a significant factor\ncontributing to the properties of ULVZs. For instance, it has been demonstrated that iron-\nrich (Mg,Fe)O exhibits low sound velocities47. The diminished seismic wave velocities\nobserved in FeO-rich mantle minerals quantitatively align with those of ULVZs8,9.\n(b) Other mechanisms have been proposed for production of iron enrichment at the mantle\nbase. These include in particular a basal magma ocean (Labrosse et al. 2007 Nature) and\nsubduction of banded iron formations (Dobson and Brodholt 2005 Nature). These should\nat least be mentioned in the introduction as another possible way to further enrich the\nmantle base in iron, beyond the mechanism proposed in this study. Authors should also\ninclude Knittle and Jeanloz (1991) Science as an earliest experiment proposing iron\nenrichment at the CMB due to reaction of silicates with the core.\nReply: As per your suggestion, we have included references to the basal magma ocean\nmodel (Labrosse et al., 2007) and the subduction of banded iron formations (Dobson and\nBrodholt, 2005) in lines 4951 of the revised manuscript as follows:\nSeveral hypotheses have been proposed to explain the origin of iron-rich ULVZs,\nincluding a basal magma ocean10 and the subduction of banded iron formations11, both of\nwhich involve the increase in iron content.\nWe have also incorporated the reference to Knittle and Jeanloz (1991), who conducted\npioneering experiments proposing iron enrichment at the CMB due to reactions between\nsilicates and the core (line 55).\n3. Sound velocity calculations\n(a) The choices made for sound velocity calculations are not very clear to me. I understand\nthe point about mass balance due to differing amounts of water. However, the elastic\nproperties used for (Mg,Fe)O are listed only as those calculated computationally for pure\nMgO and for (Mg0.75Fe0.25)O. This is a much lower iron content that what the authors\nthemselves found experimentally, where the experimental reaction produced an\n(Mg,Fe)O composition with 65% Fe. This experimental value is much closer to the iron-\nrich (Mg,Fe)O compositions proposed previously to explain ULVZ signatures (see\nreference list in my comment 2), such as iron levels of 75%-90%. In addition, those\ncompositions have experimentally determined sound velocities and densities. So it is not\nclear to me why those values are not being used for example calculations. It’s further not\nclear what the compositions of (Mg,Fe)O are in the calculations for high amounts of water\ncontent. Do these exceed the Fe=25% level? It seems they must given how low the\ncalculated bulk sound velocities are. In that case, it is likely that a linear extrapolation\nfrom MgO and (Mg0.75Fe0.25)O is no longer valid at higher Fe contents. Our community\ndoes not have a good understanding, if any, of iron content effect on sound velocities at\nintermediate compositions between Fe~25% and Fe~75%. If the authors want to model\niron-rich (Mg,Fe)O sound velocities above the Fe=25% compositions, then I believe they\nshould in some way use the experimentally reported sound velocities for very iron-rich\n(Mg,Fe)O as in the references above. See a suggestion below in comment 3c.\nReply:\nWe have revised the manuscript to be clearer regarding the elastic wave velocity\ncalculation in Fig. 4. In our calculation, the composition of (Mg,Fe)O and (Mg,Fe)SiO\n3\nchange as a function of input water, as shown in the original supplementary Fig. 3. Based\non the reviewer’s comments, we recognized that this figure is very important for\nclarifying our calculation model, and we have incorporated it into Fig. 4b, in the revised\nmanuscript.\nWe totally agree with the reviewer’s consideration. However, as shown in Fig. 4b\n(originally Supplementary Fig. 3), the required compositions of (Mg,Fe)O are continuous,\nwith Fe/(Mg+Fe) ratios ranging from 0.2 to 0.6. Although it is known that the FeO\ncomponent reduces the Vp/Vs value for ferropericlase, experimental data are missing for\nsuch specific values, as the reviewer pointed out.\nSince previous experimental data do not cover the required compositions, we have used\nthe reference values taken from the theoretical calculations in the previous studies, and a\nlinear extrapolation for different Fe components was performed (in lines 293-298 of the\nrevised manuscript and revised supplementary Table 3). Fortunately, this estimate\nroughly reproduces the V and V obtained from experiments with a composition of\np s\nFe Mg O, which is the closest available experimental data to the required\n0.78 0.22\ncomposition range (see the figure below and supplementary Fig. 6). Thus, we believe this\napproach is the most appropriate given the current limitations in experimental data, and\nit ensures the reliability of our sound velocity estimations.\nEven if we used largely different values of sound velocities, our model to explain ULVZs\nremains valid because the effect of iron lowering sound velocity is consistent in all\nexperimental and theoretical studies. Nevertheless, as written in lines 190192 of the\nrevised manuscript, further experimental studies on sound velocity measurements of\nferropericlase with different Fe components, combined with seismological studies related\nto ULVZ volume fractions, will contribute to better water quantification.\n(b) Further, it is not clear from the methods how exactly the sound velocities are\ncalculated. The authors state in line 308 that “Linear compositional dependence on the\nelastic parameters of given phases was assumed.” There is not enough detail provided.\nIs this being done for elastic moduli? What assumptions are being made for stress and\nstrain partitioning? Calculating velocities of bulk rock requires decisions on how to\naverage elastic moduli, as in the common Voigt and Reuss bounds or the Voigt-Reuss-Hill\naverage. Authors may refer to calculations detailed in e.g., Dobrosavljevic et al., (2019)\nMinerals and Lai et al. (2022) G3. This point is especially important for rocks comprised\nof minerals with very different elastic moduli, as in bridgmanite and iron-rich (Mg,Fe)O.\nIn such a case, the difference between Voigt and Reuss bounds (or uncertainties on V-R-\nH averages) can be incredibly large (again, refer to e.g., Lai et al. (2022) G3). These\ndetails need to be explicitly stated at least in the supplementary material if not in the\nMethods.\nReply: We acknowledge that the original manuscript did not provide sufficient detail on\nhow the sound velocities were calculated. To address this, we have expanded the Methods\nsection (lines 265267, 278282, 297299) and the supplementary material to include a\nmore detailed explanation. Velocities of bulk rock are made by Voigt-Reuss-Hill average\nas described in line 299 of the revised manuscript. (We used Voight bounds in the original\ncalculations.). As written in the previous reply, the required compositions of (Mg,Fe)O\nare continuous, with Fe/(Mg+Fe) ratios ranging from 0.2 to 0.6. We have used the\nreference values taken from the theoretical calculations in the previous study (Table S3)\nand a linear extrapolation for different Fe components was performed. While we did not\nuse directry the experimental values suggested in the referee’s comments for our\ncalculations, we have cited these papers in lines 4648 and 297299 to provide context\nand support for our approach.\n(c) It seems to me that the goal of the authors is to show that reasonable amounts of\nsubducted water can produce a high level of iron enrichment through reaction with the\ncore and explain ULVZ properties. In light of my comments above, I think a stronger\napproach (or at least a strong complement to the current approach) would be to\ngraphically show how much iron enrichment in (Mg,Fe)O is produced due to such a\nreaction. If the authors can plot (Mg,Fe)O composition and concentration of (Mg,Fe)O in\nthe bulk rock as a function of reacted water, then the authors could simply refer to existing\nliterature on FeO-rich rocks as an explanation for ULVZs (see reference list in my\ncomment 2 above). This would be a much cleaner message in the discussion and in Figure\n4 without needing to go into the many details of sound velocity modeling done in previous\nwork. Authors can also consider plotting the x-axis as units of fraction of ocean mass\ninstead of in units of kg (as mentioned in line 168), which are hard to interpret.\nReply: We appreciate the careful consideration of our study and the constructive\ncomments provided. The figure illustrating the composition as a function of reacted water\nwas already included in supplementary Fig. 3 of the original manuscript. Recognizing the\nimportance of this figure, we have now moved it to Figure 4b in the revised manuscript.\nThis adjustment aims to enhance the clarity of our discussion. As per your suggestion, we\nhave modified the units on the X-axis from kilograms (kg) to the fraction of ocean mass.\nWe also added the sentence “The mass of seawater corresponds to 1.38 × 10²¹ kg.” in the\ncaption of Figure 4.\n4. Discussion around CMB temperature and melting in the context of ULVZs: recent\nseismic studies are increasingly finding evidence for ULVZ existence in areas of active\nsubduction where temperatures are presumed to be relatively low (e.g., Su et al. 2024\nNature Geoscience, Wolf et al. 2024 Nature Geoscience). I think these studies actually\nstrengthen the findings of this paper. Instead of focusing the discussion so much on high\ntemperature and partial melting, the authors could consider discussing how their findings\nactually provide a compelling mechanism to create iron enrichment in regions of\nsubducted slab debris by reaction of hydrous silicate with core fluid. My question to the\nauthors here: do the experimental findings actually require higher temperatures to\npromote enough reaction for iron enrichment? Or can the authors apply the results even\nto colder regions similarly well to give a viable mechanism for CMB iron enrichment and\nproduction of iron-rich ULVZs in slab regions? It seems to me the latter can be true. This\nthen is actually basically a novel idea, that water reaction with the core could produce\niron-rich ULVZ material in cold subduction regions, perhaps even in the absence of partial\nmelting. I believe such a discussion would improve the impact and significance of these\nfindings quite strongly.\nReply: We thank the reviewer for insightful comments and suggestions and for\nreferencing the recent seismic studies by Su et al. (2024) and Wolf et al. (2024), which\nindeed provide compelling evidence for the existence of ULVZs in regions of hydrous\nplate subduction. As the reviewer pointed out, our model provides a reasonable\nexplanation for the formation of iron-rich layers through the simple mechanism of water\ntransport by subducted slabs and their reaction with the outer core. We have included\nthese papers and perspectives in lines 169172 of the revised manuscript, with the\nstatement, \"Seismological observations imply that some ULVZs are related to plates\nsubducted to the deep lower mantle (Su et al., 2024; Wolf et al., 2024). This supports our\nhypothesis that iron-rich layers can form through the transport of water by subducted\nslabs and their reaction with the outer core.\"\nHowever, we also recognize that incorporating partial melting is advantageous when\nconsidering the quantitative aspects of seismic wave velocities and reaction kinetics.\nTherefore, we have retained the discussions on partial melting at high temperatures in the\nrevised manuscript. By addressing both the simpler mechanism and the more quantitative\napproach involving partial melting, we believe our revised manuscript provides a\ncomprehensive and balanced explanation for the formation of iron-rich ULVZs in\ndifferent thermal regimes.\nIn response to the reviewer's comments, we have removed the sentence \"ULVZs are\nprimarily observed in relatively hot regions with large temperature gradients, such as near\nlarge low-velocity provinces, rather than in the coldest regions directly beneath\nsubduction zones\" from the revised manuscript, as this part is difficult to assert\ndefinitively.\n5. The authors cite the work of Otsuka and Karato (2012) (ref. 8). However, in discussion\nof the results (e.g., line 133) where the authors find penetration of molten iron into FeO-\nrich (Mg,Fe)O, the authors do not compare their results to the earlier publication. There\nshould be some comment on whether the findings agree with the previous work, which\nalso observed molten iron penetration into (Mg,Fe)O. In particular, is the length-scale of\npenetration similar or greater than in Otsuka and Karato? If it is higher here due to water,\nthat point should be made. If it is similar, then the point should be clear that a\nmechanism for molten iron penetration has previously been demonstrated. The key difference here is\nthat such iron enrichment can be promoted by the presence of water even in situations\nwhere (Mg,Fe)O grains are likely to represent only a small fraction of the bulk rock and\nthus may be unlikely to interconnect and promote iron penetration / enrichment at the\nmantle base without the presence of water.\nReply: Although direct comparison is difficult due to the different experimental\nconditions and limited ferropericlase fraction, we observed the iron enrichment with a\nthickness of 100 m, which is comparable to the previous study as written in lines\n149152 of the revised manuscript. Although we consider that the length should be\nlimited by water mass, further experimental studies are required to conclude the reaction\nmechanism and kinetics in such a complex system.\n6. Possible iron disproportionation and the role of nickel: The authors mention that the\npresence of iron metal in the silicate matrix for experiments where the iron was solid and\nnot molten is evidence for disproportionation of iron in the silicate (Fe2+ -> Fe3+ + Fe0).\nHowever, in Fig. 1, there is a faint signature of nickel in the silicate matrix in identical\nlocations as where iron enrichment (perhaps as metal, though this is not obvious with\noxygen and magnesium maps). The amount of nickel visually seems to be lower than in\nthe case of molten iron, but it is not really clear given that the color scheme of the\ncomposition maps can be somewhat arbitrary. Can the authors comment on the presence\nof nickel in the silicate in Fig. 1e? And could Ni be implicated in the reaction? What about\nthe production of NiO and NiH and possible solid solution of FeO with NiO? Have the\nauthors looked into any possible literature on such an idea? This could be an interesting\npoint to speculate on.\nReply: We have added maps for Mg and O as well as Ni in Fig. 1e and 1f of the revised\nmanuscript (In the revised manuscript, we have swapped Fig. 1e and Fig. 1f.). The maps\nshow the presence of Ni in the metal. Additionally, compositional analysis (revised\nsupplementary Table 2) has revealed that a small amount of Ni is present in ferropericlase.\nThe Fe/Ni ratio in the ferropericlase is much lower than that in the starting metal material.\nWe consider that when the Fe-Ni alloy is solid (Fig. 1e), the flux of Ni from the metal to\nthe FeO-rich layer via the diffusion was limited due to the higher siderophility of Ni\nrelative to Fe (e.g., Campbell et al., 2009 EPSL) as described in lines 146–147 of the\nrevised version of the manuscript. On the contrary, when we utilised the Fe–Ni alloy as\nthe starting material and melted it, its particles migrated into the FeO-rich layer while\nretaining their original composition. Based on our data and the absence of relevant\nliterature, further discussion regarding Ni, such as the formation of Ni-H, is difficult."}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review of the revised manuscript by Kawano et al.\nIn the revised version, Kawano et al. carefully corrected the wrong citation in the original ms. for the\ncalculation of sound velocities, which is a root for the implication of their study, and made a clear\nexplanation as well in the reply letter. They also corrected the partition coefficient of FeO between post-\nperovskite and ferropericlase ((Fe/Mg)ppv / (Fe/Mg)fp) from 0.4 to 0.2, which is reasonable to their\nexperimental conditions. I would like to recommend this manuscript for publication in Nature\nCommunications.", "reviewer_2": "I would like to thank Kawano and co-authors for their careful work revising the manuscript and\naddressing comments from myself and the other reviewer. I believe that all reviewer comments have\nbeen convincingly addressed and that this manuscript is well-suited for publication in Nature\nCommunications. I include just a few small points and questions for the authors to consider and address\nbefore publication.\n1. The topic of FeO2Hx is something brought up by both reviewers, and the new discussion does a nice\njob addressing these details in a compelling way. In the rebuttal, the authors mention that neither\nepsilon FeOOH nor FeSi are observed in the reaction products, tested using Mg and Si maps. This is an\ninteresting finding that I think is worth mentioning explicitly in the main text, maybe at the end of the\nsecond section (~line 115).\n2. The addition of more EDS maps is very helpful and now clearly demonstrates the presence of both Fe\nmetal and iron-rich (Mg,Fe)O in the bridgmanite matrix. One suggestion and one comment here:\na. The authors could further consider combining Fig. S4 into Fig. 1 to make a large 2-column figure.\nShowing 1c, 1d, and Fig. S4 together in the main text would be even more compelling for readers to\nclearly see the different phases produced. Alternately, authors could even consider making both of these\ntwo separate main figures. There should be plenty of room for an additional figure in Nat. Comm.\nb. I note in Fig. 1e the presence of regions in the quenched Fe-Ni liquid that appear to be something like\nFeO? These are regions with no Ni and no Mg. Can the authors comment on this?\n3. The explanation of sound velocity calculations and revisions made to the manuscript on this topic are\nnicely done. The authors included a figure in the rebuttal showing that modeled sound velocities for\niron-rich (Mg,Fe)O agree well with values extrapolated from experimental work, as reported eg in Lai et\nal. (2022). I would suggest the authors consider including this figure in the supplement. They already\nmention this agreement in the Methods text, and having this figure could be a nice additional illustration\nthat supports the credibility of their approach for modeling sound velocities.\n4. How is total ULVZ volume determined? I see in the Methods that an assumption is made of a 20 km\nULVZ covering 12% of the CMB. This seems somewhat reasonable, but it would be nice to have a\nreference mentioned for this (maybe looking at Yu and Garnero 2018 G3?). It is likely that the true value\ncould be higher due to limited seismic ray coverage of the CMB in existing observations (and active new\ndiscoveries of ULVZs at new locations). This would mean that the total water mass involved in the\nproposed reaction could be higher. Authors could consider mentioning this in the main text (maybe\n~line 189)."}, "author_response": "Response to review of the manuscript “Extensive iron–water exchange\nat Earth’s core–mantle boundary can explain seismic anomalies”\nThe reviewer’s comments are shown in black, and our responses are in red.\nResponse to Referee #2\n> I would like to thank Kawano and co-authors for their careful work revising\nthe manuscript and addressing comments from myself and the other reviewer.\nI believe that all reviewer comments have been convincingly addressed and\nthat this manuscript is well-suited for publication in Nature Communications.\nI include just a few small points and questions for the authors to consider and\naddress before publication.\nReply: We are pleased to hear that the reviewer comments have been convincingly\naddressed and that the manuscript is well-suited for publication in Nature\nCommunications. We appreciated your additional points and questions.\n> 1. The topic of FeO2Hx is something brought up by both reviewers, and the new\ndiscussion does a nice job addressing these details in a compelling way. In the rebuttal,\nthe authors mention that neither epsilon FeOOH nor FeSi are observed in the reaction\nproducts, tested using Mg and Si maps. This is an interesting finding that I think is worth\nmentioning explicitly in the main text, maybe at the end of the second section (~line 115).\nReply: We have added this information at the end of the second section (~line 115). \" As\nconfirmed through elemental maps (Fig. 1), neither FeSi phase and hydrous FeOOH\nphases were observed.\"\n> 2. The addition of more EDS maps is very helpful and now clearly demonstrates the\npresence of both Fe metal and iron-rich (Mg,Fe)O in the bridgmanite matrix. One\nsuggestion and one comment here:\na. The authors could further consider combining Fig. S4 into Fig. 1 to make a large 2-\ncolumn figure. Showing 1c, 1d, and Fig. S4 together in the main text would be even more\ncompelling for readers to clearly see the different phases produced. Alternately, authors\ncould even consider making both of these two separate main figures. There should be\nplenty of room for an additional figure in Nat. Comm.\nReply: We have combined Fig. S4 into Fig. 1 to create a comprehensive figure. This new\nfigure arrangement now includes 1c, 1d, and new 1e, allowing readers to clearly see the\ndifferent phases produced.\n> b. I note in Fig. 1e the presence of regions in the quenched Fe-Ni liquid that appear to\nbe something like FeO? These are regions with no Ni and no Mg. Can the authors comment on this?\nReply: We have added the following sentence to the manuscript to clarify this point in\nthe caption of Fig.1 : \"The quenched Fe-Ni liquid forms metal dendrites due to the\nseparation of FeO.\" This indicates that the FeO regions observed are a result of the\nseparation process during quenching.\n> 3. The explanation of sound velocity calculations and revisions made to the manuscript\non this topic are nicely done. The authors included a figure in the rebuttal showing that\nmodeled sound velocities for iron-rich (Mg,Fe)O agree well with values extrapolated\nfrom experimental work, as reported eg in Lai et al. (2022). I would suggest the authors\nconsider including this figure in the supplement. They already mention this agreement in\nthe Methods text, and having this figure could be a nice additional illustration that\nsupports the credibility of their approach for modeling sound velocities.\nReply: We agreed with the reviewer’s suggestion and have added the Supprimentary\nFig.6 illustrating the agreement between the modeled sound velocities for iron-rich\n(Mg,Fe)O and the experimental values from Lai et al. (2022).\n> 4. How is total ULVZ volume determined? I see in the Methods that an assumption is\nmade of a 20 km ULVZ covering 12% of the CMB. This seems somewhat reasonable, but\nit would be nice to have a reference mentioned for this (maybe looking at Yu and Garnero\ncoverage of the CMB in existing observations (and active new discoveries of ULVZs at\nnew locations). This would mean that the total water mass involved in the proposed\nreaction could be higher. Authors could consider mentioning this in the main text (maybe\n~line 189).\nReply: We have included a reference to Yu and Garnero (2018 G3) and Dobson and\nBrodholt (2005) in the Methods section to provide a basis for the assumption.\nAdditionally, we have mentioned the quantification of water contents, noting that\nseismological studies related to ULVZ volume fractions including the discovery of new\nULVZs will contribute to better water quantification, in lines 191193 of the revised\nmanuscript."}]

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