A role for subducting clays in the water transportation into the Earth’s lower mantle

Domain: earth_science
Published: 2024-05-24
Editorial outcome: minor_revision

Abstract
Subducting sedimentary layer typically contains water and hydrated clay minerals. The stability of clay minerals under such hydrous subduction environment would therefore constraint the lithology and physical properties of the subducting slab interface. Here we show that pyrophyllite (Al2Si4O10(OH)2), one of the representative clay minerals in the alumina-silica-water (Al2O3-SiO2-H2O, ASH) system, breakdowns to contain further hydrated minerals, gibbsite (Al(OH)3) and diaspore (AlO(OH)), when subducts along a water-saturated cold subduction geotherm. Such a hydration breakdown occurs at a depth of ~135 km to uptake water by ~1.8 wt%. Subsequently, dehydration breakdown occurs at ~185 km depth to release back the same amount of water, after which the net crystalline water content is preserved down to ~660 km depth, delivering a net amount of ~5.0 wt% H2O in a phase assemblage containing δ-AlOOH and phase Egg (AlSiO3(OH)). Our results thus demonstrate the importance of subducting clays to account the delivery of ~22% of water down to the lower mantle.

Introduction
Subduction zones impose an important constraint on the global circulation of H2O between the surface and the interior of the Earth1,2. A subducting slab is generally composed of sediments, oceanic crust, and a part of the upper mantle in about 1:6:50 volumetric ratio3, featuring distinctive petrological and geochemical processes in each layer. While the net H2O flux into the Earth’s deep interior is dominated by the oceanic crust and upper mantle composed of hydrated altered lithology such as serpentinites2, water transportation and/or mineral-water interaction by the topmost sedimentary layer is of significant importance as it occurs along the interface between the subducting slab and the overriding mantle wedge with a high water-to-sediment ratio, e.g., by about 2:3 (0.9×1015 g/yr:1.4×1015 g/yr) in the fluid-rich sediment in the vicinity of subduction trench4. There are various fluid migration pathways in a subduction zone, i.e., across the slab-mantle interface into the mantle wedge as well as within the subducting slab itself (see below)5. Many studies have revealed the existence of a narrow and highly strained mixing zone, so-called, subduction channel, between the subducting slab and the overlying mantle wedge6,7. The slab-mantle interface preserves metastable hydrous phases and/or fluids generated by a sequence of dehydration reactions during subduction6. Such a slab-mantle interface extends from the Earth’s surface to depths exceeding ~100 km, i.e., beneath volcanic arcs toward the deeper mantle6,7. Within the subducting slab, fluid flux becomes channelized into veins as prograde mineral reactions form open fractures due to negative volume change and associated brittle deformation8. Especially, water could be trapped in a subducting slab as free fluid and subsequently be liberated through some mechanical processes, phase transformations, or differences in dihedral angle due to changing P-T conditions or fluid composition9. In the slab-mantle interface, fluid flux is constrained by the degree of viscous deformation and becomes channelized in ductile shear zones to be aligned to the slab-mantle interface5. In the mantle wedge, fluid flux occurs through pervasive network of microfractures and grain boundaries owing to retrograde and rehydration reactions accompanying positive volume change10. According to Schmidt and Poli11, a substantial portion, ranging from 18 to 37%, of the total subducted water will be lost by the subarc depths of ~80–150 km through dehydration reactions. The released fluid could be trapped as an interstitial component and subsequently transported by the descending slab to depths of at least ~150 km12. Therefore, fluids in subduction zones could persistently infiltrate into the overlying layers and mantle wedge8. Accordingly, within a subducting oceanic plate, metasediments will interact with fluids originating from the dehydration processes in both the crust and mantle layers8.

In a subducting slab, the sediments themselves are mainly composed of hydrated clay minerals abundant in alumina and silica of ~17–21 wt% and ~55–63 wt%, respectively13–16. In contrast, a typical mid-ocean ridge basalt contains less alumina and silica to ~13–17 wt% and ~45–50 wt%, respectively, while these contents are further reduced to ~3–4 wt% and ~40–45 wt%, respectively, in the altered peridotite in the upper mantle3,13,17. Therefore the hydrated oceanic sediments can be represented by the simplified ternary system of Al2O3-SiO2-H2O (ASH)18,19 and thus suggested as the potential source materials for deep water transportation since, in recent studies, breakdown products in the ASH system have been demonstrated to have extended stabilities down to the lower mantle and the core-mantle boundary conditions (>150 GPa)20–23. However, it is surprising that high-pressure and high-temperature (HP-HT) studies of clay minerals are scarce, compared to those of crustal minerals and serpentines, although understanding the stabilities of clay minerals, especially along the water-rich subduction interface, would provide new insights into the origins of the ASH system and related deep H2O transport into the Earth24,25. In our previous work, we have demonstrated that subducting kaolinite (Al2Si2O5(OH)4), one of the representative oceanic clay sediments in the ASH system, increases its H2O transport capacity via super-hydration, i.e., a counter-intuitive mineral transformation in which a hydrated mineral uptakes more water to form a further hydrated mineral; kaolinite becomes super-hydrated (Al2Si2O5(OH)4·3H2O) at a depth of about 75 km along a water-rich cold subduction interface, which subsequently breaks down near 200 km depth to form other minerals in the ASH system26. This work called for the need for reevaluating the overall impacts of subducting clays for the origins of the ASH minerals and water transportation into the deep Earth.

Pyrophyllite (Al2Si4O10(OH)2) is a hydrous clay mineral consisting of a sheet of aluminum dioctahedra sandwiched between two layers of silicon tetrahedra, representing the so-called 2:1 clay mineral group in the ASH system. Similar to kaolinite, a representative clay for the 1:1 group26, pyrophyllite does not possess interlayer cations nor water molecules at ambient conditions and hence is nominally ‘non-expandable’, making it an ideal candidate to examine the possible intercalations of water, i.e., super-hydration, under the subduction interface environment. Pyrophyllite usually occurs in low-grade metamorphosed Al-rich sediments and also in high-pressure/low-temperature metamorphic rocks27. Interestingly, it has been established that pyrophyllite is formed by the reaction between kaolinite and quartz in the pressure and temperature range of 0.1–0.2 GPa and 250–260 °C, respectively28. Previous works also reported that pyrophyllite decomposes into the Al2SiO5-SiO2 polymorph assemblage (andalusite/sillimanite/kyanite-quartz/coesite) at ~2.5–5.0 GPa and 500–900 °C ranges29,30. Pyrophyllite has, however, not yet been investigated under subduction conditions as a reaction product of kaolinite (and quartz) and hence the combined impact of subducting clays in deep water transportation has been unknown. In this work, by using the combination of a resistively-heated diamond-anvil cell (RH-DAC) and in-situ synchrotron X-ray powder diffraction (XRD), we have investigated the stability of pyrophyllite up to ~23 GPa and ~900 °C corresponding to 600–700 km depth range, following cold subduction thermal models, i.e., in a range of cold slab surface geotherms from South Mariana and Kermadec subduction with <5 °C/km to Izu-Bonin subduction with ~6 °C/km models31–35. Typically, cold slab geotherms could represent ~28.5% of the global subduction system35,36, and the P-T conditions applied in this study can account for approximately half of the cold slabs in the contemporary Earth. We observed sequential breakdowns of pyrophyllite involving the formation of further hydrated minerals and stable hydrous phases in the ASH system, to deliver ~5.0 wt% of water across the mantle transition zone (MTZ) down to the lower mantle.

Results
Hydration breakdown of pyrophyllite along cold subduction geotherm
In-situ HP-HT synchrotron XRD experiments on pyrophyllite were performed up to ~23 GPa and 700–900 °C following the cold subduction geotherms of the South Mariana, Kermadec, and Izu-Bonin thermal models35 (Figs. 1 and 2, Supplementary Table 1, and Supplementary Figs. 1–8). In water-rich cold subduction conditions, pyrophyllite transforms into gibbsite (Al(OH)3) and diaspore (AlO(OH)) above 4.4(4) GPa and 405±60 °C, i.e., ~135 km depth, by releasing the silica component as coesite. The co-formation of these hydrated minerals can be explained by their close relative enthalpies of formation at the corresponding pressure range (Supplementary Text 1 and Supplementary Fig. 9a). From a crystal-chemical point of view, both hydrous phases form by the removal of the silicon tetrahedral sheets and then different degrees of hydration and connectivity of the remaining aluminum octahedral sheets (Fig. 2 and Supplementary Text 2). Upon further pressure and temperature increase above 5.9(5) GPa and 495±70 °C, equivalent to ~185 km depth conditions, selective dehydration of gibbsite initiates resulting in the formation of a diaspore + coesite assemblage (Fig. 1). This is also in line with the calculated enthalpies where a crossover in the relative stabilities between gibbsite and diaspore occurs near 5.0 GPa (Supplementary Text 1 and Supplementary Fig. 9a). Under moderately cold slab surface geotherm, i.e., following the thermal model of Izu-Bonin subduction35, for which data have been independently measured and analyzed, pyrophyllite showed the breakdown into diaspore + coesite assemblage above 4.0(2) GPa and 535±40 °C, i.e., ~125 km depth (Fig. 1), without undergoing a phase assemblage region having gibbsite. Under these relatively low thermal conditions, the actual depths for phase change could be altered when reaction kinetics are considered, together with other chemical/phase components existing in a real subducting layer. Above 7.4(5) GPa and 550±80 °C, i.e., ~230 km depth, topaz (Al2SiO4(OH)2) starts to appear in the diaspore + coesite assemblage37,38 (Fig. 1). However, if the temperature increases beyond the diaspore + coesite stability region to >900±50 °C near 6.3(4) GPa, mimicking the stagnation of the cold subducting slab near 195 km depth, dehydration of diaspore is induced resulting in the formation of a different phase assemblage containing kyanite (Al2SiO5) (Fig. 1)37. On the other hand, when the cold slab continues to subduct without being stagnated below 255 km depth, coesite turns to stishovite and forms a phase assembly of topaz + diaspore. Stishovite formed in our experiments is considered to be anhydrous as recent experimental studies have shown that anhydrous stishovite is preferred above 600 °C39–41. After the slab penetrates the upper part of the MTZ near 490 km depth, i.e., ~17(1) GPa and 760±90 °C, a phase assemblage containing phase Egg (AlSiO3(OH)) and δ-AlOOH are formed together with stishovite, which persists down to the bottom of the MTZ near 600–700 km depth conditions, i.e., ~21–23(1) GPa and 730–790±110 °C (Fig. 1).

In order to simulate a more realistic natural subduction conditions, another set of in-situ HP-HT experiments was performed using a solution containing NaCl-MgCl2 (Supplementary Table 1 and Supplementary Figs. 10 and 11). Numerous studies demonstrated that major and trace elements (or solutes) are dissolved in subduction zone fluids (SZFs) and recycled in the deep mantle8,12,42–44. In the SZFs, salts such as NaCl and MgCl2 are identified as important components42–44, and the salinity of such fluid ranges between 5 and 15 wt%45,46. Furthermore, Holland and Ballentine (2006) revealed that subduction of sediment and seawater-dominated pore fluids play an important role in understanding the elemental abundance pattern of heavy noble gases (Ar, Kr, and Xe) in the mantle to be remarkably close to that of seawater47, i.e., recycling of seawater is responsible for the observed heavy noble gas pattern in the convecting mantle as subduction occurs in a seawater-dominated fluid environment. Under such ternary seawater composition, breakdown into the diaspore + coesite assemblage was confirmed above 5.4(2) GPa and 415±30 °C, i.e., ~170 km depth, which was followed by the formation of the topaz + coesite assemblages above 7.1(2) GPa and 500±40 °C, i.e., ~220 km depth, confirming the overall breakdown sequence of subducting pyrophyllite in pure water conditions. In contrast, when the experiment is performed under water-free condition using silicone oil as a pressure-transmitting medium (PTM) along the same cold subduction geotherm, pyrophyllite remained stable up to ~7.0(4) GPa and 585±40 °C, i.e., ~220 km depth, without undergoing the observed hydration breakdown under pure and saline water media (Supplementary Figs. 10 and 11).

Changes in the net water contents of the phase assemblages in the ASH system
As the subducting plates reach the base of the upper mantle, some becomes stagnated while others penetrate, depending on the physical and chemical resistance encountered in the mantle transition zone (MTZ). Nearly ~70% of cold subducting slabs are found to interact with the MTZ as observed by the transition zone-slab morphologies from tomographic studies and their Benioff stress state48,49. In such a case, a subducting channel, which is defined as the topmost sedimentary layer with a thickness of up to ~2–3 km at the interface between the subducting crust and the base of the upper mantle7,50, becomes tectonically distributed and metamorphically deformed in the MTZ51,52. In the upper mantle region, we observed that pyrophyllite in a water-rich cold subducting channel undergoes sequential breakdowns/transformation to form gibbsite + diaspore + coesite near 135 km depth, diaspore + coesite near 185 km depth, and then topaz + diaspore + coesite assemblages near 230 km depth (Figs. 1 and 3a, and Table 1). In the form of structural hydroxyls (OH), pyrophyllite contains ~5.0 wt% H2O, whereas gibbsite represents the most hydrated mantle mineral with ~34.6 wt% H2O, thereby making the formation of the gibbsite + diaspore + coesite assemblage a hydration breakdown reaction, leading to a net water content increase by ~90%, i.e., an increase from net ~5.0 wt% to net ~9.5 wt% (or to net 26.1 wt% counting only the ASH phases, Fig. 4b and Table 1). This would remove a portion of subducting fluid by ~1.8 wt% near 135 km depth (Table 1 and 2). The subsequent disappearance of gibbsite and the formation of the diaspore + coesite assemblage indicates that a dehydration breakdown reaction occurred as the net water content decreased by ~47% back to ~5.0 wt% (Fig. 4b and Table 1). This would then release the same amount of fluid stored from the hydration breakdown near 135 km depth back into the subduction interface near 185 km depth. Throughout the region of the topaz + diaspore + coesite assemblage from ~230 km depth, the water content is preserved and net ~5.0 wt% H2O is transported down to the MTZ (Fig. 4b and Table 1). Under the moderately cold slab surface geotherms, the initial water content of ~5.0 wt% is preserved through the formation of the diaspore + coesite assemblage down to ~125–135 km depth.

When the subducting slab or channel bends within the upper mantle parallel to the MTZ, a phase assemblage containing anhydrous kyanite would be formed from the diaspore + coesite assemblage below ~195 km depth, which would constitute a dehydration breakdown reaction to release H2O by ~2.7 wt% (Fig. 1 and Table 1). On the other hand, when the subducting slab penetrates through the MTZ, another water-preserving transformation would occur to form the δ-AlOOH + phase Egg + stishovite assemblage, which is stable to deliver the net ~5.0 wt% H2O towards the lower mantle (Fig. 4b and Table 1). This is in contrast to the previous results where pyrophyllite has been known to undergo facile dehydration under water-free condition to form kyanite + coesite assemblage near 5.0 GPa and 1200 °C29. We therefore demonstrate that the presence of water/fluid plays a dictating role in the stability of pyrophyllite and induces sequential phase transformations along the ASH system, transporting the initial amount of water as contained in the original pyrophyllite, i.e., ~5.0 wt% H2O, down to the lower mantle in the form of the final phase assemblage of δ-AlOOH + phase Egg + stishovite (Fig. 4b and Table 1).

Changes in the net crystalline density of the phase assemblages in the ASH system
It is also important to correlate the changes in the net water contents to the net crystalline density in the observed phase assemblages (Fig. 4b and Table 1). Throughout the observed breakdown sequence, the net crystalline density increases by a total of ~49.6% to compete with the density of the corresponding depth regions. During the hydration breakdown reaction, the net crystalline density increases discontinuously by ~5.3% from 2.82 g/cm3 in pyrophyllite to 2.97 g/cm3 in the gibbsite + diaspore + coesite assemblage. As gibbsite disappears with the dehydration breakdown into the diaspore + coesite assemblage, the net density increases further by ~6.1% to 3.15 g/cm3, which then decreases marginally to 3.14 g/cm3 upon the water-preserving breakdown to the topaz + diaspore + coesite assemblage. A significant increase in the net density by ~30.2% to 4.10 g/cm3 is then driven by the transformation of coesite to stishovite in the above assemblage. The net density of the topaz + diaspore + stishovite assemblage would then be ~11.3% higher than the average density of 3.72 g/cm3 below the 410 km discontinuity, allowing the assemblage to penetrate into the MTZ. Within the MTZ, the net density increases further by ~1.7% to 4.21 g/cm3 upon the formation of the δ-AlOOH + phase Egg + stishovite assemblage. The expected density of such an assemblage would then be higher by ~2.4% than the average density of 4.19 g/cm3 in the region below the 660 km discontinuity, enabling the subducting channel to be gravitationally favorable to drive its subduction into the lower mantle region.

It has long been known that either of the two representative petrological models of the MTZ, i.e., pyrolite53 and piclogite models54, cannot firmly explain the observed seismological vs models (PREM: Preliminary Reference Earth Model)55,56. With this regard, it has been suggested that the role of a subducting slab may account for the anomalous seismic wave velocity and density changes within the region51,56,57. If the subduction of a sedimentary layer continues down to the MTZ, hydrous mineral assemblages in the ASH system, as observed in our study, could play an important role in impacting the physicochemical characteristics of the region, at least on a regional scale, and account in part the observed seismic anomalies. Our experimental results should be further examined to identify its potential relationship to earthquake nucleation as well as seismic anomalies along the subducting channel down to the MTZ region.

New estimate on the H2O flux from subducting clays in the ASH system
While the H2O flux from hydrous minerals in the subducting crust and upper mantle has been estimated down to ~150–230 km depth range to be in the range of ~1.98–3.16×1014 g/yr (ref. 2), the net H2O contribution from the subducting sedimentary layer has so far been underestimated or overlooked. Based on the average mineral makeup of oceanic sediments (Tables 2 and 3), the initial H2O flux by sediments can be estimated to be ~9.79×1013 g/yr (Fig. 3a). By an assumption that pyrophyllite would be formed from subducting kaolinite and quartz28 and these clay minerals undergo (super-) hydration breakdowns as observed in this and previous studies26 along the coldest thermal models, we can estimate the changes in the average H2O flux of a subducting sedimentary layer down to ~185 km depth (Table 2). By the depth of ~6–15 km, kaolinite and quartz, contained in the initial sedimentary layer by ~30 and ~10 wt%, respectively, would react to form pyrophyllite to account ~30 wt% of the sedimentary layer (Fig. 3). This would change the H2O flux by sediments to ~8.17×1013 g/yr (Table 3 and Fig. 3b). Super-hydration of the remaining kaolinite (~9 wt%) would occur at ~75 km depth to increase the H2O flux of sediments by ~24% (net ~7% increase to ~8.75×1013 g/yr when considering dehydration reactions of other minerals composing sediments, see Supplementary Text 3 and Table 3), which will increase further by ~19% (net ~16% increase to ~1.01×1014 g/yr, see Supplementary Text 3 and Table 3) via the hydration breakdown of pyrophyllite at ~135 km depth (Tables 2 and 3). While super-hydrated kaolinite would remain stable down to ~205 km depth, the dehydration breakdown of the phase assemblage formed from pyrophyllite would occur at ~185 km depth and reduce the H2O flux of sediments by ~19% to ~6.75×1013 g/yr (Tables 2 and 3). Therefore, by the depth of ~185 km, the H2O flux of a subducting sedimentary layer increases in steps and becomes ~8.37×1013 g/yr (maximum ~1.01×1014 g/yr between ~135 and 155 km, see Supplementary Text 3 and Table 3) where contribution from clay minerals in the ASH system, i.e., kaolinite and pyrophyllite, becomes ~64% (Fig. 4 and Tables 2 and 3). Such an amount is ~2/3 of the total water transport capacity by a subducting sedimentary layer, which would then account ~40% of the average water transport capacity by a subducting crust. At depths below ~185 km, breakdowns would occur from both super-hydrated kaolinite and the phase assemblage from pyrophyllite to form assemblages containing diaspore and topaz to deliver ~1.62–2.26×1013 g/yr H2O (Fig. 4 and Tables 2 and 3). This H2O flux would remain preserved through the MTZ region down to the lower mantle whereas dehydration of other hydrous minerals in the sediment is completed by ~350 km (equivalent to ~11 GPa conditions in Table 3).

While the total H2O flux in a subducting slab decreases down to ~60% by the depth of ~185 km (or ~150 km in the case of subducting crust and upper mantle), the H2O flux by the ASH system. i.e., kaolinite, pyrophyllite, and their derivatives, increase by ~122% due to the hydration breakdown of pyrophyllite and super-hydration of kaolinite (Table 2 and Fig. 4c). By the depth of ~250 km (or ~230 km in the case of subducting crust and upper mantle), the total H2O flux of a subducting slab is reduced to ~10% by dehydration (breakdown) of minerals within respective layers. The H2O flux of the composing layers, however, reveals the increasing role of a sedimentary layer as its H2O transport capacity per unit volume increases from ~43% to ~72% while that of a crust decreases from ~54% to ~28% (Fig. 4c and Tables 2 and 3). Within the sedimentary layer, the H2O flux also reveal the increasing role by the minerals in the ASH system as its H2O transport capacity per unit volume increases from ~15% to ~54% while that by non-ASH minerals decreases from ~27% to ~18% by the depth of ~250 km.

Modern-style subduction, as characterized by the formation of blueschist facies, has been operating since Paleoproterozoic (~2.2–2.0 Ga)58. We may infer then about 2.3–2.5% of the current ocean mass could have been transported by the subducting clays in the ASH system into the lower mantle (Tables 2 and 3). Further studies will be necessary to fully understand the role of the subducting sedimentary layer on the global H2O flux into the interior of the Earth as several uncertainties remain pertaining to the possibly overlooked and unknown hydration and dehydration processes of other mineral phases and their geochemical and geodynamical behaviors under diverse subduction environments.

Methods
Sample and initial characterization
Initial characterization of the pyrophyllite (Nowhado, South Korea, Al2Si4O10(OH)2) sample used in this study was performed using synchrotron X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM, JEOL-7800F) equipped with energy-dispersive spectrometer (EDS, Oxford Instruments) (Supplementary Fig. 1). Two-dimensional intensity data from synchrotron XRD were converted into one-dimensional data using the Dioptas Program59, which were then analyzed using the Le Bail method60 implemented in the GSAS suite of programs61. Pyrophyllite at ambient conditions was indexed with the space group C-1 resulting in refined cell parameters of a=5.1506(5) Å, b=8.9468(11) Å, c=9.3792(6) Å, α=92.37(2) °, β=100.13(2) °, γ=88.56(1)°.

In-situ high-pressure and high-temperature synchrotron X-ray powder diffraction
In-situ synchrotron X-ray powder diffraction measurements were performed at the Extreme Conditions Beamline (ECB) P02.2 at PETRA-III, Germany, 3D-XRS and 6D beamlines at PLS-II, South Korea, and 13-BMC beamline and 16-BMD beamline at APS, USA. At beamline P02.2, the X-ray beam from the undulator source was tuned to a wavelength of 0.4834(1) Å (25.650 keV) and focused to 8 × 4 μm2 in size (FWHM) using Compound Refractive Lense (CRL) optics. A PerkinElmer XRD 1621 detector was used to collect diffraction data at distance of ~401 mm from the sample with 10 sec of exposure time. At beamline 3D, the X-ray beam from the bending magnet source was tuned to a wavelength of 0.6886(1) Å (18.005 keV) and focused to 100 μm in size using a double crystal monochromator of bent Si(111) and Si(311) crystals. A Mar345 imaging plate detector was used to collect diffraction data at distance of ~310 mm from the sample with 120 sec of exposure time. At beamline 6D, the X-ray beam from the bending magnet was tuned to a wavelength of 0.6530(1) Å (18.986 keV) and focused to 100 μm in size using a double crystal monochromator and a toroidal mirror. A 2D CCD detector (MX225-HS, Rayonix L.L.C., USA) was used to collect diffraction data at distance of ~253 mm with 60 sec of exposure time. At beamline 13-BMC, the X-ray beam from the bending magnet source was tuned to a wavelength of 0.4340(1) Å (28.568 keV) and focused to 12 × 18 μm2 in size (FWHM) using a KB-mirror. A PILATUS 1M detector was used to collect diffraction data at distance of ~169 mm from the sample with 150 sec of exposure time. At beamline 16-BMD, the X-ray beam from the bending magnet source was tuned to a wavelength of 0.4959(1) Å (25.000 keV) and focused to 4 × 4 μm2 in size (FWHM) using a KB-mirror. A PILATUS 1M detector was used to collect diffraction data at distance of ~250 mm from the sample with 150 sec of exposure time. With a small beam size at DESY and APS, we collected the data with sample movement (3×3 or 5 × 5 grid measurements) at intervals of ~10 µm. As a high-pressure vessel, a symmetric-type diamond-anvil cell (DAC) equipped with a pair of type-I anvils of culet diameter of 300 µm or 500 µm was used in combination with a membrane device for online pressure control. A rhenium gasket of 250 µm in thickness was indented to 40–60 µm, and a hole of 150 µm or 200 µm in diameter was drilled in the center as a sample chamber using an electric discharge machine (EDM)62.

Simultaneous P-T condition was created by using external resistive electrical heaters (a graphite foil heater (at PETRA-III) and a coil heater (at PLS and APS) surrounding the diamond anvils). The RH-DAC setup offers the advantage of providing homogeneous and stable temperature across the entire sample chamber63. During the experiment, we followed the P-T conditions proposed for the South Mariana-Kermadec subduction and Izu-Bonin subduction models to simulate the coldest and moderately cold subduction geotherms, respectively35. Temperature was monitored using a R-type or K-type thermocouple attached to the pavilion of the diamond anvil close to the sample with the maximum uncertainties of ±3 °C. The overall uncertainty in the sample temperature has been estimated by the differences between the recorded thermocouple temperature and the calculated temperature based on the unit-cell volume and the known thermal expansion coefficient of standard materials63. The pressure was calculated using the equation of state of Au pressure marker64,65 included in the sample chamber. More detailed description of this type of RH-DAC setup can be found in the literature63,66. Initially, we compressed the sample to ~1.0–2.0 GPa and then increased the pressure and temperature in increments of ~0.5 GPa and 50 °C up to ~23 GPa and ~900 °C, respectively. During our RH-DAC experiments, the samples were heated for about 12 h (or more) to reach target temperature, where the sample temperature was kept for at least ~20–30 min to ensure no further changes are occurring (Supplementary Table 1).

Computational calculations
The relative forming enthalpies of the mineral phases observed in our experiments were calculated using first-principles method (Supplementary Table 2). We performed density function theory (DFT) calculations implemented in Vienna Ab Inito Simulaton Package (VASP)67. We used atomic potentials generated based on a projector augmented-wave method (PAW)68 and Generalized Gradient Approximations (GGA). In our calculations, a plane wave cutoff energy for the wavefunction was set to 1000 eV. Geometry optimizations were performed using conjugate gradients minimization until all the forces acting on the ions were less than 0.01 eV/Å per atom. K-point mesh with a spacing of about 0.03 Å-1 was adopted. Structural relaxations were performed at various constant volumes, and the calculated energy-volume data at zero Kelvin were fitted to a third-order Birch-Murnaghan equation of state (EOS)69: E(V)=E0 + 9V0B0/16 * {[(V0/V)^(2/3)-1]^3 * B0' + [(V0/V)^(2/3)-1]^2 * [6-4*(V0/V)^(2/3)]} (1)

The fitted parameters at zero kelvin are summarized in Supplementary Table 2. The relationship between the pressure and volume at zero kelvin can be expressed as: P(V)=3B0/2 * [(V0/V)^(7/3)-(V0/V)^(5/3)] * {1+3/4*(B0'-4)*[(V0/V)^(2/3)-1]} (2)

The enthalpies of the starting sample, its breakdown products, and the hydrous pressure medium, i.e., pyrophyllite (Al2Si4O10(OH)2), gibbsite (Al(OH)3), diaspore (AlO(OH)), topaz (Al2SiO4(OH)2), kyanite (Al2SiO5), phase Egg (AlSiO3(OH)), δ-AlO(OH), coesite-stishovite (SiO2) and ice VII, respectively, at pressures in the range of 0–20 GPa were calculated using the equation H=E+PV. Subsequently, the stabilities of these minerals were evaluated based on the relative enthalpies in the following reactions (Supplementary Fig. 9):
Al2Si4O10(OH)2 + 2H2O = 2Al(OH)3 + 4SiO2 (3)
Al2Si4O10(OH)2 = 2AlO(OH) + 4SiO2 (4)
Al2Si4O10(OH)2 = Al2SiO4(OH)2 + 3SiO2 (5)
Al2Si4O10(OH)2 = Al2SiO5 + 3SiO2 + H2O (6)
Al2Si4O10(OH)2 = 2AlSiO3(OH) + 2SiO2 (7)
Al2Si4O10(OH)2 = 2δ-AlO(OH) + 4SiO2 (8)

In addition, we have performed computational structure search to locate the hydrogen positions in pyrophyllite because there is no experimental data for the crystal structure of pyrophyllite that includes the location of hydrogen atoms. We have fixed experimentally driven atomic positions of Al, Si, and O atoms as well as the lattice constants and generated hydrogen positions randomly within the unit cell, adopting the method implemented in Ab Initio Random Structure Searching (AIRSS)70 module. Amongst ~200 predicted crystal structures, we have identified one that maintains a well-stabilized crystal structure based on the lowest total energy (Supplementary Table 4).

Net crystalline density and net fluid volume calculations71
The density of each phase was calculated using ρcalc (g cm−3) = (M Z V−1) × (avogadro’s number)−1, where M is the molecular weight, Z is the number of the formula unit per unit cell, and V (Å3) is the volume derived from the profile fitting of XRD data using GSAS program (Supplementary Figs. 1–7). Net crystalline density accounts the proportions of the composing crystalline phases. Net crystalline density is described here as:
ρNet crystalline density = aρa + bρb + ... + zρz (9)
where a, b, and z mean the proportions of the phase assemblages (Table 1 and Supplementary Table 5). The calculated density of each phase, ρ, is derived from XRD data.

To determine the changes in the net fluid volume after the dehydration, as defined as: the initial volume of pyrophyllite and water per mole is
avogadro’s number * atom * unit cell volume of albite cm3 / mol / the number of the formula unit per unit cell (atom) (10)
Based on the phase assemblages (Table 1 and Supplementary Table 5), changes in the net fluid volume are then calculated.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Remark to the Author\nThis paper presents the phase transition of pyrophyllite, one of the representative hydrous clay minerals\nin sedimentary rocks, under hydrous subduction environment in the alumina-silica-water system. In-situ\nsynchrotron X-ray diffraction measurements combined with RH-DAC and theoretical approach using first-\nprinciples method showed that sequential breakdowns of pyrophyllite involving the formation of further\nhydrous minerals through the cold subduction of sediments. Based on the results, H2O flux along\nsubducting slab down to the lower mantle\nwas discussed.\nThe discussion potentially provides some contributions to the debate regarding the mineralogy under\nhydrous conditions. However, from the perspective of water transport to the deep mantle, I do not think\nthat this discussion has advanced much from previous works showing some hydrated minerals and their\ntransitions under pressure (e.g., Hwang et al., 2021, Hwang et al., 2017). Furthermore, I afraid that the\nexperimental results obtained under water-saturated and very cold conditions may not be applicable to\nthe natural system. Thus, I do not consider that it meets the requirement of covering a significant\nadvance to be published in Nature communications.\nMain comments are in the following.\n(1) Geotherms along the subducting slab shown in Fig.1 are from South Mariana Trench and Kermadec\nTrench (W1300 model by Syracuse et al.). Since these slabs are extremely cold compared to the other\nregions, the data along these geotherms may not be applied to the discussion of the global water\nsubduction. Furthermore, the experimental temperatures are lower than the slab serface through the\ndepth (Fig.1). To convince the discussion in this study, further explanation espetically for the relation\nbetween the temperature applied in this study and the geotherm of subductiong slab in several regions,\nare needed.\n(2) Because temperature conditions applied in the experiments were low to assert phase equilibrium for\nminerals, it is unclear that the reaction paths and structural evolution of pyrophyllite reflect actual phase\ntransitinons through subduction. It is reasonable to consider that phase change of pyrophyllite at\ntemperatures below 200C up to 5 GPa was kinetically inhibited. Actually, it seems that breakdown of\npyrophyllite to gibbsite and diaspore occurs during temperature increase from 200C to 400C, at\na constant pressure of 4 GPa. In such low temperatures, even stishovite also can cotain large amount of\nhydrogen under metastable state (Spektor et al. 2011 PNAS). Discussion on the reaction kinetics is\nrequired to determine the depth of phase change through subduction in the geological time scale.\n(3) I think H2O-saturated condition applied in this experiment may be far from the actual Eatth’s interior.\nWhy is free-H2O component maintained at the slab surface in global? If we assume that free-H2O is\nmaintained in the slab for some reasons, water transportation is possible without hydrous mineral. Some\ndiscussion should be required to convincing the water subduction proposed in this study.\n(4) Line 78: “dry condition” is ambiguous expression since the referred previous paper studied the\nminerals with ASH system.\n(5) Table 1 and S1: As written in line 615, temperature was monitored using thermocouples with\nmaximum uncertainties of 3 K. Please explain how the error values shown in Table 1 are estimated.\n(6) Fig.2(a): What do the gray and white symbols mean in this figure?\n(7) Supplementary Table S3: A theoretical study (Mookherjee et al., 2016) was referred.", "reviewer_2": "The manuscript reports the reaction of pyrophyllite with water or NaCl-MgCl2 solution at the P-T\nconditions of the sudducting slabs. The study found that pyrophyllite breaks down into\ngibbsite+diaspore+coesite at ~135 km, followed by dehydration and water-preserving processes up to\nmantle transition zone (MTZ) and lower mantle. It is concluded that pyrophyllite could transport\nsignificant amount of H2O along subduction zone to MTZ and the lower mantle. The experiments\nemploying externally-heated diamond anvil cell appear to be properly performed and the data\nadequately analyzed. The results have important implications on the transport of H2O into the Earth's\nlower mantle through the subducting clays. I recommend publication of the manuscript after addressing\nthe comments listed below.\nMain comment:\n- Pyrophyllite was loaded together with water or NaCl-MgCl2 solution in the resistively-heated diamond\nanvil cell. So it was under water-saturated conditions. It is unclear whether sea water can reach to the\ndepth below 100 km along the suducting slabs. Can authors elaborate the processes that could bring sea\nwater to that depth so that it can react with pyrophyllite in the subducting slabs.\nMinor comments:\n- Line 82: change \"resistive-heated diamond-anvil cell\" to \"resistively-heated diamond-anvil cell\".\n- Fig. 1: In Fig.1b, the second pattern from the top is for 5.5(3) GPa and 570 ºC. Line 291 refers to\nSupplementary Figures 1-7 for detailed fitting results. However, the supplementary Figure 2 is for 4.5(5)\nGPa and 450 ºC. Please replace the figure with the one for 5.5 GPa and 570 ºC for consistency. Some\nmajor XRD peaks collected at 5.5 GPa and 570 ºC in Fig. 1b are not labeled. For example, the peak at Q\n<1.0.\n- Line 114 & supplementary Figs. 6-7: Stishovite was observed to form at below 255 km depth. Does\nstishovite formed during the reactions contain Al or H? This can be verified by comparing the unit cell\nparameters/volumes of the stishovite phases with those of pure SiO2 stishovite in the literature. If\nstishovite contains water or H, it may be a major host of H2O and thus influence the estimates of H2O\ntransport into the MTZ and lower mantle.\n- Line 611: Replace \"internal resistive electrical heaters\" with \"external resistive electrical heaters\", as\n\"internal heaters\" are typically referred to as a mini-heater placed between diamond anvils in the sample\nchamber."}, "author_response": "Dear Reviewers,\nWe appreciate all of the insightful comments and constructive suggestions for our\nmanuscript to Nature Communications (NCOMMS-23-10747) entitled “A role for subducting\nclays in the water transportation into the Earth’s lower mantle”. Please find attached our revised\nversion of the paper (all the changes made in the revised manuscript are marked in red). Our\npoint-by-point responses to all the comments and criticisms are summarized below (the\nreviewers’ comments are marked in blue).\nReviewer #1 (Remarks to the Author):\nThis paper presents the phase transition of pyrophyllite, one of the representative hydrous clay\nminerals in sedimentary rocks, under hydrous subduction environment in the alumina-silica-\nwater system. In-situ synchrotron X-ray diffraction measurements combined with RH-DAC\nand theoretical approach using first-principles method showed that sequential breakdowns of\npyrophyllite involving the formation of further hydrous minerals through the cold subduction\nof sediments. Based on the results, H2O flux along subducting slab down to the lower mantle\nwas discussed.\nThe discussion potentially provides some contributions to the debate regarding the mineralogy\nunder hydrous conditions. However, from the perspective of water transport to the deep mantle,\nI do not think that this discussion has advanced much from previous works showing some\nhydrated minerals and their transitions under pressure (e.g., Hwang et al., 2021, Hwang et al.,\n2017). Furthermore, I afraid that the experimental results obtained under water-saturated and\nvery cold conditions may not be applicable to the natural system. Thus, I do not consider that\nit meets the requirement of covering a significant advance to be published in Nature\ncommunications.\nReply: We appreciate the reviewer for the overall positive appraisal on the quality of our work.\nFor this revision, we have performed additional in-situ HP-HT experiments to follow\n‘moderately’ cold slab surface geotherms, e.g., Izu-Bonin subduction model, for our results to\nbe applicable to a general cold subduction system. We have confirmed that pyrophyllite\ndecomposes into diaspore + coesite assemblage as we have reported in the original manuscript\nwhich mainly followed the South Mariana and Kermadec subduction models. We advocate that\nour results contribute significant advancement to the previous findings as we have re-evaluated\nthe overall role of clays, major components in subducting sedimentary layer, on the water\ntransport capacity/efficiency and linked them down to the lower mantle, that is found to be\ncomparable to the water transport by subducting crust.\nMain comments are in the following.\n(1) Geotherms along the subducting slab shown in Fig.1 are from South Mariana Trench and\nKermadec Trench (W1300 model by Syracuse et al.). Since these slabs are extremely cold\ncompared to the other regions, the data along these geotherms may not be applied to the\ndiscussion of the global water subduction. Furthermore, the experimental temperatures are\nlower than the slab serface through the depth (Fig.1). To convince the discussion in this study,\nfurther explanation espetically for the relation between the temperature applied in this study\nand the geotherm of subductiong slab in several regions, are needed.\nReply: As stated above, we have conducted additional HP-HT experiment up to 4.3(3) GPa\nand 600±50 ºC to follow ‘moderately’ cold slab surface geotherms e.g., Izu-Bonin subduction\nmodel with ~6 ºC/km. The result shows that pyrophyllite decomposes into diaspore + coesite\nassemblage, which is in line with the result from the original manuscript that simulated South\nMariana and Kermadec subduction surface with < 5 ºC/km. The new experimental details and\nresults have been added in p.5, line 110-114 and p.6, line 136-142 together with revised Figure\n1a, Supplementary Table 1, Supplementary Figs. 8 and 11, references #35-36.\n(2) Because temperature conditions applied in the experiments were low to assert phase\nequilibrium for minerals, it is unclear that the reaction paths and structural evolution of\npyrophyllite reflect actual phase transitinons through subduction. It is reasonable to consider\nthat phase change of pyrophyllite at temperatures below 200C up to 5 GPa was kinetically\ninhibited. Actually, it seems that breakdown of pyrophyllite to gibbsite and diaspore occurs\nduring temperature increase from 200C to 400C, at a constant pressure of 4 GPa. In such low\ntemperatures, even stishovite also can cotain large amount of hydrogen under metastable state\n(Spektor et al. 2011 PNAS). Discussion on the reaction kinetics is required to determine the\ndepth of phase change through subduction in the geological time scale.\nReply: We believe the issue on the stability of pyrophyllite has been addressed, at least in part,\nby our additional experiment. We point out that the breakdown of pyrophyllite to gibbsite and\ndiaspore occurs during temperature increase from 400 ºC to 600 ºC near 4 GPa (p.5-6, line 124-\n127 and line 136-139, revised Fig. 1). During our RH-DAC experiments, the samples were\nheated for about 12 hours (or more) to reach target temperature, where the sample temperature\nwas kept for at least ~20–30 minutes to ensure no further changes are occurring. The RH-DAC\nsetup that we used in this study offers the advantage of providing homogeneous and stable\ntemperature across the entire sample chamber (Method section p.30-31, line 744-745 and 757-\n759, reference #96). We understand that the actual depths for phase transition could change\nwhen reaction kinetics are considered, together with other chemical/phase components existing\nin a real subducting layer. Although we have considered the composition of subduction fluid\nas an additional variable, i.e., using three different fluid compositions of pure water, salt water,\nand anhydrous silicone oil, to access their role as a catalyst as well as a reactant to affect kinetic\nbarriers and breakdown scheme, further research is desired to fully understand the mineral/rock\nstabilities in a more realistic subduction environments (p.6 line 140-142, p.7, line 156-167,\nreference #42-47).\n(3) I think H2O-saturated condition applied in this experiment may be far from the actual\nEatth’s interior. Why is free-H2O component maintained at the slab surface in global? If we\nassume that free-H2O is maintained in the slab for some reasons, water transportation is\npossible without hydrous mineral. Some discussion should be required to convincing the water\nsubduction proposed in this study.\nReply: To address the issue on free water in a subduction system, we have added new sentences\nwith references to showcase major fluid migration pathways and fluid-mineral interaction\nprocesses in a subduction zone. In the vicinity of subduction trench, water/seawater can be\ntransported as sedimentary pore fluids (fluid-rich sediment) whereas in a deeper region down\nto > 100 km (beneath volcanic arcs and into the deeper mantle), fluid flux is channelized\nalong open fractures and ductile shear zones. In line of this, we have revised the inset in Fig. 3a to\nillustrate the above two fluid migration pathways along relatively shallow and intermediate\nsubduction interfaces (revised Fig. 3a, p.2-3, line 46-70, reference #5-12).\n(4) Line 78: “dry condition” is ambiguous expression since the referred previous paper studied\nthe minerals with ASH system.\nReply: We have deleted the phrase “under dry condition” to clarify the experimental conditions\n(p.5, line 104).\n(5) Table 1 and S1: As written in line 615, temperature was monitored using thermocouples\nwith maximum uncertainties of 3 K. Please explain how the error values shown in Table 1 are\nestimated.\nReply: We have added new sentences (with a new reference) to explain the estimation scheme\nof errors in temperature measurements in Table 1 and Supplementary Table 1. The overall\nuncertainty in the sample temperature has been estimated by the differences between the\nrecorded thermocouple temperature and the calculated temperature based on the unit-cell\nvolume and the known thermal expansion coefficient of standard materials (Method section\np.30-31, line 749-752, reference #96).\n(6) Fig.2(a): What do the gray and white symbols mean in this figure?\nReply: We have added the sentences “The reaction products from pyrophyllite and kaolinite\nare denoted by colored and white circles, respectively, while the other aluminosilicate minerals\nwithin the ASH system are marked with gray symbols.” (revised caption of Fig. 2).\n(7) Supplementary Table S3: A theoretical study (Mookherjee et al., 2016) was referred.\nReply: We have replaced the theoretical study (Mookherjee et al., 2016) with more relevant\nexperimental study (Komatsu et al., 2003) (revised Supplementary Table 3).\nReviewer #2 (Remarks to the Author):\nThe manuscript reports the reaction of pyrophyllite with water or NaCl-MgCl2 solution at the\nP-T conditions of the sudducting slabs. The study found that pyrophyllite breaks down into\ngibbsite+diaspore+coesite at ~135 km, followed by dehydration and water-preserving\nprocesses up to mantle transition zone (MTZ) and lower mantle. It is concluded that\npyrophyllite could transport significant amount of H2O along subduction zone to MTZ and the\nlower mantle. The experiments employing externally-heated diamond anvil cell appear to be\nproperly performed and the data adequately analyzed. The results have important implications\non the transport of H2O into the Earth's lower mantle through the subducting clays. I\nrecommend publication of the manuscript after addressing the comments listed below.\nReply: We appreciate the overall positive appraisal of our work by the reviewer #2.\nMain comment:\n- Pyrophyllite was loaded together with water or NaCl-MgCl2 solution in the resistively-heated\ndiamond anvil cell. So it was under water-saturated conditions. It is unclear whether sea water\ncan reach to the depth below 100 km along the suducting slabs. Can authors elaborate the\nprocesses that could bring sea water to that depth so that it can react with pyrophyllite in the\nsubducting slabs.\nReply: We have added new sentences (with new references) to showcase major fluid migration\npathways and fluid-mineral interaction processes in a subduction zone. In the vicinity of\nsubduction trench, seawater can be transported as sedimentary pore fluids (fluid-rich sediment)\nwhereas in a deeper region down to ~100 km (beneath volcanic arcs and into the deeper mantle),\nfluid flux is channelized along open fractures and ductile shear zones. In line of this, we have\nrevised the inset in Fig. 3a to illustrate the above two fluid migration pathways along relatively\nshallow and intermediate subduction interfaces (revised Fig. 3a, p.2-3, line 46-70, reference\n#5-12). In fact, numerous studies demonstrated that major and trace elements (or solutes)\ndissolved in subduction zone fluids (SFZs) are recycled in the deep mantle. As NaCl and MgCl\n2\nare identified as important components in SZFs (Frezzotti & Ferrando, (2015)) as well as in\nseawater, we have considered the composition of subduction fluid as an additional variable,\ni.e., using three different fluid compositions of pure water, salt water, and anhydrous silicone\noil to access their role as a catalyst as well as a reactant to affect kinetic barriers and breakdown\nscheme. Previous studies have further appraised the role of subduction fluid (deep recycling of\nseawater-derived fluids), as Holland and Ballentine (2006) revealed that subduction of\nsediment and seawater-dominated pore fluids play an important role in understanding the\nelemental abundance pattern of heavy noble gases (Ar, Kr, and Xe) in the mantle to be\nremarkably close to that of seawater (i.e., recycling of seawater is responsible for the observed\nheavy noble gas pattern in the convecting mantle as subduction occurs in a seawater-dominated\nfluid environment) (p.7, line 156-167, reference #42-47).\nMinor comments:\n- Line 82: change “resistive-heated diamond-anvil cell” to “resistively-heated diamond-anvil\ncell”.\nReply: We changed the phrase to “resistively-heated diamond anvil cell” (p.5, line 107).\n- Fig. 1: In Fig.1b, the second pattern from the top is for 5.5(3) GPa and 570 ºC. Line 291 refers\nto Supplementary Figures 1-7 for detailed fitting results. However, the supplementary Figure\n2 is for 4.5(5) GPa and 450 ºC. Please replace the figure with the one for 5.5 GPa and 570 ºC\nfor consistency. Some major XRD peaks collected at 5.5 GPa and 570 ºC in Fig. 1b are not\nlabeled. For example, the peak at Q <1.0.\nReply: As pointed, we have replaced Supplementary Figure 2 with the correct one measured\nat 5.5 GPa and 570 ºC and added the label in the XRD pattern in Figure 1b (new Supplementary\nFig. 2 and revised Fig.1b).\n- Line 114 & supplementary Figs. 6-7: Stishovite was observed to form at below 255 km depth.\nDoes stishovite formed during the reactions contain Al or H? This can be verified by comparing\nthe unit cell parameters/volumes of the stishovite phases with those of pure SiO2 stishovite in\nthe literature. If stishovite contains water or H, it may be a major host of H2O and thus influence\nthe estimates of H2O transport into the MTZ and lower mantle.\nReply: We appreciate such an insightful comment by the reviewer #2. Accordingly, we have\nadded new sentences (with new references) to address the issue on possible hydration in\nstishovite. Stishovite formed in our experiments shows differences in the refined unit cell\nvolume by about +1–2 % compared to anhydrous SiO at the corresponding HP-HT conditions\n2\n(Angel et al., 2005; Kulik et al., 2018; Nishihara et al., 2005) (see Table below). However,\nrecent experimental studies have established that hydrous stishovite can be formed at ~9–10\nGPa and 350–550 ºC while above 600 ºC anhydrous stishovite is preferred (Spektor et al., 2011\nand 2016; Nisr et al., 2017, references #39-41). The temperature range where stishovite formed\nin our experiments exceeds 550 ºC, and the refined unit cell volume of the SiO phase below\n2\n550 ºC showed similar degree of expansion, which made us conclude that the apparent increase\nin the unit cell volume is due to systematic deviation within experimental uncertainty and the\nstishovite formed in our experiment is anhydrous (p.6-7, line 150-151).\nTable | Comparison of the refined unit-cell volume of the SiO phases to the values in the literature.\nHPHT HPHT\n5.5(3) GPa and 570±35 ºC 529.64 521.64b 1.53\n6.9(3) GPa and 700±45 ºC 528.61 516.16b 2.41\n7.6(4) GPa and 850±50 ºC 528.15 513.86b 2.78\n6.3(4) GPa and 900±55 ºC 531.54 519.50b 2.32\n13.6(10) GPa, 725±75 °C 45.98 45.25c 1.62\n20.7(10) GPa, 790±110 °C 45.30 44.33c 2.20\naRefined unit cell volume of the SiO2 phase at P-T conditions in our experiments as in Supplementary Fig. 2–7\nbCalculated unit cell volume of coesite at the corresponding P-T conditions using P-V and V-T equation of state (Angel et al.,\n2005; Kulik et al., 2018)\n(3rd-order Birch-Murnaghan EoS: V0 = 546.7(12) Å3, KT0 = 97.4(6), K’T = 4.3(2))\n(Mie-Grüneisen-Debye EoS based on Suzuki model: θ0 = 1004(43) K, γ0 = 0.271(16))\ncCalculated unit cell volume of stishovite at the corresponding P-T conditions using P-V-T equation of state (Nishihara et al.,\n2005)\n(Mie-Grüneisen-Debye EoS: V0 = 46.547(12) Å3 , KT0 = 299(4), K’T = 4, θ0 = 1160(120) K, γ0 = 1.33(6), q = 6.1(8))\ndΔV/VHPHT % = (V - VHPHT) / VHPHT × 100\n- Line 611: Replace \"internal resistive electrical heaters\" with \"external resistive electrical\nheaters\", as \"internal heaters\" are typically referred to as a mini-heater placed between diamond\nanvils in the sample chamber.\nReply: As suggested, we have replaced the phrase (p.30, line 742).\nBased on the above, we sincerely believe that we have addressed all the criticisms and\ncomments of the reviewers and therefore request our revised manuscript to be considered for\npublication in Nature Communications."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The revised manuscript has addressed my major comment on the fluid migration pathways in subduction\nzones. The authors have also adequately made relevant revisions in response to the minor comments\nfrom the first round of review. Additional experiments were performed at higher temperature up to 600\nºC to address the main comments on the discrepancy between the highest temperature achieved in\nexperiments and the real slab surface temperature at depth. Therefore, I do not have any further\ncomments on the revised manuscript and recommend its publication in Nature Communications.", "reviewer_3": "This paper describes the phase transformation sequences of the ASH system within cold subducting\nslabs starting from pyrophyllite, and their implication for deep water transport within the Earth. I believe\nthe paper is worth to be considered for publication in Nature Communication. Here are my comments on\nthe author's reply to Reviewer #1 comments.\n(1) Author's Reply: As stated above, we have conducted additional HP-HT experiment up to 4.3(3) GPa\nand 600±50 ºC to follow ‘moderately’ cold slab surface geotherms e.g., Izu-Bonin subduction model with\n~6 ºC/km. The result shows that pyrophyllite decomposes into diaspore + coesite assemblage, which is in\nline with the result from the original manuscript that simulated South Mariana and Kermadec\nsubduction surface with < 5 ºC/km. The new experimental details and results have been added in p.5,\nline 110-114 and p.6, line 136-142 together with revised Figure 1a, Supplementary Table 1,\nSupplementary Figs. 8 and 11, references #35-36.\n(1) Reviewer comment: I consider that the additional HP-HT experiment effectively reinforced the result\nand the discussion in the original manuscript. To clarify that the \"moderately cold slab surface\ngeotherms, i.e., following the thermal model of Izu-Bonin subduction\" (lines 136 to 137) were\nindependently analyzed from that of the coldest slab ones, it is necessary for the revised manuscript to\nexplicitly note that the experimental P-T path for \"up to 4.3(3) GPa and 600±50 ºC” was independently\nanalyzed from that for \"~23 GPa and 700-900 ºC following the cold subduction geotherms of the South\nMariana and Kermadec thermal Models\" (lines 122 to 124).\n(2) Author's Reply: We believe the issue on the stability of pyrophyllite has been addressed, at least in\npart, by our additional experiment. We point out that the breakdown of pyrophyllite to gibbsite and\ndiaspore occurs during temperature increase from 400 ºC to 600 ºC near 4 GPa (p.5-6, line 124-127 and\nline 136-139, revised Fig. 1). During our RH-DAC experiments, the samples were heated for about 12\nhours (or more) to reach target temperature, where the sample temperature was kept for at least ~20–\n30 minutes to ensure no further changes are occurring. The RH-DAC setup that we used in this study\noffers the advantage of providing homogeneous and stable temperature across the entire sample\nchamber (Method section p.30-31, line 744-745 and 757- 759, reference #96). We understand that the\nactual depths for phase transition could change when reaction kinetics are considered, together with\nother chemical/phase components existing in a real subducting layer. Although we have considered the\ncomposition of subduction fluid as an additional variable, i.e., using three different fluid compositions of\npure water, salt water, and anhydrous silicone oil, to access their role as a catalyst as well as a reactant to\naffect kinetic barriers and breakdown scheme, further research is desired to fully understand the\nmineral/rock stabilities in a more realistic subduction environments (p.6 line 140-142, p.7, line 156-167,\nreference #42-47).\n(2) Reviewer comment: The response seems satisfactory to address the given issue, and the manuscript\nwas appropriately revised.\n(3) Author's Reply: To address the issue on free water in a subduction system, we have added\nnew sentences with references to showcase major fluid migration pathways and fluid-mineral interaction\nprocesses in a subduction zone. In the vicinity of subduction trench, water/seawater can be transported\nas sedimentary pore fluids (fluid-rich sediment) whereas in a deeper region down to > 100 km (beneath\nvolcanic arcs and into the deeper mantle), fluid flux is channelized along open fractures and ductile shear\nzones. In line of this, we have revised the inset in Fig. 3a to illustrate the above two fluid migration\npathways along relatively shallow and intermediate subduction interfaces (revised Fig. 3a, p.2-3, line 46-\n70, reference #5-12).\n(3) Reviewer comment: The response seems satisfactory to address the given issue, and the manuscript\nand Figure 3 were appropriately revised.\nThe other remarks were answered as just requested by the referee #1.\nAdditional reviewer comment: \"SFZs\" should rather be \"SZFs\" (if following Mannning and Frezzotti, 2020)."}, "author_response": "Dear Reviewer,\nWe appreciate all of the valuable comments and constructive suggestions for our\nmanuscript to Nature Communications (NCOMMS-23-10747A) entitled “A role for\nsubducting clays in the water transportation into the Earth’s lower mantle”. Please find attached\nthe 2nd revised version of the paper (all the changes made in the revised manuscript are marked\nin red). Our point-by-point responses to all the comments are summarized below (the\nreviewers’ comments are marked in blue).\nReviewer #2 (Remarks to the Author):\nThe revised manuscript has addressed my major comment on the fluid migration pathways in\nsubduction zones. The authors have also adequately made relevant revisions in response to the\nminor comments from the first round of review. Additional experiments were performed at\nhigher temperature up to 600 ºC to address the main comments on the discrepancy between the\nhighest temperature achieved in experiments and the real slab surface temperature at depth.\nTherefore, I do not have any further comments on the revised manuscript and recommend its\npublication in Nature Communications.\nReply: We thank Reviewer #2 for the appreciation of our work and efforts to improve our\nmanuscript. We are very pleased to hear from Reviewer #2 that our revised manuscript has\naddressed the major concerns regarding the fluid migration pathways in subduction zones, as\nwell as the minor comments raised in the first round review. We appreciate very much the\nencouraging comments and support of Reviewer #2 for publication of our manuscript in Nature\nCommunications.\nReviewer #3 (Remarks to the Author):\nThis paper describes the phase transformation sequences of the ASH system within cold\nsubducting slabs starting from pyrophyllite, and their implication for deep water transport\nwithin the Earth. I believe the paper is worth to be considered for publication in Nature\nCommunication. Here are my comments on the author's reply to Reviewer #1 comments.\nReply: We appreciate the encouraging remarks and insightful comments by Reviewer #3. We\nare thankful for the review and recommendation for publication of our manuscript in Nature\nCommunications. We hope we have addressed all the concerns raised by reviewer #3 in this\nrevision. Our point-by-point responses to the comments are as follows.\n(1) Author's Reply: As stated above, we have conducted additional HP-HT experiment up to\n4.3(3) GPa and 600±50 ºC to follow ‘moderately’ cold slab surface geotherms e.g., Izu-Bonin\nsubduction model with ~6 ºC/km. The result shows that pyrophyllite decomposes into diaspore\n+ coesite assemblage, which is in line with the result from the original manuscript that\nsimulated South Mariana and Kermadec subduction surface with < 5 ºC/km. The new\nexperimental details and results have been added in p.5, line 110-114 and p.6, line 136-142\ntogether with revised Figure 1a, Supplementary Table 1, Supplementary Figs. 8 and 11,\nreferences #35-36.\n(1) Reviewer comment: I consider that the additional HP-HT experiment effectively reinforced\nthe result and the discussion in the original manuscript. To clarify that the \"moderately cold\nslab surface geotherms, i.e., following the thermal model of Izu-Bonin subduction\" (lines 136\nto 137) were independently analyzed from that of the coldest slab ones, it is necessary for the\nrevised manuscript to explicitly note that the experimental P-T path for \"up to 4.3(3) GPa and\n600±50 ºC” was independently analyzed from that for \"~23 GPa and 700-900 ºC following the\ncold subduction geotherms of the South Mariana and Kermadec thermal Models\" (lines 122 to\n124).\nReply: We have revised the manuscript to improve the clarity regarding the additional\nexperiment under “moderately cold slab surface geotherms (Izu-Bonin thermal models)” by\nadding “Izu-Bonin” and “for which data have been independently measured and analyzed” in\np. 6 line 130-131 and line 144-145, respectively.\n(2) Author's Reply: We believe the issue on the stability of pyrophyllite has been addressed, at\nleast in part, by our additional experiment. We point out that the breakdown of pyrophyllite to\ngibbsite and diaspore occurs during temperature increase from 400 ºC to 600 ºC near 4 GPa\n(p.5-6, line 124-127 and line 136-139, revised Fig. 1). During our RH-DAC experiments, the\nsamples were heated for about 12 hours (or more) to reach target temperature, where the sample\ntemperature was kept for at least ~20–30 minutes to ensure no further changes are occurring.\nThe RH-DAC setup that we used in this study offers the advantage of providing homogeneous\nand stable temperature across the entire sample chamber (Method section p.30-31, line 744-\n745 and 757- 759, reference #96). We understand that the actual depths for phase transition\ncould change when reaction kinetics are considered, together with other chemical/phase\ncomponents existing in a real subducting layer. Although we have considered the composition\nof subduction fluid as an additional variable, i.e., using three different fluid compositions of\npure water, salt water, and anhydrous silicone oil, to access their role as a catalyst as well as a\nreactant to affect kinetic barriers and breakdown scheme, further research is desired to fully\nunderstand the mineral/rock stabilities in a more realistic subduction environments (p.6 line\n140-142, p.7, line 156-167, reference #42-47).\n(2) Reviewer comment: The response seems satisfactory to address the given issue, and the\nmanuscript was appropriately revised.\nReply: We are grateful for acknowledging that our response and the revised manuscript have\naddressed the given issue.\n(3) Author's Reply: To address the issue on free water in a subduction system, we have added\nnew sentences with references to showcase major fluid migration pathways and fluid-mineral\ninteraction processes in a subduction zone. In the vicinity of subduction trench, water/seawater\ncan be transported as sedimentary pore fluids (fluid-rich sediment) whereas in a deeper region\ndown to > 100 km (beneath volcanic arcs and into the deeper mantle), fluid flux is channelized\nalong open fractures and ductile shear zones. In line of this, we have revised the inset in Fig.\n3a to illustrate the above two fluid migration pathways along relatively shallow and\nintermediate subduction interfaces (revised Fig. 3a, p.2-3, line 46-70, reference #5-12).\n(3) Reviewer comment: The response seems satisfactory to address the given issue, and the\nmanuscript and Figure 3 were appropriately revised.\nThe other remarks were answered as just requested by the referee #1.\nAdditional reviewer comment: \"SFZs\" should rather be \"SZFs\" (if following Mannning and\nFrezzotti, 2020).\nReply: We have corrected “SFZs” to “SZFs” in the manuscript (p. 7 line 167-168).\nBased on the above, we sincerely request our manuscript to be published in Nature\nCommunications."}]

Decision letter