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Abstract We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle.

We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle. Plain Language Summary: Water on our planet is constantly exchanged between the oceans and the interior of the Earth. This happens at subduction zones where hydrated oceanic crust is sinking into the Earth's mantle, thus bringing water to depths exceeding 300 km. We quantify this amount of water that is globally recycled into the deeper Earth in subduction zones through computer models that combine the temperature structure of the subducted rocks with the stability of hydrous phases. Knowledge about the global water budget is important to quantify the Earth's total water amount, to understand the water release from the subducting oceanic plate, and to interpret sea level changes in Earth's history. We compared thermodynamic datasets for two minerals, namely Mg‐sursassite and phase A, in the oceanic plate that can transfer large amounts of water into the deeper Earth. Our thermodynamic models show that the stability of phase A in the down going oceanic plate controls much of the recycled water in most subduction zones. Key Points: Mg‐sursassite has a limited influence on the global amount of subducted waterMg‐sursassite can transfer water to depths of up to 250 km and lead to deep dehydration and water release from the slabThe Clapeyron slope of the phase A dehydration reaction is sub‐parallel to many slab P‐T paths and mainly controls the deep water subduction [ABSTRACT FROM AUTHOR]

We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods. [ABSTRACT FROM AUTHOR]

The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of ideal composition Mg4Al5Si7O23(OH)5, that was produced by sub-solidus reaction at 24 GPa and 1400 °C in an experiment using a model sedimentary bulk composition, has been determined by single-crystal X‑ray diffraction. The phase was found to be topologically identical to Mg-sursassite, Mg5Al5Si6O21(OH)7, and has space group P21/m and lattice parameters a = 8.4222(7), b = 5.5812(3), c = 9.4055(9) Å, β = 106.793(8)°, V = 423.26(6) Å3, and Z = 1. The empirical formula determined by electron microprobe analysis of the same crystal as was used in the X‑ray experiment is [Mg3.93(3)Fe0.03(1)]S3.96[Al4.98(3)Cr0.04(1)]S5.02 Si7.02(4)O23(OH)5, with hydroxyl content implied by the crystal-structure analysis. The most significant aspect of the structure of Si-rich Mg-sursassite is the presence of octahedrally coordinated Si. Its structural formula is M 1 , VII M g 2 M 2 VI Mg 2 2 + M 3 ,VI ( Al 0.5 Si 0.5 ) 2 M 4 ,VI Al 2 M 5 ,VI Al 2 T 1 ,IV Si 2 T 2 ,IV Si 2 T 3 ,IV Si 2 O 23 (OH) 5. $^{M1,\text{VII}}M{{g}_{2}}{{\,}^{{{M}_{2}}\text{VI}}}\text{Mg}{{_{2}^{2+}}^{\,M3\text{,VI}}}{{\left(\text{A}{{\text{l}}_{\text{0}\text{.5}}}\text{S}{{\text{i}}_{\text{0}\text{.5}}} \right)}_{2}}{{\,}^{M4\text{,VI}}}\text{A}{{\text{l}}_{2}}{{\,}^{{{M}_{5}}\text{,VI}}}\text{A}{{\text{l}}_{2}}^{T1\text{,IV}}\text{S}{{\text{i}}_{\text{2}}}{{\,}^{T2\text{,IV}}}\text{S}{{\text{i}}_{2}}{{\,}^{T3\text{,IV}}}\text{S}{{\text{i}}_{2}}\,\,{{\text{O}}_{\text{23}}}{{\left(\text{OH} \right)}_{\text{5}}}.$ Si-rich Mg-sursassite joins the group of hydrous ultrahigh-pressure phases with octahedrally coordinated Si that have been discovered by experiment, and that may play a significant role in the distribution and hosting of water in the deep mantle at subduction zones. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally diferent from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A. [ABSTRACT FROM AUTHOR]

This study uses a 2D high-resolution thermo-mechanical coupled model to investigate the dynamic processes of deep plate hydration, dehydration, and subsequent magmatic activity in ocean-continent subduction zones. We reveal the pathways and temporal evolution of water transport to the deep mantle during the subduction process. Plate dehydration plays a critical role in triggering partial melting of the deep mantle and related magmatic activity. Our study shows significant differences in the volumes of melt produced at different depths, with dehydration reactions in deeper regions being weaker compared to shallower ones. It takes a longer time to reach the suitable P-T conditions for hydrous melting in the deep mantle. The results highlight the geophysical significance of water transport in deep subduction zones and its role in magmatic processes, particularly in the formation of magma chambers beneath continental plates. [ABSTRACT FROM AUTHOR]

A series of experiments was conducted on the decomposition of natural and chemically mixed chlorites to examine the stable hydrous phases in the MgO–FeO–Al 2 O 3 –SiO 2 –H 2 O (MFASH) system under 5–12 GPa and 700–1100 °C. The upper pressure and temperature limits of the stability region of chlorite are consistent with those observed in previous studies. The hydrous aluminum bearing pyroxene (phase HAPY) and Mg-sursassite (Sur) were observed just above the temperature stability region of chlorite (Chl); clinohumite (cHm) was observed coexisting with phase HAPY at 6 GPa and 800 °C and coexisting with the 23-Å phase at 7 GPa and 800 °C, which may suggest the transportation of water through Chl → (HAPY → cHm) → 23-Å phase along a relatively warm slab. The 23-Å phase has a wider stability region in the pure MASH system (up to 12 GPa and 1100 °C) than it does in the MFASH system (7–10 GPa, up to 1000 °C). The stability of the 23-Å phase beyond the chlorite breakdown pressure indicates that it may play an important role in transporting water into the deep Earth and even into the mantle transition zone. [ABSTRACT FROM AUTHOR]

Hydrous minerals in the subducting slabs are potential water carriers into the deep mantle, and thus the synthesis of new hydrous phases is significant in our understanding of water circulation throughout the Earth's interior. In this study, we report the two new hydrous phases, Al2SiO6H2 and Al5.5Si4O18H3.5 (hereafter referred to simply as phases Psi and Phi, respectively), which are synthesized in the Al2O3-SiO2-H2O system at 15.5 GPa, 1400°C and 17.5 GPa, 1600°C by using Sakura 2500-ton multi-anvil apparatus. The luminescence spectra of Cr3+ show the phase Psi has characteristic peaks at 687, 693 and 705 nm, while phase Phi has characteristic peaks at 691, 696 and 708 nm. Single-crystal X-ray diffraction (SCXRD) refinements yield a monoclinic structure of both phases (space group P21) with ideal chemical formulae of Al2SiO6H2 and Al5.5Si4O18H3.5, respectively. The determined lattice parameters for phase Psi are a=9.4168±0.0016 Å, b=4.3441±0.0007 Å, c=9.4360±0.002 Å, and β=119.726±0.005° at ambient pressure and 300 K, while the phase Phi has a=7.2549±0.0018 Å, b=4.3144±0.001 Å, c=8.0520±0.002 Å, and β=01.740±0.009° at ambient pressure and 250 K. Electron microprobe analyses (EPMA) show the chemical compositions of phases Psi and Phi to be Al1.99Si0.85O6H2.62 and Al5.58Si2.81O18H8.03, respectively, which slightly deviate from the ideal formulae inferred from SCXRD measurements. This may result from the disorder or substitution of Al and Si by H in the crystal structures under our synthesis conditions. Our study suggests that phases Psi and Phi are the two potential water carriers at the upper part of the mantle transitions zone, providing new insights into how deep water is stored in this region. [ABSTRACT FROM AUTHOR]

Hydrated ultramafic rocks play a key role in the transport of water from the surface to the mantle along subduction zones. Many key geochemical and geophysical features of Earth depend on this deep water cycle. Previous studies have demonstrated that antigorite and chlorite are the dominant water carriers in subducted slabs up to pressures corresponding to 100-150 km depth. Beyond 200 km depth hydrous Mg-silicates ("alphabet phases" A, E, D) as well as wadsleyite and ringwoodite potentially can host weight per cent of H2O. The phase relations of hydrous ultramafic rocks and the geotherms in the critical depth range of 120-200 km will determine how much water is cycled through arc magmatism back to the Earth surface and how much water is subducted to the deeper mantle. To date, very limited information is available to constrain this crucial aspect of the deep water cycle. We have conducted piston cylinder experiments in the range of 4-6.2 GPa and 600-850°C to address subduction water recycling in chlorite-rich ultramafic rocks using natural starting materials. Chlorite is stable up to 800°C at 4 GPa and then there is a moderate backbend of chlorite stability to 5.6 GPa, 740°C. Above 5.6 GPa, chlorite reacts to a hydrous Mg-Al-silicate, the 11.5 Å phase (12 wt% H2O) + garnet + olivine. Above 6.2 GPa, 600-700°C chlorite breaks down to Mg-sursassite (7.2 wt% H2O) + 11.5 Å phase + olivine. The transition of chlorite to these other hydrous Mg-Al-silicates occurs at significantly higher temperature and lower pressure conditions than previously reported, with important consequences for the deep water cycle.The new results show that along warm and intermediate slab geotherms, all hydrous phases in subducted ultramafic rocks will break down. Also in the warmer mantle wedge, no hydrous phases are stable. Thus, for these conditions, a few 100 ppm of water will be stored solely in the nominally anhydrous minerals garnet, pyroxenes and olivine. However in cold subduction zones and even more so in the cold interior of subducted slabs, water can be transferred from the hydrous phase chlorite via the hydrous 11.5 Å phase and Mg-sursassite to the hydrous "alphabet phases", providing an efficient mechanism to transport significant amounts of water to the deeper mantle along subduction zones. Thus, water incorporated in hydrous ultramafic rocks in the uppermost part of the subducted slab and in the hydrated forearc will not be able to be transported beyond 150 km depth and will be mostly recycled through arc magamatism at timescales of a few million years. In contrast, water incorporated in the slab lithospheric mantle due to hydration at bending faults will likely be transported to the deeper mantle and might return to the surface only after residence times of several 100 million years. [ABSTRACT FROM AUTHOR]

δ-AlOOH is regarded as a potential water carrier that is stable in the Earth's lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth's interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A1 mode, which corresponds to a lattice vibration of the AlO6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm−1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P21nm-to-Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P21nm-to-Pnnm transition. The continuous decrease in R2-R1 splitting with pressure indicated that the distortion of the AlO6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron difraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa. [ABSTRACT FROM AUTHOR]

Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as serpentine or chlorite, Mg–sursassite and/or the 10 Å phase, and phase A can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. The transition zone has a large water storage capacity because of the high solubility of water in wadsleyite and ringwoodite. The recent discovery of hydrous ringwoodite and phase Egg as inclusions in ultra deep diamonds from Juina, Brazil suggests that the transition zone may indeed contain water. Seismic tomographic studies and electrical conductivity observations suggest that the transition zone may contain large amount of water, at least locally, beneath the subduction zones. The discovery of a new hydrous phase H, MgSiO 2 (OH) 2 , and its solid solution with isostructural phase δ-AlOOH, suggests that a significant amount of water could be stored in this hydrous magnesium silicate phase which is stable down to the lower mantle. Water may be transported into the bottom of the lower mantle via phase H–δ solid solution in descending slabs. This new high pressure hydrous phase solid solution has a high bulk modulus and sound velocity owing to strong O–H bonding due to hydrogen bond symmetrization in the lower mantle. Therefore, water stored in this hydrous phase would not reduce the seismic wave velocity in the lower mantle, and is seismically invisible. Dehydration melting could then occur at the base of the lower mantle, providing a potential explanation for the ultralow-velocity zone at the core–mantle boundary. When this hydrous magnesium silicate phase or hydrous melt makes contact with the metallic outer core at the core–mantle boundary, then hydrogen is likely to dissolve into the core. [ABSTRACT FROM AUTHOR]

Four ultramafic bulk compositions comprising only natural minerals were used to constrain the stability field of chlorite in a variety of subducted, chlorite-rich rocks through an examination of key chlorite dehydration reactions relevant to the sub-arc. Seventy-four piston cylinder experiments were conducted at a range of pressures (1.0–5.0 GPa) and temperatures (500°C–1150°C). Bulk 1 represents a chlorite mélange (Mg# = 0.94) typically formed in the subduction channel. This composition was used to examine the terminal chlorite reactions to olivine, orthopyroxene, and spinel at low pressure and to olivine, garnet, and spinel at high pressure. Chlorite achieves a thermal maximum stability at 2.0 GPa, 850°C; at 3.0 GPa, 850°C; and at 5.0 GPa, 760°C. The terminal chlorite breakdown reaction rises at a much steeper Clapeyron slope than shown in previous studies. Bulk 2 contains additionally antigorite and tremolite, to constrain phase relations in more fertile compositions. Chlorite reacts with clinopyroxene at ~100°C lower temperatures and with orthopyroxene at ~20°C–60°C lower temperatures than the terminal chlorite breakdown. The reactions have a subparallel Clapeyron slope and none of the three chlorite dehydration reactions crosses the antigorite breakdown reaction up to 5 GPa. This demonstrates that chlorite is the most stable carrier of H2O to high temperatures in subducted ultramafic rocks. Chlorite mélanges that form at the subduction plate interface will dehydrate at 850°C–800°C, 80–120 km depth for intermediate to hot subduction geotherms and liberate 10–12 wt.% of H2O, triggering wet melting in associated sediments. For cold subduction geotherms, chlorite dehydration occurs at 780°C–740°C, 120–170 km depth. Interaction of such fluids with sediments will likely produce a supercritical fluid phase. No melting in the ultramafic rocks has been observed at the chlorite breakdown reactions. Wet melting of the chlorite mélange at 3 GPa occurred between 1100°C and 1150°C. The stability of chlorite in more Fe-rich mélanges (bulk Mg# = 0.50 and 0.68, respectively) were conducted at 3.0 GPa and revealed thermal maxima at 650°C and 765°C, respectively. Collectively, the thermal stability of chlorite is dependent upon the Mg# of the bulk composition and spans over 200°C at sub-arc depths. The density of run products was calculated to test the validity of the chlorite mélange diapir model. With the progressive breakdown of chlorite, ultramafic chlorite mélanges transform into garnet peridotite, thereby losing any buoyancy they initially possessed. This makes the likelihood of mélange diapirs as a major transport mechanism through the sub-arc unfeasible. [ABSTRACT FROM AUTHOR]

Hypervelocity impacts are among the fundamental phenomena occurring during the evolution of the solar system and are characterized by instantaneous ultrahigh pressure and temperature. Varied physicochemical changes have occurred in the building blocks of celestial bodies under such extreme conditions. The constituent material has transformed into a denser form, a high-pressure polymorph. The high-pressure polymorph is also thought to be the constituent of the deep Earth's interior. Hence, experiments using a high-pressure and temperature generating apparatus have been conducted to clarify its crystal structure, pressure–temperature stability range, and transformation mechanisms. A natural high-pressure polymorph (mineral) is found from terrestrial and extraterrestrial rocks that experienced a hypervelocity impact. Mineralogists and planetary scientists have investigated high-pressure minerals in meteorites and rocks near terrestrial craters over a half-century. Here, we report brief reviews about the experiments producing high-pressure polymorphs and then summarize the research histories of high-pressure minerals occurring in shocked meteorites and rocks near terrestrial craters. Finally, some implications of high-pressure minerals found in impact-induced shocked rocks are also mentioned. [ABSTRACT FROM AUTHOR]

The behavior of chrysotile Mg3(Si2O5)(OH)4 in water medium at simultaneously high pressure and high temperature was studied by in situ synchrotron X-ray diffraction using a diamond anvil cell. In contrast to previous 'dry' experiments, chrysotile in water-saturated conditions undergoes two-phase transitions and exhibits higher thermal stability. At 260 °C / 3.7 GPa the initial chrysotile (phase I) transforms to the 'chrysotile-like' phase II, followed by the appearance of the 'chrysotile-like' phase III at 405 °C / 5.25 GPa. Phase III is characterized by enlarged interlayer distances, presumably resulting from the H2O intercalation into the interlayer space. During further compression, the 'chrysotile-like' phase III is decomposed to the 10 Å phase Mg3(Si4O10)(OH)2·xH2O, the 3.65 Å phase MgSi(OH)6, phase D, forsterite, enstatite and coesite or stishovite. The 3.65 Å phase appears at 8.8 GPa / 500 °C. The series of transformations leads to a water deficiency in the system, restricting the complete transformation from the 10 Å phase to the 3.65 Å phase. These data emphasize the crucial role of excess water in the stabilization of the high-pressure hydrous phases. The present study is the first in situ observation of sequential transformations of hydrous phases: serpentine → 10 Å phase → 3.65 Å phase, important as a potential water transport mechanism to the deep mantle. [ABSTRACT FROM AUTHOR]

The distribution of intermediate‐depth and deep intraslab earthquakes with respect to subducting slabs offers a unique insight into seismogenesis at high pressures and temperatures that should inhibit brittle failure. This study constrains the surface of the subducting Pacific Plate beneath Japan at depths between 100 and 380 km based on a previous continental‐scale adjoint tomography model. Earthquake distributions relative to the slab surface reveal double seismic zones located within the top 60 km of the Pacific Plate. Thermal modeling suggests that the lower‐plane seismicity corresponds to temperatures between 400 and 900 °C. The seismogenic pressure and temperature conditions correlate approximately with the conditions of dehydration reactions of several hydrous minerals, that is, antigorite (serpentine) and chlorite at depths between 100 and 200 km and phase A at greater depths between 200 and 380 km. These correlations indicate that at these depths water released from dehydration processes may facilitate triggering slab mantle earthquakes. Plain Language Summary: Most earthquakes below 70 km are thought to occur within subducting oceanic plates. The cause of these intermediate‐depth and deep earthquakes is still enigmatic because at high pressure‐temperature environments brittle fractures are not expected to happen. Various earthquake mechanisms have been proposed, but differentiating them is challenging, partly due to the resolution limitations in seismic imaging of both the subducting plate structure and the earthquake sources. This study defines the deep interface of the subducting Pacific Plate beneath Japan using an improved model, obtained with an advanced seismic imaging technique. We discover vertically varying double seismic zones located within the top 60 km of the plate at 100‐ to 380‐km depths. The calculated pressure‐temperature conditions of the lower plane of the double seismic zone are correlated with the conditions at which a series of hydrous minerals break down into water and solids. This correlation suggests that water released from breakdown of hydrous minerals likely raises pore‐fluid pressure and triggers intermediate‐depth and deep earthquakes similar to the process of hydraulic fracturing. Key Points: Seismic model EARA2014 beneath Japan shows intermediate‐depth and deep earthquakes occur in the top 60 km of the subducted Pacific PlateThe lower plane of the double seismic zones at 100‐ to 380‐km depths corresponds to a slab mantle with temperatures between 400 and 900 degrees CelsiusDehydration of hydrous minerals in the slab mantle may facilitate deep earthquake genesis through the released water [ABSTRACT FROM AUTHOR]

Phase relations for a natural serpentinite containing 5 wt% of magnetite have been investigated using a multi-anvil apparatus between 6.5 and 11 GPa and 400-850 °C. Post-antigorite hydrous phase assemblages comprise the dense hydrous magnesium silicates (DHMSs) phase A (11.3 wt% H2O) and the aluminous phase E (Al-PhE, 11.9 wt% H2O). In addition, a ferromagnesian hydrous silicate (11.1 wt% H2O) identified as balangeroite (Mg,Fe)42Si16O54(OH)40, typically described in low pressure natural serpentinite, was found coexisting with Al-PhE between 650 and 700 °C at 8 GPa. In the natural antigorite system, phase E stability is extended to lower pressures (8 GPa) than previously reported in simple chemical systems. The reaction Al-phase E = garnet + olivine + H2O is constrained between 750 and 800 °C between 8 and 11 GPa as the terminal boundary between hydrous mineral assemblages and nominally anhydrous assemblages, hence restricting water transfer into the deep mantle to the coldest slabs. The water storage capacity of the assemblage Al-PhE + enstatite (high-clinoenstatite) + olivine, relevant for realistic hydrated slab composition along a relatively cold temperature path is estimated to be ca. 2 wt% H2O. Attempts to mass balance run products emphasizes the role of magnetite in phase equilibria, and suggests the importance of ferric iron in the stabilization of hydrous phases such as balangeroite and aluminous phase E. [ABSTRACT FROM AUTHOR]

Abstract: We used two‐dimensional simulations to investigate the effects of a hydrous lithospheric mantle (HLM) on subduction dynamics. It was demonstrated that the thickness of a subducting HLM strongly controls the motion and deformation of converging plates. When the HLM thickness is lower than a critical value, for example, ∼5 km for a 100‐Ma slab, all serpentine in the HLM decomposes, and the liberated fluid enters the mantle wedge. The fluid weakens the overlying plate, causing trench retreat, rapid plate convergence, and slab stagnation. In contrast, when the HLM thickness exceeds the critical value, dense hydrous magnesium silicates appear in cold parts of the HLM at high pressures. The buoyant dense hydrous magnesium silicates reduce the slab pull force, weakening trench retreat, rapid convergence, and slab stagnation. A low HLM thickness may account for tectonics in some actual subduction zones characterized by trench retreat, rapid convergence, and stagnant slabs, for example, in Northeast Japan. [ABSTRACT FROM AUTHOR]

The Earth's deep interior is a hidden water reservoir on a par with the hydrosphere that is crucial for keeping the Earth as a habitable planet. In particular, nominally anhydrous minerals (NAMs) in the silicate Earth host a significant amount of water by accommodating H point defects in their crystal lattices. Water distribution in the silicate Earth is highly heterogeneous, and the mantle transition zone may contain more water than the upper and lower mantles. Plate subduction transports surface water to various depths, with a series of hydrous minerals and NAMs serving as water carriers. Dehydration of the subducting slab produces liquid phases such as aqueous solutions and hydrous melts as a metasomatic agent of the mantle. Partial melting of the metasomatic mantle domains sparks off arc volcanism, which, along with the volcanism at mid-ocean ridges and hotspots, returns water to the surface and completes the deep water cycle. There appears to have been a steady balance between hydration and dehydration of the mantle at least since the Phanerozoic. Earth's water probably originates from a primordial portion that survived the Moon-forming giant impact, with later delivery by asteroids and comets.Water could play a critical role in initiating plate tectonics. In the modern Earth, the storage and cycling of water profoundly modulates a variety of properties and processes of the Earth's interior, with impacts on surface environments. Notable examples include the hydrolytic weakening effect on mantle convection and plate motion, influences on phase transitions (on the solidus of mantle peridotite in particular) and dehydration embrittlement triggering intermediate- to deep-focus earthquakes. Water can reduce seismic velocity and enhance electrical conductivity, providing remote sensing methods for water distribution in the Earth's interior. Many unresolved issues around the deep water cycle require an integrated approach and concerted efforts from multiple disciplines. [ABSTRACT FROM AUTHOR]

The dehydration reactions of minerals in subduction zones strongly control geological processes, such as arc volcanism, earthquakes, serpentinization, or geochemical transport of incompatible elements. In aluminum-bearing systems, chlorite is considered the most important hydrous phase at the top of the subducting plate, and significant amount of water is released after its decomposition. However, the dehydration mechanism is not fully understood, and additional hydrates are stabilized by the presence of Al beyond the stability field of chlorite. We applied here a cutting-edge analytical approach to characterize the experimental rocks synthesized at the high pressures and temperatures matching with deep subduction conditions in the upper mantle. Fast electron diffraction tomography and high-resolution synchrotron X-ray diffraction allowed the identification and the successful structure solution of two new hydrous phases formed as dehydration product of chlorite. The 11.5 Å phase, Mg6Al(OH)7(SiO4)2, is a hydrous layer structure. It presents incomplete tetrahedral sheets and face-sharing magnesium and aluminum octahedra. The structure has a higher Mg/Si ratio compared to chlorite, and a significantly higher density (ρ0 = 2.93 g/cm3) and bulk modulus [ K0 = 108.3(8) GPa], and it incorporates 13 wt% of water. The HySo phase, Mg3Al(OH)3(Si2O7), is a dense layered sorosilicate, [ρ0 = 3.13 g/cm3 and K0 = 120.6(6) GPa] with an average water content of 8.5 wt%. These phases indicate that water release process is highly complex, and may proceed with multistep dehydration, involving these layer structures whose features well match the high-shear zones present at the slab-mantle wedge interface. [ABSTRACT FROM AUTHOR]

A new Al-bearing hydrous Mg-silicate that we named as 23 Å phase was synthesized at 10 GPa and 1000 °C, while also coexisting with diaspore and pyrope in the following system: phase A [Mg7Si2O8(OH)6] + Al2O3 + H2O. The chemical composition of this new 23 Å phase is Mg11Al2Si4O16(OH)12, and it contains about 12.1 wt% water. Powder X-ray diffraction and electron diffraction patterns show that this new 23 Å phase has a hexagonal structure, with a = 5.1972(2), c = 22.991(4) Å, and V = 537.8(2) Å3, and the possible space group is P6̅c2, P63cm, or P63/mcm. The calculated density is 2.761 g/cm3 accordingly, which was determined by assuming that the formula unit per cell (Z) is 1. This crystal structure is quite unique among mantle minerals in having an extraordinarily long c axis. Several experiments revealed that its stability region is very similar to that of phase A. We further confirmed that this new 23 Å phase was stable in the chlorite composition at 10 GPa and 1000 °C. The present results indicate that this new 23 Å hydrous phase will form in an Al-bearing subducting slab, and transport water together with Al into the deep upper mantle or even into the upper part of the transition zone. [ABSTRACT FROM AUTHOR]

Dehydration and fluid circulation are integral parts of subduction tectonics that govern the dynamics of the wedge mantle. The knowledge of the elastic behavior of aqueous fluid is crucial to understand the fluid–rock interactions in the mantle through velocity profiles. In this study, we investigated the elastic wave velocities of chlorite at high pressure beyond its dehydrating temperature, simulating the progressive dehydration of hydrous minerals in subduction zones. The dehydration resulted in an 8% increase in compressional (Vp) and a 5% decrease in shear wave (Vs) velocities at 950 K. The increase in Vp can be attributed to the stiffening of the sample due to the formation of secondary mineral phases followed by the dehydration of chlorite. The fluid-bearing samples exhibited Vp/Vs of 2.45 at 950 K. These seismic parameters are notably different from the major mantle minerals or hydrous silicate melts and provide unique seismic criteria for detecting mantle fluids through seismic tomography. [ABSTRACT FROM AUTHOR]

Talc and 10-Å phase are hydrous phases that are implicated in fluid processes and rheological behaviour in subduction zones. Natural samples of talc show limited compositional variation away from the MgO–SiO2–H2O (MSH) endmember, with only substitution of Fe2+ for Mg occurring in significant amounts. In experiments at 2 GPa, talc containing 0.48 apfu Fe2+ begins to break down in the divariant field talc + anthophyllite + quartz at ~ 550 °C, a temperature ~ 270 °C lower than in the MSH system. At 4 GPa, Fe-bearing talc breaks down over a wide temperature interval in the divariant field talc + enstatite + coesite. The large decrease in temperature of the beginning of talc breakdown shows that Fe2+ is partitioned strongly into enstatite and anthophyllite with respect to talc. In phase reversal experiments at 6.5 GPa, the beginning of the dehydration of 10-Å phase containing 0.48 apfu Fe2+ was bracketed between 575 °C and 600 °C, a temperature ~ 100 °C lower than the MSH endmember reaction. The relative positions of the talc and 10-Å phase dehydration reactions indicate that the latter is able to accommodate greater Fe substitution, and is, therefore, more stable in Fe-bearing systems. In experiments at 6.2 GPa, 650 °C in the systems MgO–Al2O3–SiO2–H2O (MASH) and Na2O–MgO–Al2O3–SiO2–H2O (NMASH), 10-Å phase was synthesised that contains up to 0.5 apfu Al in the system MASH (compared to 0.8 in the starting material) and up to 0.4 apfu Al + 0.4 apfu Na in the system NMASH (compared to 0.7 of each of Al and Na in the starting material). Further experiments are required to determine if higher Al and Na contents in 10-Å phase are possible. The much higher Al and Na contents than found in talc indicate that, as with Fe, substitution of these elements enlarges the 10-Å phase stability field with respect to talc. In contrast to the effect of Fe, Al and Na also increase the stability of 10-Å phase relative to its thermal breakdown products enstatite + coesite. [ABSTRACT FROM AUTHOR]