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Fluid flow in subduction zones is one of the essential factors of seismic activity in subduction zones. However, the timescale of fluid flow and fluid flow velocity in subduction zones is unclear. In this study, we report antigorite veins with brucite-rich reaction zones in the crust-mantle transition zone of the Oman ophiolite and estimate the timescale and fluid flow velocity during vein formation.In this study, we observed 28 samples in the lower crust to upper mantle section obtained from the Oman Drilling Project Hole CM1A (Kelemen et al., 2020). The lithology of borehole CM1A consists of lower crust (0-160 m depth), crust-mantle transition zone (160-310 m depth), and mantle section (310-404 m depth), which were mainly composed of altered gabbroic rocks (olivine gabbro, troctolite: 5-95% altered) and completely serpentinized dunite and 70-100% serpentinized harzburgite, respectively. Antigorite-chrysotile (Atg-Ctl) vein network was found in dunite at 160-180 m in Hole CM1A. The matrix of the dunite (lizardite, brucite, and magnetite) is cut by antigorite-chrysotile (Atg-Ctl) vein network. Trace element analysis using LA-ICP-MS revealed that the Atg-Ctl vein is enriched in As and Sb compared to the matrix lizardite, suggesting that the Atg-Ctl veins were formed by fluids interacting with subducting sediments. Some of the Atg-Ctl veins are accompanied by brucite-rich reaction zones. The brucite-rich reaction zone was developed at both sides of the antigorite veins with widths of 0.5 – 4 mm. Elemental mapping of the reaction zone using EPMA and TEM show sharp reaction front at scale from micro- to nano- meter.Mass balance calculations and thermodynamic considerations of the reaction zone suggest that the formation of the reaction zone was caused by the removal of silica from the host rock during the precipitation of antigorite in the veins. Based on a diffusion model, we estimated the fluid activity is short-lived (2.1 × 10–1 to 1.1 × 101 yr), and the fluid flow velocity of 2.7 × 10–3 to 4.9 × 10–2 m s-1, which is much faster than those observed for the intact mantle and crustal rocks. This fluid flow velocity along the fractures within the mantle wedge is similar to the observed propagation velocities of seismic events in subduction zones. These results suggest that fluid flow in the overlying plate occurs as episodic pulses as observed as the migration of seismicity in the present subduction zones.

Pore fluid pressure on subduction zone megathrusts is lowered by fluid drainage into the overlying plate, affecting subduction zone seismicity. However, the spatial and temporal scales of fluid flow through suprasubduction zones are poorly understood. We constrain the duration and velocity of fluid flow through a shallow mantle wedge based on the analyses of vein networks consisting of high-temperature serpentine in hydrated ultramafic rocks from the Oman ophiolite. On the basis of a diffusion model and the time-integrated fluid flux, we show that the channelized fluid flow was short-lived (2.1 × 10 −1 to 1.1 × 10 1 years) and had a high fluid velocity (2.7 × 10 −3 to 4.9 × 10 −2 meters second −1 ), which is close to the propagation velocities of seismic events in present-day subduction zones. Our results suggest that the drainage of fluid into the overlying plate occurs as episodic pulses, which may influence the recurrence of megathrust earthquakes.

Talc is widely distributed over the Earth's surface and is predicted to be formed in various tectonic settings. Talc is a very soft and anisotropic sheet silicate showing very low friction behavior. Therefore, the formation of talc is expected to weaken the strength of talc-bearing rocks and may be associated with the initiation of subduction, and with a decrease in the coupling coefficient resulting in aseismic movements along faults and shear zones within subduction zones. For these reasons, understanding the crystallographic preferred orientation (CPO) of talc is important to quantify the anisotropy and physical properties of the host rock. However, it is difficult to measure a significant number of talc crystal orientations and to evaluate the accuracy of the measurements using electron-backscattered diffraction (EBSD). Therefore, talc CPO has not been reported, and there is uncertainty regarding the estimation of the strength of deformed talc-bearing rocks. Using methods developed for antigorite, we report the first successful EBSD measurements of talc CPO from a talc schist formed due to Simetasomatism of ultramafic rocks by subduction zone fluids. We used a combination of W-SEM and FE-SEM measurements to examine domains of various grain sizes of talc. In addition, we used TEM measurements to evaluate the accuracy of the EBSD measurements and discuss the results of talc CPO analysis. Talc CPO in the present study shows a strong concentration of the pole to the (001) plane normal to the foliation. The strongest concentration of the [100] direction is parallel to the lineation. The talc schist produces similar S-wave splitting and P- and S-wave anisotropy as antigorite schist in deeper domains, thus identifying talc-rich layers in subduction zones may require a combination of geophysical surveys, seismic observations, and anisotropy modeling. The presence of strong talc CPO in rocks comprising the slab–mantle interface boundary may promote spatial expansion of the slip area during earthquakes along the base of the mantle wedge.

The slab-mantle interface is one of the most active sites of fluid-rock interaction, which affects the mass transfer and mechanical properties along subduction zone megathrust. However, the effects of CO2 fluids and carbonation/decarbonation reactions on seismic activity are still poorly understood. In addition, although mantle peridotite is known as a large sink of CO2, the nature of carbonation at mantle wedge condition remains unconstrained. In this study, we show the characteristics of carbonation of serpentinite body from the Sanbagawa metamorphic belt, Kanto Mountain, Japan [1]. The Higuchi serpentinite body (8 x15 m) is mainly composed of antigorite, and has not relics of olivine and pyroxenes. Massive antigorite parts are segmented by talc + carbonates (magnesite, dolomite and calcite) veins. At the boundary between serpentinite body and pelitic schists, actinolite-chlorite schist and chlorite rocks were formed. The location, depleted composition of Cr-rich spinel in the Higuchi body and temperature of ~400 ˚C of carbonation suggest that this body was originated from the leading edge of the mantle wedge. The carbon and oxygen stable isotope compositions of carbonates within the Higuchi body reveal that carbonic fluid was derived from carboniferous materials in sediments. Carbonation of serpentinite body is characterized by gains of CO2, silica and Ca, and losses of H2O and Mg. The thermodynamic calculations on mineral-fluid equilibria reveal that (1) the carbonic fluid produced under the oxidizing conditions (QFM +0.3) explains the systematic mineralogical variations within the Higuchi body, and that (2) carbonation of serpentinite proceeded with solid volume contraction, high fluid pressure and high mobility of Mg, which is largely consistent with the experimental carbonation at the mantle wedge condition [2]. This is consistent with the tree-like patterns of carbonate veins within the Higuchi body. Brittle failure to form carbonate veins was followed by a viscos flow of carbonate and talc. We infer that episodic infiltration of oxidizing fluids causes self-promoting carbonation of mantle wedge with solid volume change, which could affect the mechanical properties of slab-mantle interface.[1] Okamoto, A, et al., 2021. Com Env Earth, 58, 4831-4839. doi.org/10.1038/s43247-021-00224-5[2] Sieber et al. 2020. J. Petrol., 1-24. doi: 10.1093/petrology/egaa035

Fluid flow in subduction zones is related to geological processes such as seismic and volcanic activities. However, the timescale of fluid flow and its flux in the supra-subduction setting is unclear. In this study, we report the novel texture of the antigorite veins with a brucite-rich reaction zone in dunite in the crust-mantle transition zone of the Oman ophiolite, and investigated the timescale and time-integrated fluid flux during the vein formation.The antigorite veins occur in the drilling cores of serpentinized dunite in the crust-mantle transition zone taken from the Oman Drilling Project CM1 site (Wadi Zeeb, Northern Sharqiyah). The dunite samples are completely serpentinized and consist mainly of lizardite, brucite, magnetite, and Cr-rich spinel and are cut by the antigorite vein networks and later chrysotile. These features indicate that the antigorite veins formed at the stage of obduction of the Oman ophiolite. The antigorite veins, which are distinct by ‘bright’ networks by X-ray CT due to magnetite, occur preferentially in dunite (at 160 – 313 m along the CM1A). The veins are filled with a mixture of randomly orientated antigorite crystals and fine chrysotile. Some antigorite vein contains fragments of the host rock. The brucite-rich reaction zone was developed at both sides of the antigorite veins with a thickness of 0.5 – 4 mm. The reaction zone is composed of brucite (39.4 area%), chrysotile (59.3 area%), and magnetite (1.3 area%). The microtexture of the reaction zone indicates the brucite silicification after the brucite reaction zone formation. Mass balance and thermodynamic calculation suggest that silica was leached from the host rock lizardite during antigorite vein crystallization, resulting in the formation of the brucite reaction zone. Given solution chemistry and the amount of leached SiO2 during vein formation, the time-integrated fluid flux was estimated to be 105 - 106 m3(fluid) m-2(rock). A diffusion-controlled model suggests that the reaction zone was formed in a short time, ~10-1 - 100 years. These results suggest that a large amount of high-temperature fluid passed through the fracture network over several hundred meters in a short time at the earlier stage of the obduction of the Oman ophiolite.

Silica activity in fluids is a key factor that controls reaction pathways during the hydrothermal alteration of olivine in the oceanic lithosphere. In this study, we conducted hydrothermal experiments (300 °C, 8.58 MPa) on the olivine (Ol)–quartz (Qtz)–H2O system to understand the coupling between silica transport and olivine alteration. Mineral powders were reacted with 0.5 mol kg−1 NaCl solution in a tube-in-tube type vessel, and the spatial distribution of reactant and product minerals was investigated after the experiments. Alteration zones formed in the Ol-hosted region after 2055 hours of reaction. With increasing distance from the Ol–Qtz boundary these were: talc; talc + serpentine; and serpentine + magnetite + brucite. Talc formed 0–2.3 mm from the Ol–Qtz boundary in the Ol-hosted region, and brucite formed >5 mm from the Ol–Qtz boundary in the Ol-hosted region. No secondary minerals formed in the Qtz-hosted region. The observed mineral distribution was modeled using a reactive transport model that simulated the coupling between SiO2(aq) diffusion and seven silica-controlling reactions. An inverse modeling framework, which combines a reactive transport model with an exchange Monte Carlo method, was used to parameterize the diffusivity of SiO2(aq) and the rate constants of the seven overall reactions. Our model shows that the rate of hydration in the serpentine + metastable talc zone with intermediate silica activity was higher than in the serpentine, serpentine + brucite, and talc zones, suggesting that the silica activity of the reacting fluid has a significant control on the rate of hydrothermal alteration of mantle peridotite by crustal fluids. Moreover, the model suggests that the rate-control process changed from being surface- to transport-controlled over the course of the experiments. We suggest that dynamic changes in rate control process are important contributors to the formation of metasomatic zoning and heterogeneous hydration patterns within the oceanic lithosphere.

Heterogeneous reactions are chemical reactions that occur at the interfaces of multiple phases, and often show a nonlinear dynamical behavior due to the effect of the time-variant surface area with complex reaction mechanisms. It is important to specify the kinetics of heterogeneous reactions in order to elucidate the microscopic elementary processes and predict the macroscopic future evolution of the system. In this study, we propose a data-driven method based on a sparse modeling algorithm and sequential Monte Carlo algorithm for simultaneously extracting substantial reaction terms and surface models from a number of candidates by using partial observation data. We introduce a sparse modeling approach with non-uniform sparsity levels in order to accurately estimate rate constants, and the sequential Monte Carlo algorithm is employed to estimate time courses of multi-dimensional hidden variables. The results estimated using the proposed method show that the rate constants of dissolution and precipitation reactions that are typical examples of surface heterogeneous reactions, necessary surface models, and reaction terms underlying observable data were successfully estimated from only observable temporal changes in the concentration of the dissolved intermediate products.

Heterogeneous reactions are chemical reactions with conjugation of multiple phases, and they have the intrinsic nonlinearity of their dynamics caused by the effect of surface area between different phases. In earth science, it is important to understand heterogeneous reactions in order to figure out the dynamics of rock formation near surface of the earth. We employ sparse modeling algorithm and sequential Monte Carlo algorithm to partial observation problem, in order to simultaneously extract substantial reaction terms and surface models from a number of candidates. Using our proposed method, we show that heterogeneous reactions can be estimated successfully from noisy observable data under conditions that the number of observed variables is less than that of hidden variables.

Abstract Inversion is a key method for extracting nonlinear dynamics governed by heterogeneous reaction that occur in parallel in the natural sciences. Therefore, in this study, we propose a Bayesian statistical framework to determine the active reaction pathways using only the noisy observable spatial distribution of the solid phase. In this method, active reaction pathways were explored using a Widely Applicable Bayesian Information Criterion (WBIC), which is used to select models within the framework of Bayesian inference. Plausible reaction mechanisms were determined by maximizing the posterior distribution. This conditional probability is obtained through Markov chain Monte Carlo simulations. The efficiency of the proposed method is then determined using simulated spatial data of the solid phase. The results show that active reaction pathways can be identified from the redundant candidates of reaction pathways. After these redundant reaction pathways were excluded, the controlling factor of the reaction dynamics was estimated with high accuracy. Graphic Abstract

Episodic tremor and slip (ETS) events in the forearc mantle wedge of a warm subduction zone may reflect mixed brittle–ductile deformation of serpentinite in association with high pore fluid pressures. To understand deformation mechanisms and processes occurring in the hydrated mantle wedge, we examined in the Sanbagawa metamorphic belt, SW Japan, an antigorite serpentinite shear zone derived from mantle wedge that was formed under pressure and temperature conditions that correspond to the ETS regions. The serpentinite underwent multiple extensional (mode I) and extensional–shear (mode I–II) failure events at supralithostatic pore fluid pressures ( P f ). Such failure events led to drops in P f (several MPa) and formation of a distributed ‘fault–fracture mesh’. Antigorite precipitation in the fracture openings contributed to an increase in P f until the failure condition was reached again, and thereby antigorite kinetics controlled the recurrence interval of seismic events. We also suggest that under the low- P f conditions that facilitate intracrystalline plasticity rather than cataclasis, the newly precipitated antigorite aggregates (localized along shear bands) deform by dislocation creep at a high strain rate and high shear stress, resulting in the transient, accelerated viscous creep that may characterize slow slip transients.

The Eastern Iratsu body of the Sanbagawa metamorphic terrane in central Shikoku recorded an Early Jurassic metamorphic event older than the Sanbagawa metamorphic event. This study carried out the petrochronology of eclogitic high-pressure marble in the Eastern Iratsu body to determine the corresponding conditions of metamorphism and their variation in time. A combination of trace element (Zr) thermometry, host-inclusion mineral elastic barometry, and in-situ U–Pb dating was performed on the titanite in an eclogitic marble to reveal the metamorphic pressure/temperature conditions of 2.3–2.7 GPa and 1000–1200 °C high-temperature (HT) eclogite facies metamorphic conditions, which occurred at approximately 200 Ma. Moreover, certain titanite grains exhibited a distinctly younger age of 164 Ma with a similar Zr concentration. The 164 Ma domain of titanite is observed near the Na-enriched diopside, indicating the fluid-infiltration and corresponding titanite recrystallization. Some diopside in the matrix contains high-salinity aqueous fluid inclusions (freezing point depression = 3.6–3.7 °C) in the Na-enriched domain, which may infer the source of Na. Furthermore, titanite rims containing trace amounts of Zr yielded 126 Ma, which is comparable with that of the early Sanbagawa metamorphism; however, no P-T estimation was available. This newly identified Early Jurassic metamorphic event is much older than the previously reported metamorphic age of the Sanbagawa metamorphic belt, i.e., c. 120 Ma for early Sanbagawa metamorphism. Although the origin of the HP marble is unclear, the pre-Sanbagawa HT eclogite facies metamorphism reported here could provide a new insight for the subduction of the Izanagi plate and the corresponding geotectonic evolution of eastern Asia.

Metaharzburgite and metadunite in the ultramafic body of the Naran Massif in the Khantaishir Ophiolite, western Mongolia, record multi-stage processes of serpentinization (antigorite, lizardite + brucite, then chrysotile). Bulk-rock chemistry and the compositions of primary olivine (P-olivine) and Cr-spinel suggest that the alteration occurred in the forearc mantle. In the metaharzburgite, a novel occurrence of fine-grained (10–50 μm) secondary olivine (S-olivine) takes the form of aggregates (a few millimeters across) with bands of antigorite. The S-olivine has higher Mg# values (0.96–0.98) than the P-olivine (Mg# = 0.92–0.94) and contains inclusions of clinopyroxene and magnetite. The P-olivine has been replaced by antigorite and magnetite. Mesh textures of lizardite + brucite are developed in both P- and S-olivine. The microtextures and chemical compositions of minerals suggest that S-olivine aggregates were formed by pseudomorphic replacement of orthopyroxene related to multi-stage hydration processes. Assuming the mantle wedge conditions beneath a thin crust, orthopyroxene was first replaced by S-olivine + talc at high temperatures (500–650 °C at ~ 0.5 GPa). With cooling to ca. 400–500 °C and fluid supply, talc transformed to antigorite with the release of silica. During this stage, P-olivine was also transformed to antigorite by consumption of silica released from orthopyroxene decomposition. At temperatures below 300 °C, lizardite + brucite ± magnetite formed from the remaining P- and S-olivine grains. The formation of S-olivine presented in this study contrasts with the commonly ascribed process of deserpentinization. Taking into account the geochemical data for the studied ultramafic rocks and those previously reported for mafic rocks, our results suggest that mantle wedge beneath thin crust was hydrated in response to continuous cooling and fluid supply from a subducting slab after subduction initiation.

Mineral–fluid equilibria in supercritical geothermal reservoirs is poorly understood, due to the non-applicability of the Helgeson–Kirkham–Flowers (HKF) electrostatic model for aqueous species in low-density regions. We experimentally investigated the dissolution equilibria of albite–K-feldspar–quartz in hydrothermal solutions at 400, 420 °C and 20–35 MPa, and compared the experimental results with thermodynamic calculations. The thermodynamics data based on the HKF model available through SUPCRT92 were extrapolated into the low-density regions with water density. Our results suggest that the extrapolated data are useful as a first approximation for evaluating feldspars–fluid equilibria under supercritical and low-density hydrothermal conditions.

Water&ndash, rock interaction in surface and subsurface environments occurs in complex multicomponent systems and involves several reactions, including element transfer. Such kinetic information is obtained by fitting a forward model into the temporal evolution of solution chemistry or the spatial pattern recorded in the rock samples, although geochemical and petrological data are essentially sparse and noisy. Therefore, the optimization of kinetic parameters sometimes fails to converge toward the global minimum due to being trapped in a local minimum. In this study, we simultaneously present a novel framework to estimate multiple reaction-rate constants and the diffusivity of aqueous species from the mineral distribution pattern in a rock by using the reactive transport model coupled with the exchange Monte Carlo method. Our approach can estimate both the maximum likelihood and error of each parameter. We applied the method to the synthetic data, which were produced using a model for silica metasomatism and hydration in the olivine&ndash, quartz&ndash, H2O system. We tested the robustness and accuracy of our method over a wide range of noise intensities. This methodology can be widely applied to kinetic analyses of various kinds of water&ndash, rock interactions.

Serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust. Given that such a fluid has a high silica activity compared with mantle peridotite, it is thought that serpentinization and silica metasomatism occur simultaneously at the crust–mantle boundary. In this study, we conducted hydrothermal experiments in the olivine (Ol)–quartz (Qtz)–H 2 O system at 250 °C and vapor-saturated pressure under highly alkaline conditions (NaOH aq , pH = 13.8 at 25 °C) to clarify the mechanism of silica metasomatism at the crust–mantle boundary. Composite powders consisting of a Qtz layer and an Ol layer were set in tube-in-tube vessels. After the experiments, the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the Ol–Qtz boundary. The mineralogy of the reaction products in the Ol-hosted region changed with increasing distance from the Ol–Qtz boundary, from smectite + serpentine (Smc zone) to serpentine + brucite + magnetite (Brc zone). Olivine hydration proceeded in both zones, but the total H 2 O content in the products was greater in the Brc zone than in the Smc zone. Mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone. In contrast, the Smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions. In the Smc zone, smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration, and around the boundary of the Smc and Brc zones, serpentine formation occurred by olivine hydration and brucite dehydration. The relative extent of hydration and dehydration reactions controlled the along-tube variation in the rate of H 2 O production/consumption and the rate of volume increase. Our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure, possibly affecting the mechanical behavior of the crust–mantle boundary.

Isocon analysis has been widely applied to various geoscientific problems as a simple standard tool for quantitative estimation of material transfer. Despite its usefulness, similar to all material transfer calculations, this method generally requires the presumptive specification of immobile elements or the assumption of conservation of mass or volume. However, the validity of such assumptions is particularly controversial. Here we propose a novel data-driven method that automatically estimates the mass gain or loss of elements based on compositional data of multiple samples that have been altered from the original rock without assuming immobile elements. The proposed method uses a mathematical framework, called sparse modeling, that can extract essential information from high-dimensional datasets based on the sparsity of the system. In this case, it is assumed that some elements show higher immobility than others (i.e., the material transfer of such elements is near zero). By optimizing the evaluation function, the immobile elements are automatically selected. By inputting only the bulk compositional datasets, the user can obtain the material gain or loss with total mass change ratio for each sample relative to the reference (original) rock. The effectiveness of the method is validated and discussed using synthetic and natural sample data. Software packages are available from the authors in MATLAB function and Excel workbook forms.