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Abstract Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

Abstract The slab–mantle interface in subduction zones is one of the geological boundaries with the most significant chemical potential gradients, which leads to fluid-mediated metasomatic reactions and chemical transport. As subducting sediment and basaltic crust often contain silica in various forms, the Si-metasomatism of mantle rocks is thought to occur along the subduction zone interface. However, growing evidence from the geochemistry of altered rocks and thermodynamic modelling has revealed the presence of multi-component fluids at the slab interface. Here, we review the laboratory experiments, geochemical models, and natural observations that improve our understanding of mass transport and metasomatic reactions at the crust–mantle interface, focusing on the relative mobility of Mg and Si. Hydrothermal experiments using analogues for the boundary between mantle (olivine) and crust (quartz or plagioclase) under vapor-saturated pressures indicate that Si is preferentially transported from crust to mantle, whereas Mg is immobile. This result is consistent with the distribution of talc rocks in oceanic lithosphere. On the other hand, at the contact between ultramafic (e.g., serpentinite) and crustal (pelitic schist or basaltic rocks) rocks in high-pressure metamorphic terranes, a large volume of chlorite rocks form in the crustal rocks, and the volume of chlorite often exceeds talc in serpentinites. Geochemical modeling reveals that in the shallow part of a subduction zone, the dissolved Si content of fluids in equilibrium with pelitic schist (C Si,crust) is significantly higher than the dissolved Mg content of fluids in equilibrium with mantle peridotite (C Mg,mantle); however, C Mg,mantle becomes dominant at depth, resulting in the Mg-metasomatism of crustal rocks to form chlorite rocks. This Mg-metasomatism is more widespread in warmer subduction zones (e.g., the Nankai and Cascadia subduction zones) than in colder subduction zones (e.g., in Northeast Japan). In addition, the infiltration of CO2-bearing fluid can form talc (along with carbonates) in ultramafic rocks without Si-metasomatism. Variations in the relative mobility of Si and Mg at the subduction zone interface produce variations in the overall solid volume change of mantle (expansion or contraction), the types of sheet silicates (talc versus chlorite), and the fluid budget (dehydration or hydration) during metasomatic reactions, which affects the pore fluid pressure, frictional strength of the subduction megathrust, and the location of seismicity around the mantle wedge corner.

Veins of aragonite in deep ocean trenches can record discharges of CO2-rich paleoseawater and suggest that exposed serpentinized forearcs can act as carbon reservoirs, according to residence time calculations based on geochemical analyses of mantle rocks from offshore Japan.

Carbonation of a serpentinized mantle wedge is associated with volume reduction and fluid over-pressurization which drives brittle fracture and seismicity, according to chemical analyses of carbonate veins from the Sanbagawa belt Japan and thermodynamic calculations.

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.