More than one teramole of carbon per year is subducted as carbonate or carbonaceous material. However, the influence of carbonation/decarbonation reactions on seismic activity within subduction zones is poorly understood. Here we present field and microstructural observations, including stable isotope analyses, of carbonate veins within the Higuchi serpentinite body, Japan. We find that the carbon and oxygen isotope compositions of carbonate veins indicate that carbonic fluids originated from organic materials in metasediments. Thermodynamic calculations reveal that carbonation of serpentinite was accompanied by a solid volume decrease, dehydration, and high magnesium mobility. We propose that carbonation of the mantle wedge occurs episodically in a self-promoting way and is controlled by a solid volume contraction and fluid overpressure. In our conceptual model, brittle fracturing and carbonate precipitation were followed by ductile flow of carbonates and hydrous minerals; this might explain the occurrence of episodic tremor and slip in the serpentinized mantle wedge.

The mechanisms of fluid penetration through the gabbroic lower crust are important for the hydration of oceanic lithosphere. In the Oman ophiolite, which preserves an entire sequence of oceanic lithosphere formed at a fast‐spreading ridge, the layered gabbros and dunites are extensively serpentinized. In this paper, we describe the characteristic textures of serpentinized troctolite and olivine gabbros recovered from the CM1A site of the Oman Drilling Project. In the troctolite, an olivine mesh texture is pervasively developed and is characterized by two types of veins: early lizardite + brucite + magnetite and late Al‐rich lizardite + magnetite. These veins suggest the initiation of serpentinization at <350°C and a supply of Si and Al from plagioclase during the later stages of serpentinization. Plagioclase surrounding serpentinized olivine grains commonly shows radial fracturing. Numerical simulations using the discrete element method applied to coupled fluid flow, reaction, and fracturing reveal that volume expansion of olivine grains during serpentinization results in the simultaneous fracturing of olivine and surrounding plagioclase, and that the thermal stress during cooling of oceanic lithosphere might also cause preferential olivine fracturing prior to serpentinization. The simulations also predict a self‐organizing fracture network that connects the olivine grains and passes through both olivine‐rich and olivine‐poor layers, resulting in permeability enhancement during serpentinization. Our results suggest that reaction‐induced fracturing plays an essential role in the infiltration of seawater through the lower crust and into the mantle within oceanic lithosphere.

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.

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.