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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.

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.