Recent thermodynamic modeling indicates that C–O–H fluids released from subducting metasediments beneath the forearc can react with the overlying mantle wedge, causing carbonation in regions where episodic tremor and slip (ETS) events occur. However, the influence of serpentinization and carbonation on the rheological behavior of ETS‐generating shear zones remains poorly understood. To address this issue, we investigated the shallow mantle wedge‐derived Higuchi serpentinite body in the late Cretaceous Sanbagawa belt, central Japan. Our results show that infiltration of CO ₂ ‐rich fluids into massive antigorite serpentinite caused distributed extensional and extensional–shear fracturing under supra‐lithostatic pore fluid pressures ( P _f ). Such failure events, potentially analogous to bursts of tectonic tremor, led to transient drops in P _f and subsequent sealing by newly precipitated chrysotile, talc, and carbonate minerals, predominantly magnesite. An anastomosing network of chrysotile and talc governed the viscous behavior of the shear zone through dissolution–precipitation creep, potentially influencing slip rates during short‐term slow slip events. Subsequently, infiltration of externally derived, high‐flux CO ₂ ‐ and CaO‐rich fluids triggered large‐scale, dolomite‐rich extensional and shear veining. Mutual crosscutting relationships among the veins, together with paleostress analysis, indicate fluctuating principal stress orientations, with σ ₁ and σ ₃ axes occasionally switching positions under sustained near‐lithostatic P _f during the vein‐forming cycle. These findings suggest that mantle wedge carbonation is largely controlled by episodic, fracture‐mediated fluid flow, and its extent depends on the thickness of highly fluid‐overpressurized, ETS‐generating serpentinite shear zones.

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