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The subduction interface is fluid-rich, chemically dynamic, and seismogenic. Constitutive relations and microstructural evidence suggest that talc-rich metasomatic rocks formed at this interface could host episodic tremor and slow slip (ETS) but the chemical pathways producing talc remain unclear. Using a suite of subduction interface rocks from the Catalina Schist (California) that record evidence of episodic slow slip, we investigate the pressure-temperature-time evolution of talc metasomatism and resulting rheological transformations. Metasedimentary protoliths are converted to chlorite and actinolite schists and mantle wedge ultramafic protoliths are converted to talc and actinolite schists. This metasomatism resulted from the local exchange dominated by transfer of Mg from ultramafic to metasedimentary rocks, and external infiltration of Ca-rich fluids derived from dehydration of underlying metabasalts …