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Abstract Talc-rich metasomatic rocks in subduction interface shear zones profoundly influence seismicity and arc magmatism, but their petrogenesis remains controversial. Magnesium isotope compositions of exhumed subduction interface rocks from the Catalina Schist (California, USA) record Mg exchange from ultramafic to crustal rocks. Preferential loss of isotopically light Mg from serpentinite produces isotopically heavy talc-rich metasomatic rocks. Addition of this isotopically light Mg to adjacent metasedimentary and metamafic rocks from the slab produces actinolite- and chlorite-rich metasomatic rocks, respectively, with convergent δ26Mg values relative to their protoliths. The addition of Ca to ultramafic- and metasedimentary-derived metasomatic rocks reflects a separate contribution from infiltrating metabasalt-derived fluids. Talc-rich rocks are formed by passive enrichment of Si in serpentinite during Mg loss to adjacent Mg sinks. These results and a global compilation of exhumed paleosubduction terranes suggest that talc is a common component of the subduction interface and often forms independent of Si metasomatism. Talc is likely prevalent along the interface from mantle wedge corner to subarc wherever ultramafic material is in contact with a Mg sink and where it could influence slow slip events, subduction interface rheology, and arc magmatism in modern subduction zones.