| Summary: | Abstract Meter‐ to hectometer‐size horizons of carbonate‐bearing talcschists are found along or near the contacts between the different Liguro‐Piemont subduction slices. Through mineral and bulk‐rock geochemistry, fluid inclusion analyses, and titanite U‐Pb geochronology, this study shows that these horizons formed by the transformation of serpentinites, at conditions close to peak burial, due to fluid infiltration sourced from surrounding sediments. These rocks containing large amounts of carbonates and high concentrations of fluid‐mobile elements may be significant for carbon cycling and contribute to the composition of arc magmas once the main constitutive assemblage destabilizes at high‐pressure and high‐temperature conditions. Following fluid infiltration, the formation of talc likely controlled strain localization in these horizons, enhancing in turn the ingression of external fluids and creating a positive feedback loop between deformation and fluid infiltration. We suggest that these rocks may have acted as major rheological weaknesses responsible for material offscraping from the downgoing plate.
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