Paleocene‐Eocene High‐Pressure Carbonation of Western Alps Serpentinites: Positive Feedback Between Deformation and CO2‐CH4 Fluid Ingression Responsible for Slab Slicing?

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. Plain Language Summary New rock outcrops were discovered along tectonic contacts inherited from past subduction—places where a tectonic plate sinks under another—in the Western Alps. Mineralogy, chemical compositions, and the study of fluid inclusion—small portions of fluids trapped in minerals—showed that they correspond to former mantle rocks transformed by the infiltration of sediment‐derived fluids. Absolute dating suggests their formation at depth during subduction. Such a process may contribute to bringing large amounts of carbon and water to great depths and may contribute to CO2 and H2O degassing at volcanic arcs. The weak minerals they contain helped strain localization, a prerequisite for removing fragments from the downgoing plate and recovering them at the surface. Key Points Carbonate‐rich talcschists in tectonic contacts juxtaposing subduction slices Transformation of serpentinites caused by the ingression of sediment‐derived fluids at peak burial conditions Positive feedback between deformation and fluid infiltration led to slab slicing