Carbonation and deformation of oceanic serpentinites in the Elba subduction channel: Evidence for fluid–rock interaction at seismogenic depth

• Carbonation in subduction channel serpentinites at shallow-intermediate depth. • Fluid-driven carbonation in serpentinite shear zones. • Strain localization in carbonated domains. • CO 2 -rich fluids are sourced from subducting ophicarbonates. • Evidence of W-dipping oceanic subduction in the Ligurian ophiolite. We present a multi-disciplinary investigation of the deformation and carbonation history of oceanic serpentinites involved in the plate interface of the Cretaceous-Eocene subduction complex exposed in the Elba Island —an exceptionally well-preserved example of oceanic subduction system. Using an integrated set of structural, petrographic, and geochemical analyses, we document antigorite growth and carbonation of oceanic serpentinites in the Elba subduction channel at shallow-intermediate depths (<22-26 km; P <0.6-0.7 GPa). Carbonation was associated with brecciation and shearing that produced a complex of tectonic slices (Norsi area) and a tectonic mélange (Cavo area). The carbonation process was likely triggered by the release of CO 2 -rich fluids from subducting oceanic serpentinites and ophicarbonate rocks, as indicated by C and O isotopes. The ingress of external CO 2 in shear zones likely ignited a positive feedback between deformation, carbonation, and fluid flow, promoting talc formation and leading to strain localization at the plate interface. The Elba subduction channel offers a unique window on the evolution of subduction systems at low pressure, highlighting the importance of fluid-induced reactions in controlling deformation style.