A Jurassic extensional detachment associated with carbonated serpentinites and basalts is preserved at Falotta (Platta nappe, southeastern Switzerland). Structural data indicate that fluid circulation occurred during late increments of extension along the detachment plane at the contact between serpentinites and basalts. The homogeneity of the isotopic signatures (δ18O ∼16‰ and δ13C centered on 0‰–1‰) can be best explained by a single, sudden seawater-derived carbonation event at temperatures of ∼100 °C. Carbonation was focused in the high-permeability zone along the detachment. Our model yields new insights for carbonation processes related to mantle exhumation.