Products of two mud volcanoes from the distal part of the Mediterranean Ridge accretionary complex have been investigated regarding their B, C, and O stable isotope signatures. The mud breccias have been divided into mud matrix, lithified clasts, biogenic deposits, and authigenic cements and crusts related to fluid flow and cementation. Isotope geochemistry is used to evaluate the depth of mobilization of each phase in the subduction zone. B contents and isotope ratios of the mud and mud clasts show a general trend of B enrichment and decreasing δ11B values with increasing consolidation (i.e., depth). However, the majority of the clast and matrix samples relate to moderate depths of mobilization within the wedge (1–2 km below seafloor). The carbonate cements of most of these clasts as well as the authigenic crusts, however, provide evidence for a deep fluid influence, probably associated with the décollement at 5–6 km depth. This interpretation is supported by δ13C ratios of the crust, which indicate precipitation of C from thermogenic methane, and by the δ11B ratios of pore-water samples of mud-breccia drill cores. Clams (Vesicomya sp.) living adjacent to fluid vents have δ11B and δ18O values corresponding to brines known in the area, which acted as the parent solution for shell precipitation. Such brines are most likely Miocene pore waters trapped at deep levels within the backstop to the accretionary prism, probably prior to desiccation of the Mediterranean in the Messinian (6–5 Ma). Combining all results, deep fluid circulation and expulsion are identified as the main processes triggering mud liquefaction and extrusion, whereas brines contribute only locally. Given the high B contents, mud extrusion has to be considered a major backflux mechanism of B into the hydrosphere.