Boron contents and boron, carbon, and oxygen isotope data for ODP Sites 160-970 and 160-971

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 d11B 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 d13C ratios of the crust, which indicate precipitation of C from thermogenic methane, and by the d11B ratios of pore-water samples of mud-breccia drill cores. Clams (Vesicomya sp.) living adjacent to fluid vents have d11B and d18O 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.

Organic carbon analyses of ODP Site 160-971 in the Mediterranean Sea

Mud volcanism on the Mediterranean Ridge is caused by extrusion of overpressured sediments, with consequent formation of spectacular dome-shaped features composed of mud breccias at the seafloor. The organic material in the mud breccia of the Napoli mud volcano is a mixture of different facies, stratigraphic origin and thermal maturities. One portion is synsedimentary organic material with only minor diagenetic alterations and represents sedimenting material that was embedded into the mud volcano during its extrusion. The mud breccia also contains thermally mature organic material of mainly terrestrial provenance with algae of fresh- and brackish-water origin. Vitrinite reflectance data of this maturity generation range from 0.65 to 0.90% R(oil) and thus characterize thermally mature source rocks, a rank which is corroborated by fluorescence and molecular characteristics. The predominance of vitrinite in the maceral assemblages and the occurrence of biomarkers of terrigenous origin suggest that the major part of the mud matrix derives from a lacustrine or riverine sedimentary unit in the subsurface, possibly from the Messinian stage. A third generation of organic material includes inertinites and vitrinites of high reflectance, which represent recycled organic matter present in any marine sediment. By use of the Lopatin method for modelling the thermal maturation of hydrocarbon source rocks from the vitrinite reflectance data, we calculated that the depth of mobilization ranges from 4900 m to 7500 m, depending upon the temperature gradient used.