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Several hydrothermal deposits associated with ultramafic rocks have recently been found along slow spreading ridges with a low magmatic budget. Three preferential settings are identified: (1) rift valley walls near the amagmatic ends of ridge segments; (2) nontransform offsets; and (3) ultramafic domes at inside corners of ridge transform-fault intersections. The exposed mantle at these sites is often interpreted to be a detachment fault. Hydrothermal cells in ultramafic rocks may be driven by regional heat flow, cooling gabbroic intrusions, and exothermic heat produced during serpentinization. Along the Mid-Atlantic Ridge (MAR), hydrothermal deposits in ultramafic rocks include the following: (1) sulfide mounds related to high-temperature low-pH fluids (Logatchev, Rainbow, and Ashadze); (2) carbonate chimneys related to low-temperature, high-pH fluids (Lost City); (3) low-temperature diffuse venting and high-methane discharge associated with silica, minor sulfides, manganese oxides, and pervasive alteration (Saldanha); and (4) stockwork quartz veins with sulfides at the base of detachment faults (15°05′N). These settings are closely linked to preferential circulation of fluid along permeable detachment faults. Compared to mineralization in basaltic environments, sulfide deposits associated with ultramafic rocks are enriched in Cu, Zn, Co, Au, and Ni. Gold has a bimodal distribution in low-temperature Zn-rich and in hightemperature Cu-rich mineral assemblages. The Cu-Zn-Co-Au deposits along the MAR seem to be more abundant than in ophiolites on land. This may be because ultramafic-hosted volcanogenic massive sulfide deposits on slow spreading ridges are usually not accreted to continental margins during obduction and may constitute a specific marine type of mineralization.

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Gas hydrates and gas bubbles were collected during the MARNAUT cruise (May-June 2007) in the Sea of Marmara along the North Anatolian Fault system, Turkey. Gas hydrates were sampled in the western part of the Sea of Marmara (on the Western High), and three gas-bubble samples were recovered on the Western High, the Central High (center part of the Sea of Marmara) and in the Çinarcik Basin (eastern part of the Sea of Marmara). Methane is the major component of hydrates (66.1%), but heavier gases such as C 2 , C 3 , and i-C 4 are also present in relatively high concentration. The methane contained within gas hydrate is clearly thermogenic as evidenced by a low C 1 /C 2 + C 3 ratio of 3.3, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.1‰ PDB and δD CH4 of − 219‰ SMOW). A similar signature is found for the associated gas bubbles (C 1 /C 2 + C 3 ratio of 24.4, δ 13 C CH4 of − 44.4‰ PDB) which have the same composition as natural gas fromK. Marmara-af field. Gas bubbles from Central High show also a thermogenic origin as evidenced by a C 1 /C 2 + C 3 ratio of 137, and carbon and hydrogen isotopic data (δ 13 C CH4 of − 44.4‰ PDB and δD CH4 of − 210‰ SMOW), whereas those from the Çinarcik Basin have a primarily microbial origin (C 1 /C 2 + C 3 ratio of 16,600, δ 13 C CH4 of − 64.1‰ PDB). UV-Raman spectroscopy reveals structure II for gas hydrates, with CH 4 trapped in the small (5 12) and large (5 12 6 4) cages, and with C 2 H 6 , C 3 H 8 and i-C 4 H 10 trapped in the large cages. Hydrate composition is in good agreement with equilibrium calculations, which confirm the genetic link between the gas hydrate and gas bubbles at Western High and the K.Marmara-af offshore gas field located north of the Western High. We calculate the characteristics of the hydrate stability zone at Western High and in the Çinarcik Basin using the CSM-GEM computer program. The base of the structure II hydrate stability field is at about 100 m depth below the seafloor at the Western High site, whereas in the Çinarcik Basin, P-T conditions at the seafloor correspond to the uppermost range for structure I hydrate formation from microbial gas.

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