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Geochemistry and stable isotope constraints on high-temperature activity from sediment cores of the Saldanha hydrothermal field
Authors:ÁS Dias  GL Früh-Green  SM Bernasconi  FJAS Barriga
Institution:1. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, FL 33124, USA;2. Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA;1. The Key Laboratory of Gas Hydrate, Ministry of Natural Resources, Qingdao Institute of Marine Geology, Qingdao 266071, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China;3. Third Institute of Oceanography State Oceanic Administration, Xiamen 361005, China;4. School of Scientific Research, China University of Geoscience (Beijing), Beijing 100083, China;5. College of Environmental Science and Engineering, Ocean University of China, Qingdao 266100, China;1. Qingdao Institute of Marine Geology, Qingdao, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China;3. China Key Laboratory of Submarine Geosciences and Technology, Ministry of Education, Department of Marine Geoscience, Ocean University of China, Qingdao, China
Abstract:The Saldanha hydrothermal field is hosted atop a mafic–ultramafic seamount, located at a non-transform offset on the Mid-Atlantic Ridge. Previous observations revealed a field where transparent low-temperature fluids discharge through centimeter-sized vents without the formation of chimney structures. We present geochemical and stable isotope (O and C) analyses from sediment samples collected at this field, both at and far from the vent area. Most sediments, including some directly adjacent to orifice vents, are pelagic oozes with only a weak hydrothermal overprinting. Hydrothermal precipitates are characterized by Fe–Mn oxyhydroxides and a minor amount of Cu–Zn sulphide minerals. However, one of the cores (SCD7) collected at the vent area shows a much stronger hydrothermal signature. This core is composed of a matrix of serpentine + talc ± chlorite with high porosity, where calcite + chalcopyrite + sphalerite/wurtzite ± pyrite–pyrrhotite were precipitated. In this core, metal enrichments, REE patterns, and the oxygen and carbon isotope composition of calcites indicate that mineralization must have occurred in the subsurface by high-temperature fluids, with minor mixing with seawater and with a significant magmatic contribution. Thus, while most samples confirm previous findings indicating that Saldanha hydrothermal fluid discharge is mainly diffuse and of low temperature, data from core SCD7 suggest that areas of high-temperature hydrothermal activity also occur, where temperatures of the fluids could reach > 260 °C and maximum temperatures of 330 °C. We suggest that fluids can flow through faults at the top of the mount and discharge in a more focused way through vent orifices, producing intense hydrothermal alteration of the sediments. At these locations complex hydrothermal processes occur, including reactions of the hydrothermal fluids with mafic and ultramafic rocks and magma degassing, as suggested by the carbon isotope composition of hydrothermal calcites. The high temperature of the fluid inferred from the geochemistry of the hydrothermal minerals requires a significant heat input to the system, suggesting an additional magmatic heat source to the already proposed exothermic serpentinization reactions.
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