The Logatchev hydrothermal field (MAR, 15°N): high- and low-temperature alteration of ultramafic oceanic crust ; geology, geochemistry, mineralogy

Detailed bathymetric mapping of the area of 15°N (MAR) as well as geological mapping of the Logatchev hydrothermal field at 14°45´N (MAR) leads to a very well description of the Logatchev hydrothermal field and adjacent areas. Main parts of the Logatchev hydrothermal field are located in a depression filled with talus material and pelagic sediment this causes an increase in porosity and permeability in the subseafloor influencing fluid pathways importantly and provide aquifers wherein fluids can spread out laterally in layers of mafic/ultramafic talus. The ‚smoking craters‛ of the Logatchev hydrothermal field are rare structures which have only been identified at Logatchev and the, in 2005 newly discovered, Nibelungen-Field at the southern MAR (Koschinsky at al., 2006). Bathymetric maping of the seafloor reveal that the hydrothermal activity at Logatchev mainly is fault controlled. The Logatchev hydrothermal field is characterized by five smoking craters, two hydrothermal mounds with black smokers and a couple of diffuse vent sites. The smoking craters emanate fluids with temperatures up to ~350°C. Host Rocks of the field are mainly serpentinites (>70%) and gabbroic rocks (>20%) with subordinately basalt. Serpentinites, gabbroic rocks, and altered sediments were sampled from the active ultramafic hosted Logatchev hydrothermal field (LHF) by ROV deployments and TV-guided grab. In this study we present new data of clay-size separates and whole rock chemistry as well as strontium and oxygen isotopic compositions in order to investigate the alteration processes, water-rock reactions and associated elemental exchanges. The samples of mantle peridotites are mostly serpentinized (>95 vol%) whereas some gabbroic samples are moderately to completely altered. Changes in geochemical compositions, especially trace element compositions, of the altered rocks indicate a highly heterogeneous distribution of alteration styles. Light rare earth elements are enriched in all serpentinites and suggest a strong influence of Logatchev vent fluids during some stages of serpentinization. Oxygen isotopic data of lizardites reveal formation temperatures of 130-150°C whereas chlorite formation in altered gabbroic samples occurred between 150° and 200°C in the LHF. Negative as well as positive Eu anomalies in serpentinites suggest varying alteration processes: (1) high temperature alteration in acidic, reducing conditions as well as (2) low temperature alteration in neutral, oxidizing environments. The absence of brucite and the formation of talc indicate high concentrations of aquatic silica in the Logatchev area. Petrographic investigations, high field strength element data and elevated rare earth elements indicate that melt–rock interaction is also an important factor affecting mantle peridotite prior to serpentinization processes at LHF. Therefore gabbroic intrusions are the most probable source for aquatic silica. Strontium isotopic data suggest high water/rock ratios in the range of 2.6-27 and alteration fluids which are characterized by high contents of heated seawater.Calculations of relative gains and losses of major and trace elements as well as mineralogical observations reveal that a large variety of alteration styles take place at the Logatchev hydrothermal field. A combination of serpentinization, hydrothermal fluids, melt-rock interaction and low-temperature seafloor weathering lead to significant gains and losses of major and trace elements. We found that serpentinization at Logatchev was isochemical for the most major elements (excepting a loss of TiO2 and CaO). However, the concentration of trace elements e.g., Cu, Nb, Ba, La, Sm, Eu, Th or U increases strongly in the serpentinites. In addition, gabbroic intrusions are a sink for MgO during the formation of chlorite and serpentine after clinopyroxene and play a major role as distributors of SiO2, TiO2, CaO, and Na2O as well as numerous trace elements in the circulating fluids.