Geochemical and physical structure of the hydrothermal plume at the ultramafic-hosted Logatchev hydrothermal field at 14°45'N on the Mid-Atlantic Ridge

摘要

The hydrothermal plume generated in deep waters above the Logatchev hydrothermal field (LHF) about 15°N on the Mid-Atlantic Ridge was investigated and mapped for its 3D distributions using a combination of in situ optical light scattering data, temperature and salinity data, as well as concentrations of hydrogen, methane, total dissolvable Fe, and total dissolvable Mn. Based on the results obtained for these meaningful parameters, we report the geochemical and physical characteristics of the fluids expelled from the ultramafic LHF and the chemical structure of its hydrothermal plume in the water column. The hydrothermal plume is sourced by at least seven distinct vent sites and possibly additional diffusive fluid and gas discharge. It comprises a water body characterized by strong nephelometric anomalies (expressed as ΔNTU, nephelometric turbidity units) and high concentrations of Fe and Mn (>5 times seawater concentration), and the gas plume with several times the H_2 and CH_4 concentrations of normal seawater. Up to three plume levels with a total vertical extension of about 350 m from the seafloor were classified in the hydrothermal plume. The ΔNTU plume could be followed to approximately 2.5 km to the north and to the south from the vent site while the gas plume spread several km farther from the hydrothermal source. High concentrations of H_2 (up to 1598 nmol l~(-1)) and CH_4 (up to 323 nmol l~(-1)) accompanied by relatively low dissolvable Fe concentrations (up to 270 nmol l~(-1)) as well as low concentrations of dissolvable Mn (112 nmol l~(-1)) compared to basaltic hydrothermal systems are the characteristics of the plume. The low metal/gas ratios showed a decrease with increasing distance from the vent site. Our data demonstrate that ultramafic systems such as the LHF serve both as sources and sinks for elements, with respect to metal and gas inputs into the oceanic water column. The relevance of such systems is underlined by the discovery (and postulated frequency) of further ultramafic-hosted hydrothermal systems on slow-spreading ridges.