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

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 ∆NTU, nephelometric turbidity units) and high concentrations of Fe and Mn (> 5 times seawater concentration), and the gas plume with several times the H2 and CH4 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 ∆NTU 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 H2 (up to 1598 nmol l− 1) and CH4 (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.