Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge rift valley

In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 10⁸ cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.