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1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
2 Institute of Oceanographic Sciences Deacon Laboratory, Brook Road, Wormley, Godalming, Surrey GU8 5UB, UK
The Lucky Strike hydrothermal field on the Mid-Atlantic Ridge (MAR) at 37°17'N was discovered in 1992 during dredging operations carried out as part of the FARA Program (French-American Ridge Atlantic). The vent field at Lucky Strike lies in a depression between three cones that form the summit of an axial seamount. The summit rises 400 m above the rift valley floor to a water depth of 1570 m. Hydrothermal fluids stream from several vent complexes with exit temperatures up to 333°C. This paper presents the first observations of hydrothermal plumes in the water column over Lucky Strike, including anomalies in light scattering with a nephelometer and dissolved Mn with the zero-angle photon fibre optic spectrometer. Plumes at Lucky Strike are small compared with those at other MAR hydrothermal sites such as TAG (26°N) and MARK (23°N). These small plumes are consistent with the relatively low Fe and Mn concentrations found in associated vent waters. The Sr/Ca ratios of these fluids are low, suggesting that the basaltic substrate below Lucky Strike is highly altered, but rare earth element concentrations are also low, indicating reduced mobilization. These observations together with the low fluid temperatures are suggestive of an old and waning hydrothermal system with extensive zones of subfloor alteration.
