Effects of Substrate Composition and Subsurface Fluid Pathways on the Geochemistry of Seafloor Hydrothermal Deposits at the Lucky Strike Vent Field, Mid-Atlantic Ridge

Abstract

The Lucky Strike vent field, located on the Mid-Atlantic Ridge (MAR), is hosted on enriched mid-ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid-ocean basalt (E-MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment-free basalt-hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in <20% of reduced sulfur within the upwelling hydrothermal fluid reaching the seafloor. We also compare the geochemistry of the hydrothermal deposits at Lucky Strike to other hydrothermal sites along the MAR and show that the average hydrothermal deposit Ba/Co is useful to discriminate between E-MORB and other mafic/ultramafic hosted deposits.