Hydrothermal vents near a mantle hot spot: The Lucky Strike vent field at 37 degrees N on the Mid-Atlantic Ridge

The Lucky Strike hydrothermal field occurs in the summit basin of a large seamount that forms the shallow center of a 65 km long ridge segment near 37 degrees N on the Mid-Atlantic Ridge. The depth and chemistry of the ridge segment are influenced by the Azores hot spot, and this hydrothermal field is the first Atlantic site found on crust that is dominated by a hot spot signature. Multiple hydrothermal vents occur over an area of at least 300 m by 700 m. Vent morphologies range from flanges and chimneys with temperatures of 200-212 degrees C, to black smoker chimneys with temperatures up to 333 degrees C. Cooler fluids from northern vents have higher chlorinities, and lower gas volumes, while hotter, southern fluids have chlorinities 20% below seawater with higher gas volumes, suggesting phase separation has influenced their compositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mineralogy, except that barite is a major mineral, particularly at lower-temperature sites, which contrasts with previously investigated Atlantic sites, The fluid chemistry, distribution of the relict sulfide deposits on the seamount summit in the areas investigated using DSV Alvinin, and contact relationships between active Vent sites and surrounding basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at the Lucky Strike vent sites are dominated by a new species of mussel, and include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species level and appears to be a new biogeographic province. The Lucky Strike field helps to constrain how variations in the basaltic substrate influence the composition of hydrothermal fluids and solids, because basalt compositions at Lucky Strike are 10-30 times enriched in incompatible elements compared to other Atlantic hydrothermal sites such as TAG, Snake Pit and Broken Spur. The incompatible element enrichment appears to influence the compositions of hydrothermal fluids and solid deposits: fluids are enriched in Ba and the light REE, and barite is a common mineral. For hydrothermal sites from around the world, REE ratios in fluids correlate with the REE ratios in basalts, and high Ba in the substrate is associated with barite in the hydrothermal deposits. Therefore the chemistry of the substrate exerts an important control on both fluid trace element and solid chemistry of sea floor hydrothermal systems, even for those constructed on bare rock. The deep mantle precesses that give rise to hot spots ultimately are manifested in distinctive compositions of hydrothermal fluids and solids.