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 la rge seamount that forms the shallow center of a 65 km long ridge segme nt near 37 degrees N on the Mid-Atlantic Ridge. The depth and chemistr y 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 o ver an area of at least 300 m by 700 m. Vent morphologies range from f langes and chimneys with temperatures of 200-212 degrees C, to black s moker chimneys with temperatures up to 333 degrees C. Cooler fluids fr om northern vents have higher chlorinities, and lower gas volumes, whi le hotter, southern fluids have chlorinities 20% below seawater with h igher gas volumes, suggesting phase separation has influenced their co mpositions. All gas volumes in fluids are higher than those at TAG and Snake Pit hydrothermal fields. Black smokers exhibit their typical mi neralogy, except that barite is a major mineral, particularly at lower -temperature sites, which contrasts with previously investigated Atlan tic sites, The fluid chemistry, distribution of the relict sulfide dep osits on the seamount summit in the areas investigated using DSV Alvin in, and contact relationships between active Vent sites and surroundin g basaltic and sulfide substrate suggest that the hydrothermal system has a long history and may have recently been rejuvenated. Fauna at th e Lucky Strike vent sites are dominated by a new species of mussel, an d include the first reported sea urchins. The Lucky Strike biological community differs considerably from other vent fauna at the species le vel and appears to be a new biogeographic province. The Lucky Strike f ield helps to constrain how variations in the basaltic substrate influ ence 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, S nake Pit and Broken Spur. The incompatible element enrichment appears to influence the compositions of hydrothermal fluids and solid deposit s: 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 su bstrate is associated with barite in the hydrothermal deposits. Theref ore the chemistry of the substrate exerts an important control on both fluid trace element and solid chemistry of sea floor hydrothermal sys tems, even for those constructed on bare rock. The deep mantle precess es that give rise to hot spots ultimately are manifested in distinctiv e compositions of hydrothermal fluids and solids.