Diffuse hydrothermal fluids from Lucky Strike hydrothermal vent field: Evidence for a shallow conductively heated system

Chemical analysis of 18 diffuse hydrothermal fluids collected from the Lucky Strike hydrothermal vent site reveals the presence of less than 7% high-temperature hydrothermal end-member. Temperature-Li and temperature-Mg relationships for the diffuse fluids indicated two sample groups. One group of three samples had temperatures which fell on or below the temperatures expected fur conservative mixing between seawater and high-temperature vent fluids, consistent with mixing and cooling. The second grouping had diffuse fluid temperatures greater than expected from conservative mixing of high-temperature end-member fluids and seawater. This indicates that the diffuse hydrothermal effluent was heated conductively during its formation. The Lucky Strike site is characterized by a hydrothermal slab of basaltic fragments indurated by silica and barite. It is proposed that the diffuse fluids circulate within the cracks of the slab, forming a shallow short-pass hydrothermal convection system. The source of heat is likely to be high-temperature hydrothermal fluids circulating beneath the hydrothermal slab. The chemistry of the diffuse fluids shows no evidence for silica precipitation, suggesting that the slab is not actively forming from the diffuse fluids at the present.