Near Vent Seismicity at the Tour Eiffel Vent Site, Lucky Strike Hydrothermal Field, Mid-Atlantic Ridge

Secondary hydrothermal circulation is created by local entrainment of cold seawater into the shallow subseafloor by the intense discharge of high temperature fluids through focused vents at black smoker hydrothermal fields. To improve our understanding of secondary hydrothermal circulation, we tested a new instrumentation consisting of a small-scale (~150 m aperture) four-hydrophone network connected to single datalogger, which could potentially record very small and shallow microearthquakes associated with near-seafloor upflow of hydrothermal fluids and mineral precipitation. We report on a one-year catalog of near-seafloor microearthquakes, recorded with this hydrophone network deployed around the Tour Eiffel black smoker vent site of the Lucky Strike hydrothermal field. We detect 740 shallow microearthquakes (depth between 0 and 300 m, local magnitude between -4.0 and -0.5) that may be generated by reaction-driven cracking due to anhydrite precipitation in the shallow subseafloor. The seismicity rate is low, and the events are not focused around the black smokers, suggesting that heating of entrained seawater and anhydrite precipitation is not prevalent near these vents. This hypothesis is supported by time-series analysis of diffuse fluid samples from the Tour Eiffel site, which show little to no chemical evidence for anhydrite precipitation.