We report on a 3 years monitoring experiment of low to medium temperature diffuse venting at two vent sites (Tour Eiffel and White Castle) of the Lucky Strike, black smoker-type hydrothermal field, Mid-Atlantic Ridge. Diffuse vents account for a large part of the energy flux of mid-ocean ridges hydrothermal fields and provide key habitats for the hydrothermal fauna. We document the time and space variability of diffuse venting temperature and chemistry, describe the effect of tidal loading and currents and discuss the extent of mixing, cooling of black smoker fluids, heating of entrained seawater and anhydrite precipitation/dissolution in the substratum. We emphasize the role of a thin (<2 m) volcaniclastic formation capping the brecciated basalt substratum. This formation is porous, but becomes impermeable when indurated by hydrothermal precipitates. It forms an intermediate layer between the vents at the seabed and the fluids as they discharge out of the brecciated basalts. Diffuse fluids inferred to discharge out of meter-spaced cracks in the brecciated basalts beneath this volcaniclastic layer are hot (>80°C) and contain >10% of the hot endmember fluid component, over distances of up to 25 m from the black smokers. These results provide a geologically integrated framework in which to study site-scale, near seafloor hydrothermal circulation and associated vent habitats at Lucky Strike and other black smoker-type hydrothermal fields. They suggest diffuse heat fluxes in the upper range of previously published estimates at the two studied Lucky Strike hydrothermal vent sites.

Key Points

Time variability of both fluid temperature and fluid chemistry at diffuse vents of the Lucky Strike mid-ocean ridge hydrothermal field

Hot (>80°C) and hydrothermal endmember-rich diffuse fluids (>10%) come out of the basalts up to 25 m from the black smokers

Fluids that come out of basalt substratum are modified in volcaniclastic layer before coming out at vents that host the hydrothermal fauna

Plain Language Summary

Mid-ocean ridges (MOR) are a key feature of plate tectonics, extending some 60,000 km in all the major oceans. MOR hydrothermal circulations transfer heat and chemical compounds from the solid earth to the ocean and provide habitats for the hydrothermal fauna. The vents include black smokers that expel the hottest fluids and diffuse vents that expel lower temperature fluids at lower rates but over larger surfaces. The contribution of diffuse vents to the energy and chemical fluxes of MOR hydrothermal systems is still largely an open question. In this paper, we address it by using data from a 3 years monitoring experiment of diffuse vents at two sites (Tour Eiffel and White Castle) of the Lucky Strike, a black smoker-type hydrothermal field in the Mid-Atlantic Ridge. We document the time and space variability of venting temperature and derive chemical constraints on the extent of mixing of black smoker fluids with entrained seawater and of mineral precipitation/dissolution in the substratum of the vents. Our results suggest diffuse heat fluxes in the upper range of previously published Lucky Strike hydrothermal field estimates and provide a geologically integrated framework in which to study diffuse vent habitats at Lucky Strike and other black smoker-type hydrothermal fields.