AN OFF-AXIS HYDROTHERMAL VENT FIELD NEAR THE MID-ATLANTIC RIDGE AT 30°N

Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centers.Here we report the discovery of an extensive hydrothermal field at 30°N near the eastern intersection of the Mid-Atlantic Ridge and Atlantis fracture zone.The vent field-named"Lost City"-is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Ma-old crust,nearly 15 km from the spreading axis,and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks.It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys,rather than the sulphide structures typical of"black smoker"hydrothermal fields.We found that vent fluids are relatively cool(40～75 ℃)and alkaline (pH9.0～9.8),supporting dense microbial communities that include anaerobic thermophiles.Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic,Indian and Arctic ridges,these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.