Intense CH4 plumes generated by serpentinization of ultramafic rocks at the intersection of the 15°20′N fracture zone and the Mid-Atlantic Ridge

As part of the FARA French-US Program designed to study the Mid- Atlantic Ridge (MAR) between 15 degrees N and the Azores, twenty-three dives with the submersible Nautile were conducted during the French-US Faranaut 15N cruise on the eastern and western parts of the 15 degrees 20'N Fracture Zone/Ridge axis intersection. South of the eastern ridge-transform fault intersection, nine Nautile dives were made within the rift valley and along the western rift valley wall. CH(4) concentrations in the bottom waters reach 53.2 nmol/kg along faulted zones on top and on the east flank of the ultramafic inner corner high (15 degrees 05'N, 44 degrees 59'W) where serpentinized rocks outcrop. No (3)He anomaly is associated with methane, ruling out any primary mantle component. Fourteen dives were also made in the rift valley to the north, close to the western intersection of the 15 degrees 20'N Transform. High CH(4) anomalies (up to 22 nmol/kg) are also present in the bottom waters of the rift valley northern segment on both the western and eastern valley walls and on the inner high adjacent to the eastern wall where ultramafic rocks outcrop. Seven vertical hydrocasts carried out in the axial valley (4500 m deep) show an intense CH(4) anomaly, with a maximum (35.8 nmol/kg) at 3200 m depth. This CH(4) concentration is among the highest found along the Mid-Atlantic Ridge (Charlou and Donval, 1993; Charlou et al., 1997). CH(4) concentrations of 9.9-14.9 nmol/kg are also present on the western wall along the 3200 m isobath. The high CH(4) concentrations correspond to only weak 3He anomalies. This CH(4)-rich plume is also associated with active fault zones that expose peridotite rocks. CH(4) output from ultramafic outcrops on the western and eastern intersections of the 15 degrees 20'N Fracture Zone with the MAR is believed to reflect ongoing serpentinization. These results associated with many other CH(4) anomalies measured in the water column above ultramafic outcrops found between 12 degrees N and the Azores most likely reflect serpentinization processes extending along the whole slow spreading Mid-Atlantic ridge. Copyright (C) 1998 Elsevier Science Ltd.