FAMOUS and AMAR segments on the Mid-Atlantic Ridge: ubiquitous hydrothermal Mn, CH4, δ3He signals along the rift valley walls and rift offsets

Dynamic hydrocast experiments enabled Mn (TDM), CH4 concentrations and δ3He ratio to be recorded through vertical cross-sections of hydrothermal plumes along the FAMOUS segment and the southern part of the AMAR segment on the Mid-Atlantic Ridge between 36°N and 37°N. Mn, CH4 and δ3He figures all along both segments are well above the seawater background in the open ocean: they are interpreted to be the result of time-integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends. Hydrothermal activity along the FAMOUS and AMAR segments appears to be similar. A comparison of the residence times of the three tracers from the dispersed, time-integrated signals is proposed. Although the background values in these closed basin are high, some proximal and (or) large hydrothermal inputs, overprinted on the general time-integrated plume, can be detected (i.e. the Rainbow site south of AMAR). Based on the depth and the location of plumes, hydrothermal activity is not, by far, limited to the neo-volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley. Considering the Mn and CH4 concentrations in these plumes, two types of ocean–mantle interaction may be represented: hot, focused discharges on ultramafic exposures (Rainbow site) and low-temperature diffuse serpentinisation.