FAMOUS AND AMAR SEGMENTS ON THE MID-ATLANTIC RIDGE - UBIQUITOUS HYDROTHERMAL MN, CH4, DELTA-HE-3 SIGNALS ALONG THE RIFT-VALLEY WALLS AND RIFT OFFSETS

Dynamic hydrocast experiments enabled Mn (TDM), CH4 concentrations and delta(3)He ratio to be recorded through vertical cross-sections of hy drothermal plumes along the FAMOUS segment and the southern part of th e AMAR segment on the Mid-Atlantic Ridge between 36 degrees N and 37 d egrees N. Mn, CH4 and delta(3)He figures all along both segments are w ell above the seawater background in the open ocean: they are interpre ted to be the result of time-integrated hydrothermal discharges disper sed 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 thr ee tracers from the dispersed, time-integrated signals is proposed. Al though the background values in these closed basin are high, some prox imal and (or) large hydrothermal inputs, overprinted on the general ti me-integrated plume, can be detected (i.e. the Rainbow site south of A MAR). Based on the depth and the location of plumes, hydrothermal acti vity is not, by far, limited to the neo-volcanic inner floor of the va lley and should involve the walls and complex offsets of the rift vall ey. Considering the Mn and CH4 concentrations in these plumes, two typ es of ocean-mantle interaction may be represented: hot, focused discha rges on ultramafic exposures (Rainbow site) and low-temperature diffus e serpentinisation. (C) 1998 Elsevier Science B.V. All rights reserved .