MINERAL AND GAS CHEMISTRY OF HYDROTHERMAL FLUIDS ON AN ULTRAFAST SPREADING RIDGE - EAST PACIFIC RISE, 17-DEGREES TO 19-DEGREES-S (NAUDUR CRUISE, 1993) PHASE-SEPARATION PROCESSES CONTROLLED BY VOLCANIC AND TECTONIC ACTIVITY

During the Naudur cruise in December 1993, 23 dives using the French s ubmersible Nautile were conducted on the axis of the East Pacific Rise between 17 degrees and 19 degrees S where the spreading rate is among the fastest in the ocean (14 to 16 cm/yr). Twenty hydrothermal fluids located at the topographic high of each segment in the axial domain w ere collected between 2573 and 2669 m depth on three segments centered , respectively, at 17 degrees 25'S, 18 degrees 15'S, and 18 degrees 26 'S. The fluids exhibit a very wide range of temperature, chemical, and gas compositions. On the 17 degrees 25'S and 18 degrees 26'S segments , fluids have quite uniform compositions, low chlorinities, are gas-en riched and are low in dissolved metals relative to fluids from the 18 degrees 15'S segment which show high chlorinities, are less gas-enrich ed and show high-metal concentrations. Chloride and metal depletion as sociated with gas enrichment is consistent with phase separation. Wher eas CH4 end-members show large variations between sites, the (CH4)-C-1 3 data are very similar, with C-13 values in a narrow range -22.0 to - 23.9 parts per thousand. versus pee-dee belemnite (PDB). delta(13)CO( 2) measured in fluids within the 18 degrees 15'S and 18 degrees 26'S s egments are,respectively -7.9 and -5.8 parts per thousand versus PDB, similar to C-13 of CO2 trapped in mid oceanic ridge basalts, suggestin g a magmatic origin. The variability in fluid composition is linked to the variability of the accretion process observed on the three segmen ts. The uniform venting of low-chlorinity fluids in the 17 degrees 25' S and 18 degrees 26'S segments is connected with volcanic activity whi ch causes boiling with preferential venting of vapor-enriched fluids. High-salinity fluids are emitted on the 18 degrees 15'S segment where the ridge is tectonics-dominated and subseafloor circulation controlle d by faults. Phase-separated effluents induced by volcanic and tectoni c activity are delivered to the deep ocean in this area, as previously observed on the Juan de Fuca Ridge or in the North Fiji Basin Ridge.