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 submersible 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 were 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-enriched and are low in dissolved metals relative to fluids from the 18 degrees 15'S segment which show high chlorinities, are less gas-enriched and show high-metal concentrations. Chloride and metal depletion associated with gas enrichment is consistent with phase separation. Whereas CH4 end-members show large variations between sites, the (CH4)-C-13 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 segments are,respectively -7.9 and -5.8 parts per thousand versus PDB, similar to C-13 of CO2 trapped in mid oceanic ridge basalts, suggesting a magmatic origin. The variability in fluid composition is linked to the variability of the accretion process observed on the three segments. The uniform venting of low-chlorinity fluids in the 17 degrees 25'S and 18 degrees 26'S segments is connected with volcanic activity which 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 controlled by faults. Phase-separated effluents induced by volcanic and tectonic 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.