Mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge: East Pacific Rise, 17° to 19°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° and 19°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°25′S, 18°15′S, and 18°26′S. The fluids exhibit a very wide range of temperature, chemical, and gas compositions. On the 17°25′S and 18°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°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 CH 4 endmembers show large variations between sites, the 13 CH 4 data are very similar, with 13 C values in a narrow range −22.0 to −23.9‰ versus pee‐dee belemnite (PDB). δ 13 CO 2 measured in fluids within the 18°15′S and 18°26′S segments are, respectively, −7.9 and −5.8‰ versus PDB, similar to 13 C of CO 2 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°25′S and 18°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°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.