Chemistry of hydrothermal vent fluids from 9°-10°N, East Pacific Rise:: Time zero, the immediate posteruptive period

In 1991, vent fluids were sampled from 10 high temperature hydrothermal vent sites within weeks of a volcanic eruption/dike intrusion on the East Pacific Rise, between 9 degrees and 10 degrees north latitude, and from eight high temperature vents south of the eruptive area. As a result of the eruption north of 9 degrees 45'N, the vent fluids from this area have unique compositional characteristics compared to the present global database of hydrothermal vent fluid compositions, including unusually low concentrations of Li, Na, K, Ca, Sr, Si, Cl, and Br, low pH, low alkalinity and high concentrations of H2S, and temperatures up to 403 degrees C. These compositions reflect subcritical phase separation at very shallow depths within the oceanic crust (less than or equal to 300 m) and minimal water-rock interaction. Dramatic changes in composition also occurred in the vent fluid compositions on time scales as short as a week, our minimum resampling interval. Our data suggest that the fluids that were generated were not pre-existing brines that were resident in the oceanic crust before the eruption/intrusion. Some of the fluids may be influenced by the dissolution of halite, assumed to be formed at the time of the eruption. Concentrations of Fe in these fluids are unusually high, both on an absolute and especially on a CI-normalized basis, which we attribute to the more reducing nature of these fluids. Fluids south of 9 degrees 45'N, out of the area where the eruption occurred, are generally higher in CI and have compositions more similar to those observed elsewhere: There is evidence to suggest that at least some of the vents south of the eruption area may also have been influenced by the eruption/intrusion, although no young lavas were observed at the surface at these other vent sites.