GEOCHEMISTRY OF HYDROTHERMAL SOLUTIONS FROM 9°50′ N AT THE EAST PACIFIC RISE (EPR) WITHIN TWELVE YEARS AFTER THE ERUPTION OF A SUBMARINE VOLCANO

Abstract

Hydrothermal solutions were examined in a circulation system that started to develop after the 1991 volcanic eruption in the axial EPR segment between 9°45′ N and 9°52′ N. Within the twelve years elapsed after the eruption, the diffusion outflow of hot fluid from fractures in basaltic lavas gave way to focused seeps of hot solutions through channels of hydrothermal sulfide edifices. The example of field Q demonstrates that the concentrations of H2S decreased from 86 to 1 mM/kg from 1991 to 2003, and the Fe/H2 S ratio simultaneously increased by a factor of 1.7, a fact that can explain the disappearance of the microbial mats, which were widespread at the fields before 1991. The S isotopic composition of H2S is independent of the H2S concentration, a fact testifying to the rapid evolution of the hydrothermal system in the early years of its evolution. Carbon in CH4 from the hot fluid sampled in 2003 is richer in the light 12C isotope than carbon in the fluid from the hydrothermal field at 21° N in EPR, which suggests that methane comes to field Q from more than one source. The composition of particulate matter in the hydrothermal solutions indicates that it was contributed by biological material. Experimental solutions with labeled substrates (t < 70°C) show evidence of the active processes of methane oxidation and sulfate reduction. Our results indicate that, during the 12 years of the evolution of the hydrothermal system, the composition of its solutions evolved and approached the compositions of solutions in mature hydrothermal systems in EPR. © Pleiades Publishing, Inc. 2006.