HYDROTHERMAL ALTERATION IN OCEANIC RIDGE VOLCANICS - A DETAILED STUDYAT THE GALAPAGOS FOSSIL HYDROTHERMAL FIELD

The Galapagos Fossil Hydrothermal Field is composed of altered oceanic crust and extinct hydrothermal vents within the eastern Galapagos Rif t between 85-degrees-49'W and 85-degrees-55'W. The discharge zone of t he hydrothermal system is revealed along scarps, thus providing an opp ortunity to examine the uppermost mineralized, and highly altered inte rior parts of the crust. Altered rocks collected in situ by the submer sible ALVIN show complex concentric alteration zones. Microsamples of individual zones have been analysed for major/minor, trace elements, a nd strontium isotopes in order to describe the complex compositional d etails of the hydrothermal alteration. Interlayered chlorite-smectite and chlorite with disequilibrium compositions dominate the secondary m ineralogy as replacement phases of primary glass and acicular pyroxene . Phenocrysts and matrix grains of plagioclase are unaffected during a lteration. Using a modification of the Gresens' equation we demonstrat e that the trivalent rare earth elements (REEs) are relatively immobil e, and calculate degrees of enrichment and depletion in other elements . Strontium isotopic ratios increase as Sr concentrations decrease fro m least-altered cores to most-altered rims and cross-cutting veins in individual samples, and can be modeled by open system behaviour under low fluid-rock ratio (< 10) conditions following a period of lower-tem perature weathering of volcanics within the rift zone. The complex pat terns of element enrichment and depletion and strontium isotope variat ions indicate mixing between pristine seawater and ascending hot fluid s to produce a compositional spectrum of fluids. The precipitation of base-metal sulfides beneath the seafloor is probably a result of fluid mixing and cooling. If, as suggested here, the discharge zone alterat ion occurred under relatively low fluid-rock ratios, then this shallow region must play an important role in determining the exit compositio n of vent fluids in marine hydrothermal systems.