Subsurface processes at the Lucky Strike hydrothermal field, Mid-Atlantic Ridge: Evidence from sulfur, selenium, and iron isotopes

At Lucky Strike near the Azores Triple Junction, the seafloor setting of the hydrothermal field in a caldera system with abundant low-permeability layers of cemented breccia, provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution. Coupled analyses of S isotopes performed in conjunction with Se and Fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems. These data provide a tool for resolving the different abiotic and potential biotic near-surface hydrothermal reactions. The delta(34)S (between 1.5parts per thousand and 4.6parts per thousand) and Se values (between 213 and 1640 ppm) of chalcopyrite suggest a high temperature end-member hydrothermal fluid with a dual source of sulfur: sulfur that was leached from basaltic rocks, and sulfur derived from the reduction of seawater sulfate. In contrast, pyrite and marcasite generally have lower delta(34)S within the range of magmatic values (0 +/- 1parts per thousand) and are characterized by low concentrations of Se (< 50 ppm). For Se-82/Se-76 ratios, the delta(82)Se Values range from basaltic values of near -1.5parts per thousand to -7parts per thousand,. The large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between Se leached from igneous rock and Se derived from seawater. We interpret the Se isotope signature to be a result of leaching and mixing of a fractionated Se source located beneath hydrothermal chimneys in the hydrothermal fluid. At Lucky Strike we consider two sources for S and Se: (1) the end-member hydrothermal fluid with basaltic Se isotopic Values (-1.5%) and typical S isotope hydrothermal values of 1.5parts per thousand; (2) a fractionated source hosted in subsurface environment with negative delta(34)S values, probably from bacterial reduction of seawater sulfate and negative delta(82)Se values possibly derived from inorganic reduction of Se oxyanions. Fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by S isotopes. Fe isotope systematic reveals that Se-rich high temperature samples have delta(57)Fe values close to basaltic values (similar to0parts per thousand) whereas Se-depleted samples precipitated at medium to low temperature are systematically lighter (delta(57)Fe values between -1 to -3parts per thousand). An important implication of our finding is that light Fe isotope composition down to -3.2parts per thousand may be explained entirely by abiotic fractionation, in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions. Copyright (C) 2004 Elsevier Ltd.