Analysis of stable sulfur isotopes and trace cobalt on sulfides from the TAG hydrothermal mound

Abstract

Stable sulfur isotopes (δ34S) and trace Co are analyzed in sulfide and sulfate minerals from six sample types collected from the TAG active mound, 26°N Mid-Atlantic Ridge. δ34S values range from 2.7 to 2O.9%, with sulfate minerals isotopically indistinguishable from seawater (21%), and sulfide minerals reflecting input of 1/3 seawater and 2/3 basaltic sulfur (~0%). Co concentrations in pyrite analyzed by ion microprobe primarily reflect depositional temperatures. The δ34S and Co data are combined to provide information regarding the sources and temperatures of parent fluids, the genetic relationships among sample types, and the circulation of hydrothermal fluids and seawater in the mound. δ34S values and Co concentrations vary by sample type. Chalcopyrite from black smoker samples exhibits invariant δ34S values, indicating direct precipitation from black smoker fluids. Crust samples contain chalcopyrite with a mean δ34S indistinguishable from that of black smoker samples, and pyrite with some light δ34S and moderately high Co values, consistent with crust samples precipitating from cooled black smoker fluids. Massive anhydrite samples are a mixture of anhydrite with high δ34S, and pyrite with variable δ34S and Co values, indicative of deposition from disequilibrium mixing between black smoker fluids and seawater. White smoker samples contain chalcopyrite and sphalerite with high δ34S, and pyrite with low Co values, reflecting deposition from cooler fluids formed from mixtures of seawater and black smoker fluid, with some reduction of sulfate. Mound samples contain chalcopyrite with a mean δ34S indistinguishable from that of black smoker and crust samples, and pyrite with low Co values, suggesting deposition from a fluid isotopically similar to black smoker fluid at temperatures similar to those of white smoker fluid. Massive sulfide samples exhibit pyrite with high δ34S values and very high Co, indicating deposition from and recrystallization with very hot fluids contaminated with seawater-derived sulfate. The data demonstrate that direct precipitation from black smoker fluids, conductive cooling, disequilibrium mixing with entrained seawater, sulfate reduction, and recrystallization all contribute to the formation of the TAG mound deposit. The successful preliminary Co analyses demonstrate that ion microprobe analyses are a viable technique for measuring trace elements in sulfides.