SULFUR, CARBON, AND OXYGEN ISOTOPE VARIATIONS IN SUBMARINE HYDROTHERMAL DEPOSITS OF GUAYMAS BASIN, GULF OF CALIFORNIA, USA

Sulfur, carbon, and oxygen isotope values were measured in sulfide, sulfate, and carbonate from hydrothermal chimney, spire, and mound samples in the southern trough of Guaymas Basin, Gulf of California, USA. delta-S-34 values of sulfides range from -3.7 to 4.5% and indicate that sulfur originated from several sources: (1) dissolution of 0% sulfide contained within basaltic rocks, (2) thermal reduction of seawater sulfate during sediment alteration reactions in feeder zones to give sulfide with positive-delta-S-34, and (3) entrainment or leaching of isotopically light (negative-delta-S-34) bacteriogenic sulfide from sediments underlying the deposits. Delta-S-34 of barite and anhydrite indicate sulfur derivation mainly from unfractionated seawater sulfate, although some samples show evidence of sulfate reduction and sulfide oxidation reactions during mixing within chimneys. Oxygen isotope temperatures calculated for chimney calcites are in reasonable agreement with measured vent fluid temperatures and fluid inclusion trapping temperatures. Hydrothermal fluids that formed calcite-rich chimneys in the southern trough of Guaymas Basin were enriched in O-18 with respect to seawater by about 2.4% due to isotopic exchange with sedimentary and/or basaltic rocks. Carbon isotope values of calcite range from -9.6 to -14.0% delta-C-13PDB, indicating that carbon was derived in approximately equal quantities from the dissolution of marine carbonate minerals and the oxidation of organic matter during migration of hydrothermal fluid through the underlying sediment column. Statistically significant positive, linear correlations of delta-S-34, delta-C-13, and delta-O-18 of sulfides and calcites with geographic location within the southern trough of Guaymas Basin are best explained by variations in water/rock (w/r) ratios or sediment reactivity within subsurface alteration zones. Low w / r ratios and the leaching of detrital carbonates and bacteriogenic sulfides at the southern vent sites result in relatively high delta-C-13 and low delta-S-34 in chimney carbonates and sulfides, respectively. In the north, where the depletion of alkalis in vent fluids indicates higher w/r ratios, positive delta-S-34 and more negative delta-C-13 are due to increased contributions from organic matter oxidation and sulfate reduction reactions.