Sulfur isotopic composition of sulfides in ODP Hole 111-504B (Table 1)

An ion microprobe was used to perform in-situ measurements of the sulfur isotopic compositions of sulfide minerals in seven samples of hydrothermally altered rocks from DSDP/ODP Hole 504B. The sulfides exhibit a wide range in d34S values, from -4.1 per mil to + 8.3 per mil. Heterogeneities exist among different sulfide grains in a given sample as well as within individual grains. A transect across a centimeter-size pyrite crystal reveals d34S values that span the range of the entire data set, indicating a complex growth history for this crystal. The positive d34S values are similar to those reported for seafloor sulfide deposits and are interpreted to reflect a combination of sulfur leached from basalts and 34S-rich sulfide derived from seawater sulfate. Incorporation of seawater-derived sulfur is greatest in a stockworklike sulfide mineralization in the core, where subsurface mixing occurred between hydrothermal fluids and seawater. Variations in d34S are attributed to (1) incorporation of variable amounts of seawater-derived sulfide, (2) kinetic effects during precipitation, (3) reservoir effects during precipitation, (4) and variations in pH and f(O2) .