Mineralogical studies of sulfide samples and volatile concentrations of basalt glasses from the southern Juan de Fuca Ridge

Sulfide samples obtained from the U.S. Geological Survey's DSRV Alvin dives on the southern Juan de Fuca Ridge closely resemble those from the same area described by Koski et al. (1984). Major minerals include sphalerite, wurtzite, pyrite, marcasite, isocubanite, anhydrite, and chalcopyrite. Equilibrium, if attained at all, during deposition of most sulfides was a transient event over a few tens of micrometers at most and was perturbed by rapid temperature and compositional changes of the circulating fluid. Two new minerals were found: one, a hydrated Zn, Fe hydroxy-chlorosulfate, and the other, a (Mn, Mg, Fe) hydroxide or hydroxy-hydrate. Both were formed at relatively low temperatures. Lizardite, starkeyite, and anatase were found for the first time in such an environment. Sulfide geothermometry involving the system Cu-Fe-S indicates a vent temperature of <328°C for one sample. Fluid inclusion studies on crystals from the same vicinity of the same sample give pressure-corrected homogenization temperatures of 268° and 285°C. Ice-melting temperatures on inclusions from the same sample are about −2.8°C, indicating that the equivalent salinity of the trapped fluid is about 50% greater than that of seawater. Volatile concentrations from vesicle-free basalt glass from the vent field are about 0.013 wt % CO₂ and 0.16 wt % H₂O. CO₂ contents in these samples yield an entrapment depth of 2200 m of seawater, which is the depth from which the samples were collected.