SUMMARY

Sr and S isotope data and trace element data for anhydrite samples from drill core from the PACMANUS hydrothermal field reveal remarkable similarities and differences to anhydrite from the mid-ocean-ridge TAG system. A Sr mass balance indicates that, on average, there is less seawater present in the mixed fluids from which anhydrite precipitates. Within the PACMANUS area, the site of focused venting (Site 1189) appears to show more entrainment of seawater than the site of diffuse venting (Site 1188). Apparent distribution coefficients calculated for PACMANUS anhydrite are identical to those derived for anhydrite from TAG. Again, anhydrites from Site 1188 are distinct from those from Site 1189 and reveal higher apparent distribution coefficients. These differences are probably most pronounced in the ³⁴S compositions that are dominantly <21 at Site 1188 and largely >21 at Site 1189. Rare earth element concentrations show a remarkable variability in PACMANUS anhydrites when compared to anhydrite from TAG. Most noteworthy is anhydrite with mid-REE enrichment from Site 1188. The differences in anhydrite geochemistry between PACMANUS and TAG and the differences between separate sites within the PACMANUS field can be used to infer contrasts in style and intensity of fluid flow and fluid-rock interaction.