ABSTRACT

Six anhydrite samples from the submarine hydrothermal sulphide/sulphate mineral deposits found along the crest of the East Pacific Rise at 21 °N have been examined for filling temperature of fluid inclusions and for ³⁴S/³²S and ¹⁸O/¹⁶O ratios. The anhydrite was precipitated upon mixing of hot (260–350°C), SO₄^2--poor hydrothermal fluids with cold seawater at various proportions, which is reflected in a large variation in the filling temperatures ranging from 180°C to almost the highest exit temperatures. Seawater sulphate origin for anhydrite-sulphur is indicated by the δ³⁴S values that are essentially identical with the present-day seawater sulphate value, whereas the δ¹⁸O values are up to 2‰ higher than the seawater sulphate value. The anhydrites are not in oxygen isotopic equilibrium with the mixed solution at measured exit temperatures of 260 to 350°C. However, this enrichment of ¹⁸O may be attributed to partial equilibration of fine-grained anhydrite with solutions at lower temperatures after deposition. Absence of anhydrite at dead vents suggests that SO₄^2- slightly enriched in ¹⁸O is added to seawater sulphate owing to dissolution of anhydrite after the cessation of a high temperature stage of the hydrothermal activity.