Sulphur and oxygen isotope signatures of late Permian Zechstein anhydrites, West Poland: seawater evolution and diagenetic constraints

The stable oxygen and sulphur isotope ratios of 52 anhydrite samples from three Zechstein anhydrite units (Lower Anhydrite, Upper Anhydrite and Basal Anhydrite) of West Poland show d18O values vs. VSMOW in the range of 9.4 to 15.5% (mean of 12.6 š1.3%), and d34S values vs. VCDT between 9.6 to 12.6%o (mean of 11.4 š0.6%o). A generally uniform distribution pattern of both isotopic values throughout the section, although with some random variation, implies that sulphate ions were sufficiently supplied and the basin was open during sulphate deposition. There is a slight stratigraphic differentiation of both the d18O and d34S values: the highest mean values are shown by the Upper Anhydrite and the lowest average values occur in the Basal Anhydrite. The correlation between d18 O and 8 S values is statistically significant only in case of the Basal Anhydrite. A wide range of oxygen isotopic ratios (from 11.6 to 25. l%o), with only several samples having d18O values that fall within the range of late Permian seawater, have been recorded in anhydrite cements and nodules that occur in the Main Dolomite rocks. Sulphur isotope ratios of anhydrite cements (range of 7.6 to 12.9%o, average of 10.7 š1.4%o) tend to reflect the late Permian sulphur isotopic signature of sulphate in seawater. The higher ranges of d18O and d34S values of anhydrite cements and nodules in the Main Dolomite compared to the underlying and overlying anhydrites are due to diagenetic resetting. The conversion of gypsum to anhydrite (often very early and under negligible cover) evidently did not affect the primary marine stratigraphic sulphur isotope composition of the sulphate deposits.