Oxygen and sulphur isotopes of gypsum from the Mogilno Salt Dome cap-rock (Central Poland)

The stable-isotope compositions of oxygen and sulphur from 30 gypsum samples obtained from three drill cores of the Mogilno Salt Dome cap-rock have been analysed; the Mogilno Salt Dome itself is composed of Zechstein evaporites. d 34 S values were measured for SO 4 ions, whereas d 18 O values were measured for SO 4 and H 2 O. For the analyses, bulk samples of gypsum (CaSO 4 ·2H 2 O) and its dehydrated equivalents ( CaSO4 ) were used. The d 18 O and d 34 S values in SO 4 were analysed using a direct method, but the d 18 O value in H 2 O (water of crystallisation) was determined using an indirect method, which influenced the results. The SO 4 appeared to be distinctly enriched in heavy oxygen ( d 18 O = 14.6–11.0‰), which shows that the Zechstein sea was the source of the sulphate; the d 34 S values (10.9–13.6‰) support this conclusion. Variable d 18 O values of the water of crystallisation (–11.3 to 10.7‰) indicate that diagenetic (hydration) processes affected the various samples. Accessory minerals occurring in gypsum probably also influenced the d 18 O values. Five groups of gypsum are distinguished: (1) gypsum in which the – d 18 O values of the H 2 O are less than –10.2‰ (this gypsum recrystallised in the presence of water enriched in light oxygen isotopes), (2) gypsum in which the d 18 O values of H 2 O range between –6.6 and –4.4‰ (interpreted as gypsum in equilibrium with water from the cap-rock), (3) gypsum in which the d 18 O value of the H 2 O amounts to 10.7‰ (anhydrite-bearing sandstone near the salt mirror), (4) gypsum in which the d 18 O value of the H 2 O ranges between 1.8 and –3.4‰ (samples contaminated by anhydrite; the real d 18 O content is unknown), and (5) gypsum in which the d 18 O value of the H 2 O ranges between –6.9 and –8.7‰ (interpreted as gypsum with a mixed isotopic composition due to mixing of water from cap-rock and water enriched in light isotopes of oxygen: recent meteoric water or postglacial water?, or as gypsum formed in a warm interval when the groundwater was somewhat