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

Abstract

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. δ34S values were measured for SO4 ions, whereas δ18O values were measured for SO4 and H2O. For the analyses, bulk samples of gypsum (CaSO4-2H2O) and its dehy- drated equivalents (CaSO4) were used. The δ18O and δ34S values in SO4 were analysed using a direct method, but the δ18O value in H2O (water of crystallisation) was determined using an indirect method, which influenced the results. The SO4 appeared to be distinctly en-riched in heavy oxygen (δ18O = 14.6-11.0‰), which shows that the Zechstein sea was the source of the sulphate; the δ34S values (10.9-13.6‰) support this conclusion. Variable δI8O 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 δ18O values. Five groups of gypsum are distinguished: (1) gypsum in which the -δ18O values of the H2O are less than -10.2‰ (this gypsum recrystallised in the presence of water enriched in light oxygen isotopes), (2) gypsum in which the δ18O values of H2O range between -6.6 and -4.4‰ (interpreted as gypsum in equilibrium with water from the cap-rock), (3) gypsum in which the δ18O value of the H2O amounts to 10.7%o (anhydrite-bearing sandstone near the salt mirror), (4) gypsum in which the δ18O value of the H2O ranges between 1.8 and -3.4‰ (samples contaminated by anhydrite; the real δ18O content is unknown), and (5) gypsum in which the δ18O value of the H2O 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 enriched in heavy isotopes of oxygen if compared with the groundwater at present).