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. d³⁴S values were measured for SO₄ ions, whereas d¹⁸O values were measured for SO₄ and H₂O. For the analyses, bulk samples of gypsum (CaSO₄·2H₂O) and its dehydrated equivalents (_CaSO4) were used. The d¹⁸O and d³⁴S values in SO₄ were analysed using a direct method, but the d¹⁸O value in H₂O (water of crystallisation) was determined using an indirect method, which influenced the results. The SO₄ appeared to be distinctly enriched in heavy oxygen (d¹⁸O = 14.6–11.0‰), which shows that the Zechstein sea was the source of the sulphate; the d³⁴S values (10.9–13.6‰) support this conclusion. Variable d¹⁸O 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 d¹⁸O values. Five groups of gypsum are distinguished: (1) gypsum in which the –d¹⁸O 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 d¹⁸O 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 d¹⁸O value of the H₂O amounts to 10.7‰ (anhydrite-bearing sandstone near the salt mirror), (4) gypsum in which the d¹⁸O value of the H₂O ranges between 1.8 and –3.4‰ (samples contaminated by anhydrite; the real d¹⁸O content is unknown), and (5) gypsum in which the d¹⁸O 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