An oxygen and sulfur isotopic study of gypsumfrom the Wapno Salt Dome cap-rock (Poland)

Abstract

The stable isotope compositions of oxygen and sulfur from gypsum samples derived from borehole cores of the Wapno Salt Domecap-rock have been determined. The d18O values were measured (for both the SO4 and the H2O) in bulk samples of gypsum(CaSO4 2H2O) and in its dehydrated equivalents (CaSO4). The SO4 is enriched in the heavy oxygen isotope, which unequivocally indicates the Zechstein sea as the place of origin of the sulfate. The variable d18O values of the H2O (water of crystallization) indicate different conditions (and stages) of diagenetic processes of these samples, primarily recrystallization (hydration) episodes. Two groups ofgypsum were distinguished: (1) a group in which the d18O values of H2O range between –9.0 and –10.2‰ (interpreted as gypsum in equilibrium with recent or subrecent meteoric water), (2) a group in which the d18O values of the H2O were less than –10.2‰ (this gypsum hasrecrystallized in the presence of water enriched in the light isotopes of oxygen). In addition, in one single analysis the d18O value of theH2O amounts to –5.6‰ (gypsum with mixed isotopic composition).