Preservation and modification of the isotopic composition (18O/16O and 2H/1H) of structurally-bound hydration water of gypsum (CaSO4•2H2O) in aqueous solution

Early investigations on stable isotopes (delta O-18 and delta D) of structurally-bound gypsum (CaSO4 center dot 2H(2)O) hydration water (GHW) suggested that soon after gypsum precipitation, its primary isotopic composition could be altered via gypsum re-crystallization or by isotope exchange by diffusion of water into the intact crystals. If this occurs, the use of stable isotopes of GHW as a paleoclimate proxy is compromised. Here we investigated the long-term (up to 6 years) stability of GHW in contact with aqueous solution by conducting isotopic exchange experiments at different temperatures and using varying gypsum grain size. We placed gypsum with known hydration water isotopic composition in O-1(8)/D-enriched and O-1(8)/D-depleted aqueous solutions and monitored any change in the stable isotopes of GHW with time. At low temperature (25 degrees C) and when the solution is in chemical equilibrium with the mineral, GHW preserves its primary isotopic composition after 2 years. In contrast, when the gypsum-solution system is out of chemical equilibrium, the delta(18)OGHW and delta D-GHW values are altered, first via gypsum recrystallization from the solution and later by isotope exchange via diffusion. Importantly, delta(18)OGHW and delta D-GHW stabilized after 2 years to values that represent only an 8 % change relative to the original GHW and remain constant during the subsequent 4 years. Our results suggest that at low temperature the effect of isotope exchange on GHW is limited. At higher temperature (65 degrees C), chemical equilibrium is not attained after 2 years for fine-grained synthetic gypsum (<250 mu m) and its isotopic values continues to change. In contrast, the delta(18)OGHW and delta D-GHW in experiments using fine-grained natural gypsum at 45 degrees C stabilized after 1 year and values changed less than 3 % with respect to initial conditions. Experiments with coarser-grained natural gypsum (1-2 mm) at 45 degrees C did not show measurable isotopic changes of the GHW, indicating the lack of gypsum dissolution/re-crystallization or diffusion isotopic exchange. We conclude that microcrystalline gypsum crystals (<250 mu m) are more readily affected by diagenesis resulting from changes in the gypsum saturation state of the solution, whereas larger gypsum crystals are more likely to preserve their original isotopic compositions. Our results indicate that under certain conditions, GHW can preserve the isotopic composition of its parent fluid and provide valuable information about paleoclimate and paleo-hydrologic conditions. (C) 2022 Elsevier Ltd. All rights reserved.

Automated summary

Early investigations on stable isotopes of gypsum hydration water suggested that its isotopic composition could be altered soon after precipitation. This study investigates the long-term stability of gypsum hydration water in contact with solution at different temperatures and grain sizes. The results show that at low temperature, the isotopic composition of gypsum hydration water remains stable over time, while at high temperature and with fine-grained gypsum crystals, the isotopic composition continues to change.