The stable isotopes (18O/16O, 17O/16O and 2H/1H) of structurally-bound water (also called hydration water) in gypsum (CaSO4·2H2O) and bassanite (CaSO4·0.5H2O) can be used to reconstruct the isotopico composition of paleo-waters. Understanding the variability of the isotope fractionation factors between the solution and the solid (α17Omineral-water, α18Omineral-water and αDmineral-water) is crucial for applying this proxy to paleoclimatic research. Here we predict the theoretical equilibrium fractionation factors for triple oxygen and hydrogen isotopes in the gypsum-water and bassanite-water systems between 0 °C and 60 °C. We apply first-principles using density functional theory within the harmonic approximation. Our theoretical results for α18Ogypsum-water (1.0035 ± 0.0004) are in agreement with previous experimental studies, whereas αDgypsum-water agrees only at temperatures above 25 °C. At lower temperatures, the experimental values of αDgypsum-water are consistently higher than theoretical values (e.g. 0.975 and 0.978, respectively, at 3 °C), which can be explained by kinetic effects that affect gypsum precipitation under laboratory conditions at low temperature. We predict that α18Obassanite-water is similar to α18Ogypsum-water in the temperature range of 0–60 °C. Both α18Ogypsum-water and α18Obassanite-water show a small temperature dependence of ∼0.000012 per °C, which is negligible for most paleoclimate studies. The theoretical relationship between α17Ogypsum-water and α18Ogypsum-water (θ =lnα17Olnα18O) from 0 °C to 60 °C is 0.5274 ± 0.0006. The relationship is very insensitive to temperature (0.00002 per °C). The fact that δ18O values of gypsum hydration water are greater than free water (α18Ogypsum-water > 1) whereas δD values of gypsum hydration water are less than free water (αDgypsum-water < 1) is explained by phonon theory. We conclude that calculations from first-principles using density functional theory within the harmonic approximation can accurately predict fractionation factors between structurally-bound water of minerals and free water.
Keywords: Gypsum, Bassanite, Fractionation factor, First-principles