Prediction of equilibrium isotopic fractionation of the gypsum/bassanite/water system using first-principles calculations

Abstract The stable isotopes ( 18 O/ 16 O, 17 O/ 16 O and 2 H/ 1 H) of structurally-bound water (also called hydration water) in gypsum (CaSO 4 ·2H 2 O) and bassanite (CaSO 4 ·0.5H 2 O) can be used to reconstruct the isotopic composition of paleo-waters. Understanding the variability of the isotope fractionation factors between the solution and the solid (α 17 O mineral-water , α 18 O mineral-water and αD mineral-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 α 18 O gypsum-water (1.0035 ± 0.0004) are in agreement with previous experimental studies, whereas αD gypsum-water agrees only at temperatures above 25 °C. At lower temperatures, the experimental values of αD gypsum-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 α 18 O bassanite-water is similar to α 18 O gypsum-water in the temperature range of 0–60 °C. Both α 18 O gypsum-water and α 18 O bassanite-water show a small temperature dependence of ∼0.000012 per °C, which is negligible for most paleoclimate studies. The theoretical relationship between α 17 O gypsum-water and α 18 O gypsum-water (θ = l n α 17 O l n α 18 O ) 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 δ 18 O values of gypsum hydration water are greater than free water (α 18 O gypsum-water  > 1) whereas δD values of gypsum hydration water are less than free water (αD gypsum-water