Prediction of equilibrium isotopic fractionation of the gypsum/bassanite/water system using first-principles calculations
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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 isotopic 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 (1.00347±0.00037) 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. 0.9749 and 0.9782, 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 °C to 60 °C. Both α18Ogypsum-water and α18Obassanite-water show a small temperature dependence of ∼0.0000122 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±0.00063. 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.
Liu , T , Artacho , E , Gázquez , F , Walters , G & Hodell , D 2019 , ' Prediction of equilibrium isotopic fractionation of the gypsum/bassanite/water system using first-principles calculations ' , Geochimica et Cosmochimica Acta , vol. 244 , pp. 1-11 . https://doi.org/10.1016/j.gca.2018.08.045
Geochimica et Cosmochimica Acta
© 2018 Elsevier Ltd. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at: https://doi.org/10.1016/j.gca.2018.08.045Copyright © 2018 The Authors. Published by Elsevier Ltd.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
DescriptionThis research was supported by the ERC WIHM Project [#339694] to DAH.
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