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

Tao Liu,Emilio Artacho,Fernando Gázquez,Gregory Walters,David Hodell

doi:10.1016/j.gca.2018.08.045

Tao Liu, Emilio Artacho + Show 3 more

Open Access

https://doi.org/10.1016/j.gca.2018.08.045

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Abstract

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 (1.0035 ± 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.

Highlights

Gyspum (CaSO42H2O) is a common hydrous mineral on Earth and has been shown to be abundant on Mars (Showstack, 2011)
We carried out first-principles calculations of oxygen and hydrogen isotopic fractionation factors between free water and the hydration water of gypsum and bassanite
The temperature dependence of α18Ogypsum-water is insignificant for most paleoclimate applications using gypsum hydration water, but the dependence of αDgypsum-water on temperature is significant

Summary

Introduction

Gyspum (CaSO42H2O) is a common hydrous mineral on Earth and has been shown to be abundant on Mars (Showstack, 2011). The presence of bassanite, together with gypsum and other hydrous minerals on Mars (Wray et al, 2010), has generated considerable interest in how these minerals form and their paleoenvironmental significance. Under certain conditions, the isotopic composition of hydration water in some minerals (e.g. gypsum) record the isotope values of the mother water with an offset between the free solution and mineral hydration water because of isotope fractionation (Gonfiantini and Fontes, 1963; Sofer, 1978; Hodell et al, 2012; Tan et al, 2014; Gázquez et al, 2017b; Herwartz et al, 2017). The fractionation factor ( mineral-water) can be expressed as: where R is the ratio of the heavy to light isotope (e.g., 18O/16O, 17O/16O, D/H) of the mineral hydration water and mother water, respectively. The fraction factor can be approximated by the isotopic difference between the mineral and water ( ):

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