Precise and accurate isotope fractionation factors (α17O, α18O and αD) for water and CaSO4·2H2O (gypsum)

Gázquez, Fernando and Evans, Nicholas P. and Hodell, David A. (2017) Precise and accurate isotope fractionation factors (α17O, α18O and αD) for water and CaSO4·2H2O (gypsum). Geochimica et Cosmochimica Acta, 198. pp. 259-270. ISSN 00167037 DOI 10.1016/j.gca.2016.11.001

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Official URL: http://doi.org/10.1016/j.gca.2016.11.001

Abstract

Gypsum (CaSO4·2H2O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (α17O, α18O and αD) between GHW and free water of the mother solution in the temperature range from 3 °C to 55 °C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (αDgypsum-water) increases by 0.0001 units per °C between 3 °C and 55 °C and salinities <150 g/L of NaCl. The αDgypsum-water is 0.9812 ± 0.0007 at 20 °C, which is in good agreement with previous estimates of 0.981 ± 0.001 at the same temperature. The α18Ogypsum-water slightly decreases with temperature by 0.00001 per °C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3 °C and 55 °C, α18Ogypsum-water averages 1.0035 ± 0.0002. This value is more precise than that reported previously (e.g. 1.0041 ± 0.0004 at 25 °C) and lower than the commonly accepted value of 1.004. We found that NaCl concentrations below 150 g/L do not significantly affect α18Ogypsum-water, but αDgypsum-water increases linearly with NaCl concentrations even at relatively low salinities, suggesting a salt correction is necessary for gypsum formed from brines. Unlike oxygen isotopes, the αDgypsum-water is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for 17O and 18O follows the theoretical mass-dependent fractionation on Earth (θ = 0.529 ± 0.001). We provide specific examples of the importance of using the revised fractionation factors when calculating the isotopic composition of the fluids.

Item Type: Article
Uncontrolled Keywords: 2016AREP; IA71;Bull
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
07 - Gold Open Access
Journal or Publication Title: Geochimica et Cosmochimica Acta
Volume: 198
Page Range: pp. 259-270
Identification Number: 10.1016/j.gca.2016.11.001
Depositing User: Sarah Humbert
Date Deposited: 10 Jan 2017 14:12
Last Modified: 17 Feb 2017 16:24
URI: http://eprints.esc.cam.ac.uk/id/eprint/3791

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