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

Gypsum (CaSO4 center dot 2H(2)O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (alpha O-17, alpha O-18 and alpha D) between GHW and free water of the mother solution in the temperature range from 3 degrees C to 55 degrees C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (alpha Dgypsum-water) increases by 0.0001 units per degrees C between 3 degrees C and 55 degrees C and salinities <150 g/L of NaCl. The alpha Dgypsum-water is 0.9812 +/- 0.0007 at 20 degrees C, which is in good agreement with previous estimates of 0.981 +/- 0.001 at the same temperature. The alpha O-18(gypsum-water) slightly decreases with temperature by 0.00001 per degrees C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3 degrees C and 55 degrees C, alpha O-18(gypsum-water) averages 1.0035 +/- 0.0002. This value is more precise than that reported previously (e.g. 1.0041 +/- 0.0004 at 25 degrees C) and lower than the commonly accepted value of 1.004. We found that NaC1 concentrations below 150 g/L do not significantly affect alpha O-18(gypsum-water) , but alpha O-18(gypsum-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 alpha O-18(gypsum-water) is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for O-17 and O-18 follows the theoretical mass -dependent fractionation on Earth (theta = 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. (C) 2016 The Author(s). Published by Elsevier Ltd.