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Precise and accurate isotope fractionation factors (α$^{17}$O, α$^{18}$O and αD) for water and CaSO$_{4}$·2H$_{2}$O (gypsum)

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Gázquez, F 
Evans, NP 
Hodell, DA 


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 (αDgypsumwater) increases by 0.0001 units per °C between 3 °C and 55 °C and salinities <150 g/L of NaCl. The αDgypsumwater 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 α18Ogypsumwater 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, α18Ogypsumwater 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 α18Ogypsumwater, 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 αDgypsumwater is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for 17O and$^{18}8OfollowsthetheoreticalmassdependentfractionationonEarth(\textit{θ}$ = 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.



gypsum hydration water, fractionation factor, triple oxygen isotopes, stable isotopes

Journal Title

Geochimica et Cosmochimica Acta

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European Research Council (339694)
Natural Environment Research Council (NE/I016716/1)
This research was supported by the ERC WIHM Project [#339694] to DAH.