Isotopic Composition of Hydration Water in Gypsum and Hydroxyl in Jarosite

Abstract

AbstractData on the isotopic composition of hydration water in gypsum and hydroxyl in jarosite are rare. This study was initiated to measure the δ18O and δD (deuterium) values of hydration water in gypsum and hydroxyl in jarosite and to determine their relationship with the isotopic composition of environmental water. Gypsum and jarosite in soils from different localities in Saskatchewan, Canada, were sampled for O and H isotope analyses. Two samples from Maryland and one from Thailand were included for comparison. The δD and δ18O values of hydration water in gypsum and hydroxyl in jarosite varied from ‐166 to ‐80‰ and ‐17 to ‐9‰, respectively. Gypsum and jarosite samples from Saskatchewan generally followed the gradient in isotopic composition of local precipitation with slopes of 8.9 and 8.6, respectively. Samples from Maryland and Thailand, which are located at lower latitudes, showed relative enrichment in D and 18O, compared with the samples from Saskatchewan. The δD and δ18O data of samples from Saskatchewan, Maryland, and Thailand plot along a straight line with a steep slope of 11.5 due to the relative D and 18O enrichment in samples from Maryland and Thailand. Because the isotopic composition of hydration water of gypsum and hydroxyl of jarosite reflect that of water in the soil environment, they may be useful as a proxy for the isotopic composition of soil moisture at depths where gypsum or jarosite precipitates.