The isotopic record of gypsum diagenesis in diluted solutions: Observations in natural salinas and experiments

Gypsum deposits are often considered as reliable components of evaporitic series for reconstructing their conditions of precipitation from saturated (marine or non-marine) calcium-sulfate solutions. The isotopic compositions of the hydration water and of the sulfate are thought to provide the composition and thus the origin of the parent solution, assuming that the primary signals are preserved. The present study demonstrates that dissolution-recrystallization of gypsum from natural salinas and in experiments where gypsum reacted with diluted solutions, was responsible for the rapid modification of the primary isotopic signal by the incorporation of the reacting water in the crystal lattice of the new gypsum generation. It is also shown that gypsum diagenesis may cause significant changes of the isotopic composition of the sulfate due to fractionation between the resulting dissolved and new gypsum phases.