Sulphur, sulphate oxygen and strontium isotope composition of Cenozoic Turkish evaporites

Sulphur (δ34S) and strontium isotope (87Sr/86Sr) ratios have been measured in 37 sulphate minerals (gypsum, celestite and thenardite) and 4 sulphide samples (δ34S only) from 9 Cenozoic marine and nonmarine evaporites located in Anatolia, Turkey. Oxygen isotope (δ18Osulphate) ratios were also measured in 25 gypsum and 1 anhydrite sample from these deposits. These data have been used to determine the origin of dissolved sulphate in the brines that precipitated these minerals. They show that seawater was the dominant source of sulphate and Sr in the marine evaporites, but that perturbations from contemporaneous seawater Sr and sulphur isotope compositions result from recycling of older evaporites and sulphate reduction. Although continental geothermal fluids played an important role in supplying the dissolved salts that formed the nonmarine evaporites, the δ18Osulphate, δ34S and Sr isotope compositions of many of these nonmarine evaporites are indistinguishable from the marine evaporites. As well as suggesting that recycling of marine evaporites was important for controlling the composition of the nonmarine evaporites, it also suggests that δ18Osulphate, δ34S and Sr isotope compositions are not unequivocal tracers in distinguishing between these two types of evaporite. For the Turkish evaporites considered here, the major difference between marine and nonmarine evaporites that contain similar δ34S–δ18Osulphate–87Sr/86Sr relationships is that the latter contain high concentrations of boron that reflect a geothermal contribution to the deposits.