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Using Strontium Isotopes to Determine the Age and Origin of Gypsum and Anhydrite Beds

The variation of 87Sr/86Sr in seawater with time can be used to determine the age of calcium sulfate beds precipitated from brine derived from seawater. The determination of a marine origin can be established by the consistency of strontium isotope results from samples at different stratigraphic levels in a single gypsum/anhydrite bed collected at separated sites. The scatter of strontium isotope results in the gypsum/anhydrite samples examined here is interpreted to result from the contribution of meteoric strontium to the salina. Strontium isotope results from three evaporite settings, the Jurassic Todilto Formation from New Mexico, the Permian Blaine Formation from Blaine County, Oklahoma, and the Permian Salado Formation from New Mexico, are used to determine the age and origin of the parent brine for these gypsum/anhydrite beds. The calcium sulfate beds from each of these formations precipitated from salinas that originated with a marine flooding but show, at different times and localities, volume dominance by meteoric water. The marine/meteoric mixing of ancient salinas can be modeled by using strontium concentrations and isotope ratios estimated from modern analogs. A limited comparison of strontium and sulfur isotopes shows that sulfur isotopes in the settings studied are less sensitive to meteoric influx.