Stable chlorine isotopes in the Palo Duro Basin, Texas: Evidence for preservation of Permian evaporite brines

Experimental evaporation of seawater yields brines with δ³⁷Cl from 0.0‰ (initial) to -0.9‰. In the Palo Duro Basin, brines with δ³⁷Cl values overlapping the -0.1 to 0.4‰ range of halite evaporite can be generated by a set of processes including dissolution of halite in meteoric water. Such brines occur above and below an evaporite aquitard. Brines with δ³⁷Cl values of -0.4 to -1.0‰ in the evaporite aquitard and in a deep brine aquifer cannot be generated by dissolution of halite. Considered with Br and Cl content, δD and noble gas content, the δ³⁷Cl data indicate that such brines originated as evapoconcentrated seawater. High Br evaporite brine formed directly from seawater on the eastern side of the basin, whereas low Br evaporite brines on the western side formed after an influx of meteoric water at the time of evaporite formation. There has not been detectable vertical flow of meteoric water across the evaporite aquitard. Strata beneath the evaporite contain compartments that have been isolated geochemically since the Permian.