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

Experimental evaporation of seawater yields brines with delta(37)Cl from 0. 0 parts per thousand (initial) to -0.9 parts per thousand. In the Pale Duro Basin, brines with delta(37)Cl values overlapping the -0.1 to 0.4 parts pe r thousand 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 delta(37)Cl values of -0.4 to -1.0 parts per thousand in the evaporite aquitard and in a deep brine aqui fer cannot be generated by dissolution of halite. Considered with Br and Cl content, delta D and noble gas content, the delta(37)Cl data indicate that such brines originated as evapoconcentrated seawater. High Br evaporite br ine 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 mete oric water at the time of evaporite formation. There has not been detectabl e vertical flow of meteoric water across the evaporite aquitard. Strata ben eath the evaporite contain compartments that have been isolated geochemical ly since the Permian. Copyright (C) 1999 Elsevier Science Ltd.