--> Abstract: Using Strontium Isotopes to Better Understand Gypsum/Anhydrite Beds, by R. E. Denison and D. W. Kirkland; #90928 (1999).

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DENISON, RODGER E. and DOUGLAS W. KIRKLAND
Programs in Geosciences, The University of Texas at Dallas, Richardson, TX 75083-0688

Abstract: Using Strontium Isotopes to Better Understand Gypsum/Anhydrite Beds

Gypsum records the strontium isotope ratio of the brine from which it precipitates. We interpret consistent strontium isotope results for individual gypsum or anhydrite beds, ideally from different stratigraphic levels and from different geographic localities, to indicate precipitation from a marine salina. We interpret a scatter of strontium isotope results to indicate precipitation from a lacustrine or hybrid salina. Isotopic measurements on adjacent laminations from Permian gypsum units precipitated from mixed-water salinas record examples of marked short-term shifts in balance between contribution of meteoric and marine water to the salinas. Variations in the 87Sr/86Sr are as great as 0.00015. Within hybrid evaporites, some beds may closely approach the marine ratio. An ocean-connected salina may also precipitate gypsum with a marine signature at one locality, while at another, because of poor mixing, may precipitate gypsum with an isotope ratio demanding a parent brine consisting of many volumes of meteoric water for each volume of marine water. For hybrid beds we have modeled the marine-to-meteoric mixing-ratio of the parent salina using strontium concentrations and isotope ratios estimated from modern analogs. Isotopic results from the same samples show that sulfur is much less sensitive to meteoric influence than strontium. We have not been able to identify any petrographic or trace element criteria to identify marine derived evaporites. Seven evaporite sequences we examined began with marine flooding and six were intermittently influenced by substantial amounts of meteoric water.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas