Uranium-isotope disequilibrium in brine reservoirs of the Castile Formation, northern Delaware Basin, southeastern New Mexico. I. Principles and methods

We evaluated uranium isotope activity ratios with respect to models for the origin of the brines in two brine reservoirs in the Castile Formation (ERDA No. 6 and WIPP No. 12). In Castile anhydrite, a completely closed water system that was continuously leaching Th-recoil-produced 234U from freshly exposed surfaces of fractured host rock would give rise to uranium 234/238 isotope activity ratio (α) values significantly higher than observed values. Therefore, the brine occurrences are not the result of continuous deformation. Similarly, a model assuming movement of intergranular Permian seawater into fractures was found inconsistent with observed uranium isotope systematics. The observed α-values (95% confidence limits) of ERDA No. 6 (1.34 to 1.58) and WIPP No. 12 (1.74 to 2.54), used in conjunction with an inferred intial α0 higher than observed values, allows calculation of reasonable finite minimum ages, involving no preferential leaching of 234U in the host rock. If the brine occurrences are inferred to have been connected at one time with a more extensive nearby hydrologic system, the Capitan limestone (α0 = 5.1), calculated minimum ages of isolation from that system are 700,000 to 880,000 yr for ERDA No. 6 and 360,000 to 610,000 yr for WIPP No. 12. These ranges in ages are the 95% confidence limits based on experimental determinations of α-values. The ages thus derived may reflect an episode of structural deformation in the Pleistocene that allowed water to enter the resulting fractures. 16 references, 6 figures