Geochronologic studies near the WIPP site, southeastern, New Mexico

The U.S. Department of Energy's Waste Isolation Pilot Plant (WIPP) site is currently being investigated for pilot storage of defense-generated radioactive waste. A critical facet of the overall study is the question as to whether the rocks have remained closed systems with respect to their bulk chemistry and isotopic composition since formation in the Late Permian as opposed to episodic or more-or-less continuous recrystallization during the post-Late Permian. Early attempts at K-Ar dating of sylvites mixed with other salts yielded inconclusive results, primarily because sylvite is not well suited for K-Ar study due to loss of radiogenic 40Ar. Rb-Sr study of sylvites yields a 214 +/- 15 m.y.B.P. (million years before present) isochron indicating closed system conditions to Rb and Sr since latest Permian or earliest Triassic. Pre-200 m.y.B.P. K-Ar dates have also been determined for pure langbeinites and polyhalites. When mixed with sylvite, age lowering results. The Rb-Sr systematics of eolian clay minerals known to have interacted with the evaporite brine yield a poorly defined isochron of 390 +/- 77 m.y.B.P., but the apparent date does indicate that the clay mineral-brine interactions were not so severe as to completely rehomogeize Sr isotopes despite the clay mineral alteration. A 34 +/- 1.5 m.y.B.P. lamprophyre dike intrudes the evaporite sequence some 70 km north of the WIPP site. Contact effects, including recrystallization of polyhalite, are restricted to within 10 m of the dike. Finally, polyhalite inclusions in one rubble chimney yield a pre-200 m.y.B.P. age indicating no major recrystallization effects due to this distrubance of the evaporite sequence. Collectively, the geochronologic studies argue for pre-200 m.y.B.P. formation of the evaporite mineral and stability of the rocks since that time