Radiometric Dating of Ochoan (Permian) Evaporites, Wipp Site, Delaware Basin, New Mexico, Usa

Abstract

We have attempted radiometric dating of halide-sulfate salts and clay minerals from the Delaware Basin, New Mexico, USA, as part of geochemical study of the stability of the evaporite sequence at the WIPP (Waste Isolation Pilot Plant--a US DOE facility) site. We undertook this dating to determine: (1) primary age of evaporite genesis or time(s) of recrystallization, (2) if previously undated evaporite minerals (leonite, polyhalite, kieserite) give useful data, and (3) if the detrital clay minerals have been radiometrically reset at any time following their incorporation into the evaporite medium. We have shown earlier that polyhalites can indeed be successfully dated by the K-Ar method, and once corrections are applied for admixed halide minerals, dates of 210-230 Ma for the Delaware Basin are obtained. Rb-Sr isochrons from early stage sylvites-polyhalites- anhydrites yield 220 ± 10 Ma, even when some sylvites yield lower K-Ar dates due to l1s of 40Ar*. K-Ar dates on leonites and kieserites are also low due to 40Ar* loss, but their Rb-Sr dates are higher. Detrital clay minerals from the Delaware Basin collectively yield a highly scattered isochron (390 ± 77 Ma), but samples from a local area, such as the WIPP Site, give a much better age of 428 ± 7 Ma. These dates show that the interaction between the clay minerals and the evaporitic brines was insufficient to reset the clay minerals Rb-Sr systematics. In a related study, we note that a dike emplaced into the evaporite at 34 Ma had only very limited effect on the intruded rocks; contact phenomena were all within 2 m of the dike. All of our geochemical (radio-metric and trace element) studies of the WIPP site argue for preservation of the isotopic and chemical integrity of the major minerals for the past 200 Ma.

Origin of Potash Deposits

“…For example, Obradovich et al (1982) have indicated that their and other work predicted K-Ar ages of 245 Myr for polyhalite and langbenite, generally agreeing with Brookins and Lambert (1987) and non-isotopic geological age dating estimates of 240 Myrs. For example, Obradovich et al (1982) have indicated that their and other work predicted K-Ar ages of 245 Myr for polyhalite and langbenite, generally agreeing with Brookins and Lambert (1987) and non-isotopic geological age dating estimates of 240 Myrs.…”

“…Brookins and Register (1981) and Brookins and Lambert (1987) report that both argon and Rb-Sr isotope determinations show that the deposit is 210 ± 10 million years old. Brookins and Register (1981) and Brookins and Lambert (1987) report that both argon and Rb-Sr isotope determinations show that the deposit is 210 ± 10 million years old.…”

“…According to Baadsgaard and Dodson 1964, the K-Ar method for sylvite predicts ages somewhat lower than the real age. (Brookins and Lambert 1987 suggest that Rb-Sr determinations normally give very low results for halite). The inference drawn from these results is that the sylvite was derived from carnallite in this deposit soon after it crystallized from the sea water, and that red or clear sylvite, and its location in the deposit were all of nearly the same age.…”