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Hydrologic investigations of the Wolfcamp aquifer, Palo Duro basin, Texas Panhandle, report slow flow rates and long travel paths, which lead to a prediction of very old ages for the ground water. Because the expected ages are about 10 million years, many methods of dating ground water are unsuitable. The U, Th^rarr4He and 40K^rarr40Ar clocks are suitable. All ground water samples have large amounts of radiogenic 4He and 40Ar. Using these 4He and 40Ar data and a mixture of estimated and measured values for K, U, and Th contents, water-rock ratios, and release factors, the initial age estimate for the ground water is about 100 million years at two wells, Stone and Webster 1 Sawyer in Donley County Texas, and Stone and Webster 1 Zeeck in Swisher County, Texas. At a third well, Stone and Webster 1 Mansfield in Oldham County, Texas, the concentrations of 4He and 40Ar are much higher, and the apparent ages are about 250 million years. Other isotopic and chemical data (^dgrD, ^dgr18O, Br/Cl) for ground water from this third well indicate a different origin and/or history. The current working hypothesis is that ground water sampled in the 1 Mansfield well is a mixture of meteoric water and a deep-basin ground water.
The 4He ages for the 1 Sawyer well and 1 Zeeck well samples appear to be valid and are presently assigned errors of about a factor of two, determined almost entirely by uncertainties in U and Th concentrations and water-rock ratios. The 40Ar ages remain suspect because data are currently too few to confirm or negate the possibility that a significant fraction of the 40Ar is inherited from detrital minerals.
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