THE POTASSIUM–ARGON METHOD OF GEOLOGICAL AGE DETERMINATION

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Abstract
The development of the potassium–argon method for dating minerals is discussed. The decay scheme for radioactive potassium-40 is examined. The results of 11 recent counting experiments have been averaged and give a mean value of 29.4 ± 2.7 beta emissions per second per gram of potassium. Averaging the 15 previous determinations of the rate of gamma decay gives a branching ratio of 0.090 ± 0.038 with the above beta decay rate. This is in close agreement with a branching ratio of 0.089 suggested by us in a previous paper on the basis of two potassium–argon determinations.Methods of measuring the radiogenic argon content of potassium minerals are discussed and the method in use at this laboratory is described in detail. Yield runs and grain size experiments have been carried out and are reported. Potassium–argon ages have been calculated for four perthites and two lepidolites. These ages are in excellent agreement with the best available ages obtained by other methods.