A40-K40 dating of igneous rocks and sediments

The discrepancy in A40K40 ratios between micas and feldspars reported by Wetherill has been confirmed. Assuming no A40 to be inherited by potassium minerals, a branching ratio of 0.113 is calculated from dated mica samples. This implies that the micas investigated have retained 85% or more of their argon. The branching ratio of R = 0.085 is interpreted to be an empirical calibration constant which corrects for argon loss. The A40K40 ratios in four authigenic minerals have been determined and the ages are found to be in reasonable agreement with the times assigned to the geologic periods by Holmes.