Decay Constants of K40 as Determined by the Radiogenic Argon Content of Potassium Minerals

It is shown that the potassium-argon age of young minerals depends almost linearly on the decay constant for electron capture in K40 and is very insensitive to the decay constant for beta emission. This fact permits calculation of λe by comparing the concordant uranium-lead age of cogenetic uraninite with A40/K40 ratios found in young samples of mica. It is found that λe=(0.557±0.026)×10^-10 yr^-1. Similar comparisons with older mica samples indicate that satisfactory agreement with the uraninite ages are obtained by use of this value of λe together with λβ=(0.472±0.05)×10^-9 yr^-1. It is concluded that there is no conflict between the decay constants inferred by this geological method and those found by direct counting experiments