Decay Constants ofK40as Determined by the Radiogenic Argon Content of Potassium Minerals

Abstract

It is shown that the potassium-argon age of young minerals depends almost linearly on the decay constant for electron capture in ${\mathrm{K}}^{40}$ and is very insensitive to the decay constant for beta emission. This fact permits calculation of ${\lambda }_e$ by comparing the concordant uranium-lead age of cogenetic uraninite with ${\mathrm{A}}^{40}$/${\mathrm{K}}^{40}$ ratios found in young samples of mica. It is found that ${\lambda }_e=(0.557\mathrm{}\pm 0.026)\times {10}^{-10\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$. Similar comparisons with older mica samples indicate that satisfactory agreement with the uraninite ages are obtained by use of this value of ${\lambda }_e$ together with ${\lambda }_{\beta }=(0.472\mathrm{}\pm 0.05)\times {10}^{-9\mathrm{}}$ ${\mathrm{yr}}^{-1\mathrm{}}$. It is concluded that there is no conflict between the decay constants inferred by this geological method and those found by direct counting experiments.