 Potassium-argon dating

With no initial radiogenic argon-40 and complete daughter retention, the surviving potassium-40 population is $N_K=N_{K,0}{e}^{-\lambda t}$ and the accumulated daughter population is $N_{\mathrm{Ar}}=b(N_{K,0}-N_K)$. Here $\lambda$ is the total nuclear decay constant and $b$ is the argon-producing branching fraction. Their ratio gives the displayed age. The result measures closure of the mineral's isotope system under those assumptions.