K-Ar Dating Formulas
Age = (1/lambda_total) * ln(1 + (lambda_total/lambda_Ar) * (Ar40*/K40))
lambda_total = 5.543 x 10^-10 /yr (K-40 total decay)
lambda_Ar = 0.581 x 10^-10 /yr (electron capture to Ar-40)
Half-life K-40 = 1.248 billion years
K-Ar dating is used for volcanic rocks older than about 100,000 years. K-40 decays to both Ar-40 (10.48% by electron capture) and Ca-40 (89.52% by beta decay). The method assumes all radiogenic Ar-40 was trapped in the mineral since its crystallization from lava.