Radioactive ages of micas from granitic rocks by Rb‐Sr and K‐A methods

Abstract

Analytical data and ages are given for K‐A, Rb‐Sr, U‐Pb and Th‐Pb ages from a number of locations. Comparisons of the ages obtained indicate (a) that K‐A and Rb‐Sr ages of mica usually agree when modern laboratory determined values of the decay constants of K40 and Rb87 are used; (b) that the K‐A and Rb‐Sr ages of micas agree with the U‐Pb ages of cogenetic uraninite, zircon, and monazite when the U‐Pb ages are concordant; (c) that when the U‐Pb ages of these uranium minerals are discordant, the mica ages agree best with the Pb207‐Pb206 age; (d) that the pattern of K‐A and Rb‐Sr ages of micas can be most simply explained by postulating the loss of variable but small amounts of argon from the mineral.