A comparison of the ratio, radiogenic ${\mathrm{Sr}}^{87}$/${\mathrm{Rb}}^{87}$, found in eight Rb minerals of differing Rb content from the same rock unit has shown this ratio to be constant for a given mineral assemblage. This ratio has been measured on Rb minerals from six rock units for which concordant U-Pb ages ranging from 375 to 2700 million years have also been obtained. From the ratio radiogenic ${\mathrm{Sr}}^{87}$/${\mathrm{Rb}}^{87}$ and the age of the mineral obtained from the U-Pg age, the half-life of R${\mathrm{B}}^{87}$ is calculated to be (5.0±0.2)×${10}^{10}$ yr. This lies in the range of values found by direct counting experiments.

• Received 20 March 1956

DOI:https://doi.org/10.1103/PhysRev.103.1045