Variations in Isotopic Abundances of Strontium

Isotopic abundances of mineral sources of Sr have been measured. Variations in the ratio ${\mathrm{Sr}}^{86}$/${\mathrm{Sr}}^{88}$ of unknown origin have been found from 0.1160 to 0.1220. In Rb free minerals, the variation of the ratio ${\mathrm{Sr}}^{87}$/${\mathrm{Sr}}^{88}$ (0.0832 to 0.0868) is small enough to be completely attributable to the decay of ${\mathrm{Rb}}^{87}$ in geologic time. The large ${\mathrm{Sr}}^{87}$/${\mathrm{Sr}}^{88}$ variation in Rb minerals (biotite) from 0.0840 to 0.2629 indicates their suitability for use in the determination of mineral age.