rubidium-strontium dating

a technique for determining the age of minerals based on the occurrence in natural rubidium of a fixed amount of the radioisotope ⁸⁷ Rb which decays to the stable strontium isotope ⁸⁷ Sr with a half-life of 4.7 × 10 ¹¹ years. Measurement of the ratio of these isotopes thus gives the age of a mineral, for ages of up to about 4 × 10 ⁹ years