Rubidium Strontium Dating Method

In the rubidium-strontium method, rubidium-87 decays with a half-life of 48.8 billion years to strontium-87. Strontium has several other isotopes that are stable and do not decay. The ratio of strontium-87 to one of the other stable isotopes, say strontium-86, increases over time as more rubidium-87 turns to strontium-87. But when the rock first cools, all parts of the rock have the same strontium-87/strontium-86 ratio because the isotopes were mixed in the magma. At the same time, some of the minerals in the rock have a higher rubidium/strontium ratio than others. Rubidium has a larger atomic diameter than strontium, so rubidium does not fit into the crystal structure of some minerals as well as others.

Rubidium-Strontium
87Rb37 → 87Sr38 + e−,half life ≈ 4. 7 × 1010 yr,47billion years

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Source Link:
Radiometric Dating
Isotope explanation

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Other links to see:
Rb-St daing
Rubidium -Strontium Dating
Radiometric Dating
Clocks in the Rocks
Rubidium-strontium dating