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If a rock has been a closed system since its time of formation, each particle that decays forms a single particle.
So the amount of particles measured long after the rock has formed is We are assuming that all of the rocks collected formed at the same time and thus are the same age.
is a stable isotope of rubidium Strontium-87 and Strontium-86 are two stable isotopes of strontium.
If a group of rocks are co-genetic, meaning they are formed at the same time and place and are derived from the same parent material, they will have the same ratio of when they are formed .
Making matters worse, it appears that there is no way of determining whether or not the rocks in question are ‘closed’ systems.