Rubidium strontium dating equation
Extrapolating back 4.6 billion years, the ratio just after the earth formed would have been about 0.699.Rocks derived from recycling of crustal material are derived from sources that have had time to accumulate radiogenic Sr-87, and can have initial ratios of 0.710 or greater.Fortunately, there is another isotope of strontium around, Strontium 86. Furthermore, since the two isotopes differ by only about 1% in weight, they do not undergo appreciable fractionation from physical processes.
Thus, if a sample had 100 Sr-86 atoms total, it could be assumed that 70 of the Sr-87 atoms were primordial.
A few problems with the Rb-Sr system are that both elements are mobile during weathering and metamorphism, so their concentrations can be affected by these processes.
In addition, Rb is a volatile element so the ratio of Sr-87/Sr-86 is not chondritic since some Rb was lost during the formation of the earth.
The above equation tells us the absolute amount of Strontium-87 in the rock sample.
However, because this is a small number and counting atoms isn’t easy, it is more useful to use the ratio of one isotope to another.