Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium-87 compared with the stable isotopes.
This relative abundance is expressed as the Sr ratio, where strontium-86 is chosen to represent the stable isotopes strontium-88, strontium-86, and strontium-84, which occur in constant proportions in natural materials.
Thus, if a sample had 100 Sr-86 atoms total, it could be assumed that 70 of the Sr-87 atoms were primordial.
If the sample had 100 atoms of each isotope, we could assume that 70 of the Sr-87 atoms were primordial and 30 were produced by the decay of Rb-87.
Thus, a precise measurement of the Sr ratio in a modern volcano can be used to determine age if recycled older crust is present.
A ratio for average continental crust of about 0.72 has been determined by measuring strontium from clamshells from the major river systems.
Unfortunately, there is a lot of primordial Strontium 87 around, making it very hard to tell how much was present when the rock formed and how much formed later as a result of radioactive decay.
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.
Rubidium 87 decays to Strontium 87 with a half life of xx my.
Rubidium and strontium are reasonably abundant and the decay is not branched, unlike potassium-argon.
Sr) was the first widely used dating system that utilized the isochron method.
Rubidium is a relatively abundant trace element in Earth’s crust and can be found in many common rock-forming minerals in which it substitutes for the major element .
These whole-rock ages were approximately correct, but a much more precise mathod is now in use.