Editing Rubidium–strontium dating (section)

==Calculating the age==

The age of a sample is determined by analysing several minerals within multiple subsamples from different parts of the original sample. The <sup>87</sup>Sr/<sup>86</sup>Sr ratio for each subsample is plotted against its <sup>87</sup>Rb/<sup>86</sup>Sr ratio on a graph called an [[:File:Isochron.jpg|isochron]]. If these form a straight line then the subsamples are consistent, and the age probably reliable. The slope of the line dictates the age of the sample.

Given the [[Radioactive decay|universal law of radioactive decay]] and the following rubidium [[beta decay]]: <chem>^{87}_{37}Rb ->[{\beta^-}]~^{87}_{38}Sr ~+e^-\ + \bar{\nu}_e</chem>, we obtain the expression which describes the growth of strontium-87 from the decay of rubidium-87:<math display="block">^{87}_{38}\text{Sr}(t)=~^{87}_{38}\text{Sr}(0) +~^{87}_{37}\text{Rb}(e^{\lambda t}-1)\ ,</math><chem>\lambda</chem> being the [[Exponential decay|decay constant]] of rubidium. Furthermore, we consider the number of <chem>^{86}_{38}Sr</chem> as a constant, since it is stable and not radiogenic. Hence,  <math display="block">\frac{^{87}\text{Sr}}{^{86}\text{Sr}} = \left( \frac{^{87}\text{Sr}}{^{86}\text{Sr}} \right)_0 + \frac{^{87}\text{Rb}}{^{86}\text{Sr}}(e^{\lambda t}-1)</math>is the isochron equation. After measurements of Rubidum and Strontium concentration in the mineral we can easily determine the age, the t value, of the sample.<ref>{{cite book |doi=10.1007/978-94-009-2611-0_4 |chapter=Rubidium-Strontium Dating |title=Isotopes in the Earth Sciences |date=1994 |last1=Bowen |first1=Robert |pages=162–200 |isbn=978-94-010-7678-4 }}</ref>