Editing Rubidium–strontium dating (section)

==Example==

For example, consider the case of an [[igneous rock]] such as a [[granite]] that contains several major Sr-bearing minerals including plagioclase [[feldspar]], [[orthoclase|K-feldspar]], [[hornblende]], [[biotite]], and [[muscovite]]. Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt.

The ideal scenario according to [[Bowen's reaction series]] would see a granite melt begin crystallizing a [[cumulate rocks|cumulate]] assemblage of plagioclase and hornblende (i.e.; [[tonalite]] or [[diorite]]), which is low in K (and hence Rb) but high in Sr (as this substitutes for Ca), which proportionally enriches the melt in K and Rb. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb–Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.