Rb is an alkali metal which, along with K, is concentrated in micas and clays in geological materials. Sr is an alkali earth that is excluded from the structure of high-K minerals, such as micas, but (along with Ca with which it shares a similar ionic radius) is concentrated in feldspars and carbonates.
Therefore, these minerals control the Rb-Sr radiometric dating system. Both Rb and Sr are mobile during low-grade (greenschist facies) metamorphism and hydrothermal alteration, so for primary ages, samples must be fresh and preserve the original mineralogy.
During melt extraction, Rb can’t readily be accommodated into common refractory or early-crystallising minerals as well as Sr, which can mimic Ca and fit into sites in residual and early-crystallising minerals such as feldspars, amphiboles and pyroxenes, so the melt is typically preferentially enriched in Rb.
For high-Rb/Sr minerals (e.g. micas), a typical 87Sr/86Sr initial value (~ 0.71) can be assumed to determine a model date on the basis of a single mica Rb-Sr analysis. Alternatively, the value measured in a co-genetic low Rb/Sr mineral may be used to approximate the host melt/rock initial Sr isotopic composition to provide a 2-point isochron.
Whole-rock and mineral isochrons: preconditions
- samples must have had the same initial ratio (i.e. co-genetic)
- samples must be the same age
- isotopic (parent-daughter) system must have remained closed since t = 0
If all uncertainties have been correctly estimated and the preconditions above apply, then all scatter about the isochron can be accounted for as analytical uncertainty. This enables the slope of the line to be determined. The date can be calculated from the isochron slope and its uncertainty.