The important concept in isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same Sr ratio (and often the Nd-Nd ratios as well) the geological fingerprint of an object or skeleton can be measured, allowing migration patterns to be determined.
and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, and can also be used to date a wide range of natural and man-made materials.
This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
The age of a sample is determined by analysing several minerals within the sample.
During fractional crystallization, Sr tends to be come concentrated in plagioclase, leaving Rb in the liquid phase.
Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite.
By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.