The utility of the rubidiumstrontium isotope system results from the fact that different minerals in a given geologic setting can have distinctly different ^{87}Sr/^{86}Sr as a consequence of different ages, original Rb/Sr values and the initial ^{87}Sr/^{86}Sr. For example, consider the case of a simple igneous rock such as a granite that contains several major Srbearing minerals including plagioclase feldspar, Kfeldspar, hornblende, biotite, and muscovite. If these minerals crystallized[?] from the same silicic melt, each mineral had the same initial ^{87}Sr/^{86}Sr as the parent melt. However, because Rb substitutes for K in minerals and these minerals have different K/Ca ratios, the minerals will have had different Rb/Sr ratios.
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. Highest ratios (10 or higher) occur in pegmatites. Typically, Rb/Sr increases in the order plagioclase, hornblende, Kfeldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic ^{87}Sr, measured ^{87}Sr/^{86}Sr values will be different in the minerals, increasing in the same order. The RbSr dating method has been used extensively in dating rocks. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the ^{87}Sr/^{86}Sr ratio. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered.
The important concept for isotopic tracing is that Sr derived from any mineral through weathering reactions will have the same ^{87}Sr/^{86}Sr as the mineral.
Adapted from a public domain USGS webpage (http://wwwrcamnl.wr.usgs.gov/isoig/period/sr_iig). Modify as necessary.
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