The rubidiumstrontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.
Development of this process was aided by Fritz Strassmann, who later moved onto discovering nuclear fission with Otto Hahn and Lise Meitner.
The utility of the rubidiumstrontium isotope system results from the fact that ^{87}Rb (one of the isotopes of rubidium) decays to ^{87}Sr with a halflife of 49 billion years. Different minerals in a given geologic setting can have a distinctly different ratio of strontium87 to strontium86 (^{87}Sr/^{86}Sr) as a consequence of different ages, original Rb/Sr values and the initial ^{87}Sr/^{86}Sr.
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 become 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.
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Example
For example, consider the case of an igneous rock such as a granite that contains several major Srbearing minerals including plagioclase feldspar, Kfeldspar, 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.
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