The preferential incorporation of Rb (following K) or Sr (following Ca) into different mineral structures, as well as their differing behaviors during closed system evolution of magmas by fractional crystallization, results in a wide range in Rb/Sr in mineral and whole rock compositions. The radioactive decay of⁸⁷Rb to⁸⁷Sr can then be exploited to generate an isochron and yield age information about a magmatic system, providing all the analyzed materials crystallized from a homogeneous parental melt, and the system has not been subsequently thermally or chemically disturbed. Some caution is required when applying the Rb–Sr chronometer to igneous systems; evaluation of the mobility of elements (especially Rb) and whether the strontium isotopic composition has been compromised by fluids (e.g., hydrothermal alteration and weathering) is required and the consequences of the relatively low closure temperature for Sr diffusion in many minerals, especially biotite, must be considered.