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Because rubidium is concentrated in crustal rocks, the continents have a much higher abundance of the daughter isotope strontium-87 compared with the stable isotopes.This relative abundance is expressed as the Sr ratio, where strontium-86 is chosen to represent the stable isotopes strontium-88, strontium-86, and strontium-84, which occur in constant proportions in natural materials.This situation is easily visualized on an isochron diagram, where a series of rocks plots on a steep line showing the primary age, but the minerals in each rock plot on a series of parallel lines that indicate the time since the heating event.If cooling is very slow, the minerals with the lowest blocking temperature, such as biotite mica, will fall below the upper end of the line.A more dramatic presentation of this phenomenon is found when the changes in the Sr ratio over time, whereas the value in such rubidium-rich, strontium-poor minerals as biotite increases rapidly with time.

Potassium-bearing minerals including several varieties of mica, are ideal for rubidium–strontium dating as they have abundant parent rubidium and a low abundance of initial strontium.As a liquid rock cools, first one mineral and then another achieves saturation and precipitates, each extracting specific elements in the process.Strontium is extracted in many minerals that are formed early, whereas rubidium is gradually concentrated in the final liquid phase.At the time of heating, identical Sr ratios are again achieved as described above, only to be followed by a second episode of isotopic divergence.Approaches to this ideal case are commonly observed, but peculiar results are found in situations where the heating is minimal.

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