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How does rubidium-strontium dating work for igneous rocks?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

Rubidium-strontium dating is a geochronological method that estimates the age of igneous rocks by measuring the accumulation of the stable isotope strontium-87, which forms through the radioactive decay of rubidium-87.12

Because rubidium and strontium are distributed differently as liquid rock cools, crystallization creates a range of rubidium-to-strontium ratios within a single rock body, all sharing the same initial strontium-87/strontium-86 ratio. Scientists use the isochron method to date these rocks by plotting the present-day isotopic ratios of multiple samples on a graph. The resulting data points form a straight line, where the slope of the line represents the time elapsed since the rock crystallized.32

For this method to be valid, the samples must meet three criteria:
* They must have shared the same initial strontium ratio at the time of formation.3
* They must have remained a closed system, meaning no rubidium or strontium was added or removed over geologic time.3
* All samples must be the same age.3

Unweathered, rapidly crystallized rocks are most likely to yield accurate results, as weathering or reheating can disturb the isotopic system.3