The 40Ar/39Ar method of dating rocks has its foundations in the potassium–argon (K–Ar) isotopic dating method, a widely used technique for measuring numerical ages on minerals and rocks. The method is based upon the occurrence in nature of the radioactive isotope of potassium, 40K, which has a half life of 1250 million years (Ma). This isotope of potassium has a dual decay to 40Ca and to 40Ar, and the branch yielding radiogenic argon (40Ar∗) as daughter product through its accumulation over geologic time, provides the basis for the K–Ar dating technique. In the simplest case of a volcanic rock, for example, the K–Ar method normally yields an age that is equal to the time that has elapsed since its eruption and cooling. The K–Ar dating method gives reliable ages on rapidly cooled igneous rocks, but ages on potassium-bearing minerals from basement rocks are often discordant. This phenomenon is now understood in terms of differences in diffusion behavior for 40Ar∗in different minerals...
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Harrison, T.M. (1998). 40Ar/39Ar dating method . In: Geochemistry. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4496-8_14
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DOI: https://doi.org/10.1007/1-4020-4496-8_14
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