Argon-Argon Dating

Argon-argon (40Ar/39Ar) dating is the modern, high-precision successor to conventional potassium-argon dating, in which the parent potassium is measured indirectly by converting it to a measurable argon isotope inside a nuclear reactor. A potassium-bearing sample is irradiated so that potassium-39 transmutes into argon-39, which then stands as a proxy for the parent potassium; both the radiogenic argon-40 daughter and this argon-39 proxy can be measured on the same aliquot by a single mass spectrometer, eliminating the sample-heterogeneity problem of the older method. Crucially, the gas can be released either by fusing a single crystal with a laser or by heating the sample in incremental steps, the latter producing an age spectrum that reveals argon loss, excess argon, and disturbance. Synthesized in McDougall and Harrison's monograph, the technique delivers the precise, internally checkable ages now standard for dating volcanic deposits at hominin and Palaeolithic sites.