Introduction
Argon (Ar) has three natural isotopes with masses 36, 38, and 40. 36Ar and 38Ar are stable, that is, they are not radioactive, and they are also not produced by radioactivity processes. In detail, these isotopes can be produced in tiny amounts by rare reactions such as the β−radioactive decay of 36Cl yielding36Ar. Besides such minor productions, the isotopes 36Ar and 38Ar have been essentially produced in stars before solar system formation. They are therefore mostly primordial in origin and they bear a message about the processes that occurred at and before planetary formation.
Argon-40 is not radioactive but is produced by the decay of 40K, a rare isotope of potassium. Models of nucleosynthesis indicate that 40Ar is produced in stars in tiny amount compared to 36Ar and 38Ar (e.g., 40Ar/36Ar ~10−4). Thus, practically all 40Ar present in Earth is radiogenic. The terrestrial atmosphere presents a concentration of argon of 0.934 mol.% (dry air), mainly made of 40Ar (40Ar/36Ar...
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Sarda, P. (2018). Argon Isotopes. In: White, W.M. (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-319-39312-4_206
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