Argon

Argon isotopes are used to date rock samples, especially volcanic rocks, using two related techniques (Fig. IUPAC.18.1) [101], [168], [169], [170]. –The first technique is potassium-argon dating (K-Ar), which is based on the decay of radioactive ⁴⁰K to stable ⁴⁰Ar. By comparing the concentrations of potassium and ⁴⁰Ar in a sample, it is possible to determine how long the sample has been accumulating radiogenic ⁴⁰Ar to determine the “age” of the sample. The half-life of ⁴⁰K is approximately 1.25×10⁹ years, making this a useful tool for dating rocks range in age from about 10⁶ to 10⁹ years. –A modification of the potassium-argon dating technique is the n(⁴⁰Ar)/n(³⁹Ar) isotope-amount-ratio technique, in which a sample is irradiated in a nuclear reactor to produce ³⁹Ar from ³⁹K. The isotope-amount ratio n(⁴⁰Ar)/n(³⁹Ar) is then determined, and from this, the approximate age of the rock can be calculated (Fig. IUPAC.18.2). The study of ³⁷Ar (half-life of 35 days), ³⁹Ar (half-life of 268 years), and ⁴⁰Ar concentrations in groundwater can provide information about the production and release of these isotopes from rocks and other sources into groundwater and the relative ages of different groundwaters [159], [164], [165], [171], [172], [173].

³⁸K (half-life of 7.6 min), which is produced by the reactions ³⁸Ar (p, n) ³⁸K and ⁴⁰Ar (n, 3n) ³⁸K, is a widely used blood-flow tracer. Because ³⁸Ar is more expensive, ⁴⁰Ar, which also offers many additional advantages as a target, is more commonly used to produce ³⁸K for medical purposes [176], [177]. ⁴¹Ar (half-life of 1.82 h) is used as an industrial gas-flow tracer to help track the movement of gases because its inert properties, half-life, and gamma radiation make it well suited for this purpose [177].