K-Ar

Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites. In these materials, the decay product ⁴⁰Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes). The amount of Argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure and/or open-air. Time since recrystallization is calculated by measuring the ratio of the amount of ⁴⁰Ar accumulated to the amount of ⁴⁰K remaining. The long half-life of ⁴⁰K allows the method to be used to calculate the absolute age of samples older than a few thousand years.

The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.

Potassium naturally occurs in 3 isotopes: ³⁹K (93.2581%), ⁴⁰K (0.0117%), ⁴¹K (6.7302%). Two are stable, while the radioactive isotope ⁴⁰K decays with a half-life of 7016393838848000000♠1.248×10⁹ years to ⁴⁰Ca and ⁴⁰Ar. Conversion to stable ⁴⁰Ca occurs via electron emission (beta decay) in 89.1% of decay events. Conversion to stable ⁴⁰Ar occurs via electron capture in the remaining 10.9% of decay events.

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