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Concept
K–Ar dating
Summary
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 feldspars, micas, clay minerals, tephra, and evaporites. In these materials, the decay product 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 the final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure or open air. Time since recrystallization is calculated by measuring the ratio of the amount of accumulated to the amount of remaining. The long half-life of 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: (93.2581%), (0.0117%), (6.7302%). and are stable. The isotope is radioactive; it decays with a half-life of 1.248e9years to and . Conversion to stable occurs via electron emission (beta decay) in 89.3% of decay events. Conversion to stable occurs via electron capture in the remaining 10.7% of decay events.
Argon, being a noble gas, is a minor component of most rock samples of geochronological interest: It does not bind with other atoms in a crystal lattice. When decays to ; the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal.
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