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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). 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. ==Decay series== Potassium naturally occurs in 3 isotopes – 39K (93.2581%), 40K (0.0117%), 41K (6.7302%). The radioactive isotope 40K decays with a half-life of 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.〔 〕 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 40K decays to 40Ar (argon), the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. But it can escape into the surrounding region when the right conditions are met, such as change in pressure and/or temperature. 40Ar atoms are able to diffuse through and escape from molten magma because most crystals have melted and the atoms are no longer trapped. Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again. After the recrystallization of magma, more 40K will decay and 40Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified. Calcium is common in the crust, with 40Ca being the most abundant isotope. Despite 40Ca being the favored daughter nuclide, its usefulness in dating is limited since a great many decay events are required for a small change in relative abundance, and also the amount of calcium originally present may not be known. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「K–Ar dating」の詳細全文を読む スポンサード リンク
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