Potassium—argon datingabbreviated K—Ar datingis 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 micasclay mineralstephraand evaporites. In these materials, the decay product 40 Ar 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 the sample, the composition of the mother material, and a number of other factors. Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining.
The long half-life of 40 K 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: Conversion to stable 40 Ca occurs via electron emission beta decay in Conversion to stable 40 Ar occurs via electron capture in the remaining Argon, being a noble gasis a minor component of most rock samples of geochronological interest: When 40 K decays to 40 Ar argonthe atom typically remains trapped within the lattice because grand forks bc dating is larger than the spaces between the other atoms in a mineral crystal.
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 40 K will decay and 40 Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40 Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified.
Despite 40 Ca being the favored daughter nuclide, it is rarely useful dating as calcium is common in the crust, with 40 Ca being the most abundant isotope. Thus, the amount of calcium originally present is not known with enough accuracy to be able to measure the small increase produced by radioactive decay. The ratio of the amount of 40 Ar to that of 40 K is directly related to the time elapsed since the rock was cool enough to trap the Ar by the following equation:.
The scale factor 0. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required. To obtain the content ratio of isotopes 40 Ar to 40 K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is melted in vacuum. The potassium is quantified by flame photometry or atomic absorption spectroscopy. The amount of 40 K is rarely measured directly.
The amount of 36 Ar is also measured to assess how much of the total argon is atmospheric in origin. Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample. Ar—Ar dating is a similar technique which compares isotopic how reliable is potassium argon dating from the same portion of the sample to avoid this problem.
Due to the long half-lifethe technique is most applicable for dating minerals and rocks more thanyears old. For shorter timescales, it is unlikely that enough argon will have had time to accumulate in order to be accurately measurable. K—Ar dating was instrumental in the development of the geomagnetic polarity time scale. One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits.
In the K—Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has how reliable is potassium argon dating dated from its mineral ingredients while situated on another planet. From Wikipedia, the free encyclopedia. National Nuclear Data Center. Retrieved 20 September Retrieved from " https: Use dmy dates from May Navigation menu Personal tools Not logged in Talk Contributions Create account Log in.
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