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K–Ar dating
Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on the measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element in many materials, such as feldspars, micas, clay minerals, tephra, and evaporites. In these materials, the decay product 40
Ar can 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 sample's purity, the composition of the mother material, and several 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 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: 39
K (93.2581%), 40
K (0.0117%), 41
K (6.7302%). 39
K and 41
K are stable. The 40
K isotope is radioactive; it decays with a half-life of 1.248×109 years to 40
Ca and 40
Ar. Conversion to stable 40
Ca occurs via electron emission (beta decay) in 89.3% of decay events. Conversion to stable 40
Ar 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 40
K decays to 40
Ar; the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. However, it can escape into the surrounding region when the right conditions are met, such as changes in pressure or temperature. 40
Ar atoms can 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 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 are 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 in dating because calcium is so common in the crust, with 40
Ca being the most abundant isotope. Thus, the amount of calcium originally present is unknown and can vary enough to confound measurements of the small increases 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 equation:
Where:
The scale factor 0.109 corrects for the unmeasured fraction of 40
K which decayed into 40
Ca; the sum of the measured 40
K and the scaled amount of 40
Ar gives the amount of 40
K which was present at the beginning of the elapsed period. 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.
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K–Ar dating AI simulator
(@K–Ar dating_simulator)
K–Ar dating
Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology. It is based on the measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element in many materials, such as feldspars, micas, clay minerals, tephra, and evaporites. In these materials, the decay product 40
Ar can 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 sample's purity, the composition of the mother material, and several 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 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: 39
K (93.2581%), 40
K (0.0117%), 41
K (6.7302%). 39
K and 41
K are stable. The 40
K isotope is radioactive; it decays with a half-life of 1.248×109 years to 40
Ca and 40
Ar. Conversion to stable 40
Ca occurs via electron emission (beta decay) in 89.3% of decay events. Conversion to stable 40
Ar 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 40
K decays to 40
Ar; the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. However, it can escape into the surrounding region when the right conditions are met, such as changes in pressure or temperature. 40
Ar atoms can 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 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 are 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 in dating because calcium is so common in the crust, with 40
Ca being the most abundant isotope. Thus, the amount of calcium originally present is unknown and can vary enough to confound measurements of the small increases 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 equation:
Where:
The scale factor 0.109 corrects for the unmeasured fraction of 40
K which decayed into 40
Ca; the sum of the measured 40
K and the scaled amount of 40
Ar gives the amount of 40
K which was present at the beginning of the elapsed period. 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.