Potassium-argon dating principles techniques and applications to geochronology
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Potassium-Argon Dating Methods
Potassium-argon (argon-argon), structural fabrics
Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes. Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs. However, the 40 K isotope is radioactive and therefore will be reduced in quantity over time. But, for the purposes of the KAr dating system, the relative abundance of 40 K is so small and its half-life is so long that its ratios with the other Potassium isotopes are considered constant.
Potassium argon dating range
The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K via the n-p reaction 39 K n,p 39 Ar. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.
Geochronology is the science of finding the ages of rocks , fossils and sediments. It uses a number of methods. By measuring the amount of radioactive decay of a radioactive isotope with a known half-life , geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years.