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Potassium argon dating is only done with

The potassium-argon K-Ar isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale. Potassium occurs in two stable isotopes 41 K and 39 K and one radioactive isotope 40 K. Potassium decays with a half-life of million years, meaning that half of the 40 K atoms are gone after that span of time.
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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 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 of 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.
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However, it is well established that volcanic rocks e. If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable. Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. Patterson et al. Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: "If the rock is heated or melted at some later time, then some or all the 40 Ar may escape and the K-Ar clock is partially or totally reset.
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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. Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released.
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