Regina Ramsey, 26 years old
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. Its decay yields argon and calcium in a ratio of 11 to K-ar and ar-ar dating K-Ar method works by counting these radiogenic 40 Ar atoms trapped inside minerals. What simplifies things is that potassium is a reactive metal and argon is an inert gas:
Reviews in Mineralogy and Geochemistry ; 47 1: The aim of this chapter is to present the K-Ar and Ar-Ar dating techniques in the context of noble gas studies, since there k-ar and ar-ar dating already several recent texts on K-Ar and Ar-Ar dating Dickin ; McDougall and Harrison K-ar and ar-ar dating K-Ar dating technique was one of the earliest isotope dating techniques, developed soon after the discovery of radioactive potassium, and provided an Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. Sign In or Create an Account.
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 k-ar and ar-ar dating when the rock solidifies recrystallizes.
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Some of the problems of K-Ar dating can be avoided by the use of the related Ar-Ar dating method. In this article we shall explain how this method works and why it is superior to the K-Ar method. The reader should be thoroughly familiar with the K-Ar method, as explained in the previous articlebefore reading any further. In the previous article I introduced you to 40 K, an unstable isotope of potassium which produces the daughter isotope 40 Ar by electron capture or beta plus decay. The Ar-Ar dating method k-ar and ar-ar dating crucially on the existence of two other isotopes.
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 hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K. 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. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time. The J factor relates to the fluence of the neutron bombardment during the irradiation process; a denser flow of neutron particles will convert more atoms of 40 K to 40 Ar than a k-ar and ar-ar dating dense one.