Dating techniques potassium-argon

TABLE 14 Minerals Potassium-Argon Technique... 

Guillot, P. Y. and Y. Comette (1986), The Cassignoll technique for potassium-argon dating, precision and accuracy Examples from the late Pleistocene to recent vol-canics from southern Italy, Client. Geol. (Isotopes Geoscience Section) 59, 205-222. 

Potassium-40 dating techniques can be tricky because of the properties of the decay product argon. What about argon makes it hard to measure ... 

The essential difference between K-Ar and Ar-Ar dating techniques lies in the measurement of potassium. In K-Ar dating, potassium is measured generally using flame photometry, atomic absorption spectroscopy, or isotope dilution and Ar isotope measurements are made on a separate aliquot of the mineral or rock sample. In Ar-Ar dating, as the name suggests, potassium is measured by the transmutation of to Ar by neutron bombardment and the age calculated on the basis of the ratio of argon isotopes. 

The gases helium and argon are formed by radioactive decay in various minerals, for example argon by radioactivity decay of This mechanism can be applied to age-determination of minerals, a technique known as potassium-argon dating. Radon is generally obtained as Rn from the decay of radium. Its half-life is only about 3.8 days. 

Dalrymple, G. V., and M. A. Lanphere. 1969. Potassium-Argon Dating Principles, Techniques and Applications to Geochronology. W. H. Freeman, San Francisco, xiv + 258 pp. 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

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