k/ar-40 dating

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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

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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.

How Accurate is K-Ar Dating? Messel, "A Modern Introduction to Physics" vol. Sydney, p: The radiogenic argon that builds up in potassium-rich minerals after they have crystallized, therefore, furnishes a good measure of the age of the sample. Del Mar California , p: The rubidium-strontium and uranium-lead techniques are very difficult to use with such samples, because the slow decay rates of the parent isotopes have not allowed a significant increase in the daughter isotopes. Commonly the ages of minerals from rather old rocks dated by the potassium-argon method are lower than the ages obtained by either the rubidium-strontium and uranium-lead dating. Moreover, many studies have demonstrated that argon escapes readily during metamorphic events when rocks become heated and partially crystallized. Dr Steven A.

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