Argon 40 dating
In about 89.28% of events it decays to calcium-40 ( Potassium-40 is especially important in potassium–argon (K–Ar) dating.Argon is a gas that does not ordinarily combine with other elements.Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! This dating method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.A precise amount of argon-38 is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.Then the gas sample is cleaned of all unwanted gasses such as H A variant of the K-Ar method gives better data by making the overall measurement process simpler.The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample.