Age dating of basalt
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.
Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.
Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.
Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.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.These dating methods have been under constant improvement for more than 50 years.The learning curve has been long and is far from over today.
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.