Advantages and disadvantages of radiometric dating
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.
A variant of the K-Ar method gives better data by making the overall measurement process simpler.
The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).
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 gases such as H, nitrogen and so on until all that remains are the inert gases, argon among them.The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.