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, nitrogen and so on until all that remains are the inert gasses, 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.Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.Three argon isotopes are measured: Ar is determined by comparison to it.Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.
The key is to put the mineral sample in a neutron beam, which converts potassium-39 into argon-39.
These effects must be corrected, and the process is intricate enough to require computers.
Ar-Ar analyses cost around 00 per sample and take several weeks.
Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clean indicator of the potassium content.
The advantage is that all the information needed for dating the sample comes from the same argon measurement. This method is commonly called "argon-argon dating."The physical procedure for K.