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.Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.
Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.
K-Ar analyses cost several hundred dollars per sample and take a week or two.
The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.
With each increment in quality, more subtle sources of error have been found and taken into account.
Morris cited other examples of anomalous dates produced by excess argon and falsely claimed that it is a universal problem for K-Ar dating.
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.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.
These dating methods have been under constant improvement for more than 50 years.