What is potassium argon dating used for
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 Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method.
Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The rock sample to be dated must be chosen very carefully.
With each increment in quality, more subtle sources of error have been found and taken into account.
Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in geochronology and archaeology.