K ar dating

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 gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.The abundance of Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature.

Dating of movement on fault systems is also possible with the Ar method.

Ar after cooling past the closing temperature and that this was properly sampled during analysis.

This technique allows the errors involved in K-Ar dating to be checked.

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.

That is, a fresh mineral grain has its K-Ar "clock" set at zero.