Argon argon dating definition
The trick is to irradiate the sample with neutrons along with samples of known age.
But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals.Argon-36 in the sample accounts for any air contamination.Argon-argon dating, ideally assumes that: OK I think I see!To account for any 40Ar already present (not from 40K decay) you also measure 36Ar and assume a 36Ar/40Ar ratio. The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.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.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
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.
But then, different passages in the Wikipedia article contradict each other (first section: 39-K is converted into 39-Ar by neutron bombardment; but "age equation" section: 40-K is bombarded; I think it should be 39-Ar).
There's a more basic explanation here: books.google.com/…
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.