Argon-argon dating

From Freepedia

Argon-argon dating is a radiometric dating technique similar to potassium-argon (K-Ar) dating. Argon-argon dating is most commonly used to confirm the results of K-Ar dating, by verifying how much atomspheric argon was initially in the rock when it cooled, or if the rock has been reheated and "reset".

Argon-40 and argon-39 are the two isotopes examined in this technique. While Ar-40 is a stable isotope, Ar-39 has a half-life of 269 years. Therefore, in natural rock which is thousands to millions of years old, Ar-39 should exist only in minute quantities. Ar-39 is produced in the rock by subjecting the sample to nuclear radiation of a known quantity, thus converting some potassium-39 into Ar-39.

By then observing the release of both argon types from the rock as it is heated to different temperatures, the ratio of Ar-40 to Ar-39 is measured and used to date the rock. If there was excess Ar-40 in the rock when it first hardened, it shows up in this analysis. If for some reason the rock was reheated, releasing some of the daughter Ar-40 from the rock, that too will show up. This technique allows the errors involved in K-Ar dating to be checked.