Argon-argon dating

Argon-argon dating is a radiometric dating technique similar to that of Potasium-Argon. In fact, Argon-Argon is a method to confirm the results of the K-Ar results by verifying how much Atomspheric argon was initially in the rock when it cooled, or if the rock has been reheated and "reset".

The two isotopes being examined are Ar40 and Ar39. While Ar40 is a stable Isotope, Ar39 has a half-life of only 269 days. So in natural rock thousands to million of years old, Ar39 shouldn't even exist. Instead, Ar39 is manufactured in the rock by subjecting the sample to nuclear radiation of a known quantity, and converting some normal K39 into Ar39.

By then observing the release of both argon types from the rock as it's heated to different temperatures, a ratio of Ar40-Ar39 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 Ar40 from the rock, that too will show up. In other words, the errors created from dating a rock using Potassium-Argon dating can be checked using this radiometric technique.