A method for radiometric dating in which a sample is irradiated to convert 39K to 39Ar. The sample is then heated in stages, at each stage yielding argon with a particular 40Ar: 39Ar ratio. The age of the sample is calculated by comparing that ratio with the ratio in a sample of known age. The method provides more detail than potassium–argon dating and it can be used with very small samples.
http://www.diggles.com/bturrin/40Ar.html A tutorial on the 40Ar/39Ar step-heating dating technique.