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.
However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral.
The dating method is usually performed on the mineral zircon.
The mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead.
Although the time at which any individual atom will decay cannot be forecast, the time in which any given percentage of a sample will decay can be calculated to varying degrees of accuracy.
The time that it takes for half of a sample to decay is known as the half life of the isotope.
Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C.
Ar) dating is a radiometric dating method invented to supersede potassium-argon (K/Ar) dating in accuracy.
The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.
Therefore, one can assume that the entire lead content of the zircon is radiogenic, i.e.
it is produced solely by a process of radioactive decay after the formation of the mineral.