The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time.For geological purposes, this is taken as one year.However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks.Argon-Argon dating (39Ar-40Ar) This technique developed in the late 1960s but came into vogue in the early 1980s, through step-wise release of the isotopes.Another way of expressing this is the half-life period (given the symbol T).
However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. The dual decay of potassium (K) to 40Ar (argon) and 40Ca (calcium) was worked out between 19.
This technique is good for iron meteorites and the mineral molybdenite.
This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques.
Radiocarbon dating is normally suitable for organic materials less than 50 000 years old because beyond that time the amount of 14C becomes too small to be accurately measured.
This scheme was developed in 1937 but became more useful when mass spectrometers were improved in the late 1950s and early 1960s.
The relationship between the two is: T = 0.693 / λ Many different radioactive isotopes and techniques are used for dating.