The relationship between the two is: T = 0.693 / λ Many different radioactive isotopes and techniques are used for dating.All rely on the fact that certain elements (particularly uranium and potassium) contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age.The Re-Os isotopic system was first developed in the early 1960s, but recently has been improved for accurate age determinations.

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.

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.

However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes.

This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe.

For an element to be useful for geochronology (measuring geological time), the isotope must be reasonably abundant and produce daughter isotopes at a good rate.

Either a whole rock or a single mineral grain can be dated.

Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme).

Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.

The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time.