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.
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.
This technique is used on ferromagnesian (iron/magnesium-containing) minerals such as micas and amphiboles or on limestones which also contain abundant strontium.
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.
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.
Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate.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, 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.All living organisms take up carbon from their environment including a small proportion of the radioactive isotope 14C (formed from nitrogen-14 as a result of cosmic ray bombardment).The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate.The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers.