The Re-Os isotopic system was first developed in the early 1960s, but recently has been improved for accurate age determinations.
The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes.
This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events. The decay of 147Sm to 143Nd for dating rocks began in the mid-1970s and was widespread by the early 1980s.
Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces.
These are released as radioactive particles (there are many types).
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.
Another way of expressing this is the half-life period (given the symbol T).
The half-life is the time it takes for half of the parent atoms to decay.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.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 has become more widely used since the late 1950s.