Define radioactive dating

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.

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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.

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.

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.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.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 amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.

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