The methods of radiometric dating are based on the radioactive decay of certain isotopes that have very long half-lives. The radioactive isotope is often referred to as the “parent” and the elements resulting from the decay of parent are called the “daughter products”. The age of rocks and minerals that contain radioactive isotopes is determined by measuring the accumulation of the daughter products in them. This procedure is called “Radiometric Dating”. Radiometric dating is very reliable because the radioactive decay proceeds at a constant rate and it remains unaffected by any physical or chemical agents.
A commonly used term in radiometric dating is “Half Life”. it is a common way of expressing the rate of radioactive decay. Half-life may be defined as the time required for a given amount of radioactive substance to decay to one-half of its initial value. For example, the half life period of Uranium-238 is 4500 million years. This means that in 4500 million years one gram of Uranium-238 would be reduced to half gram, the rest having been transformed into the lead-206.
Of the many radioactive isotopes that exist in nature, only three have proved useful in providing radiometric ages for ancient rocks. These three isotopes are given in the following table. The methods which are commonly used for dating rocks include Potassium-Argon, Rubidium-Strontium and Uranium-Lead. The basic equation for calculating the age of a rock sample is as follows.
Age of a rock = 3.323 T log10 [1+Nd/Np]
Where Nd is the number of atoms of the daughter product. Np is the number of atoms of the parent radioactive substance present today and T is the half life of the radioactive substance.
|Radioactive Parent||Half-life in million years||Daughter Product||Minerals/Rocks commonly dated|
|Uranium-238||4500 m.y||Lead-206||Zircon, Sphene|
|Potassium-40||11900 m.y||Argon-40||Muscovite, Biotite, Hornblende, Glauconite, Volcanic rock.|
|Rubidium-87||47000 m.y||Strontium-87||Muscovite, Giotite, Microcline, Metamorphic rocks.|
In practice mass spectrometers are used to measure precisely and accurately the quantities of parent and daughter radioactive substance in rocks. in radioactive dating, it is assumed that none of the daughter products has been leached out from the rocks and that none of the daughter product has been leeched out from the rock and that none has been introduced from out side.
Radiometric dating methods give a figure of some 3500 million years for the oldest rocks of the earth’s crust. The radioactive analysis of meteorites has yielded dates of up to 46 million years which probably correspond to the age of the earth.