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Minerals used in high-precision geochronological dating. Biotites (black mica) to left, zircons (clear) in centre, and hornblende (greenish) to right.

Geochronology is a special branch of geochemistry, related to isotope physics, that involves determining the time of formation of rocks, minerals and fossils. Geochronologists examine materials ranging in age from a few years to billions of years old, mostly utilizing the principle that radioactive isotopes present at the "birth", or formation, of a mineral will decay at a certain fixed rate. Measurement of relative abundances of the "parent" and "daughter" isotopes can determine the age of the rocks.
One example of the use of radiogenic isotopes is the uranium-lead (U-Pb) method of dating zircons. Zircon occurs in small quantities in many crustal rocks and contains a small amount of U which decays to Pb with a half-life (4,500 Ma) approximately equal to the age of the Earth. By measuring the abundances and isotopic ratios of U and Pb in zircon, the age of the rock can be determined with a precision on the order of 2 to 5 Ma.

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