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Zircon crystal. Image is from a scanning electron microscope. Red shades show high uranium content; green shades show low uranium content.

Zircon occurs in small quantities in many crustal rocks and contains a small amount of uranium (U), which decays to lead (Pb), with a half-life (of 4,500 Ma) approximately equal to the age of the Earth (4,600 Ma). 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, i.e. better than one part in a thousand.
In LITHOPROBE, geochronology is important in providing the detailed age control for surface rocks and intrusives that is essential to unraveling their local geological history.

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