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.