Geochronology

Article By:

Davis, Donald W. Department of Geology, University of Toronto, Toronto, Canada.

Last reviewed:2006

DOI:https://doi.org/10.1036/1097-8542.YB060180

Geochronology involves measuring absolute ages for ancient rock-forming events and applying these ages to understand the development of the Earth. The most precise methods use radioactive isotopes (unstable atoms of an element) that decay into a radiogenic daughter isotope of another element. Either the parent isotope is constantly replenished (for example, from cosmic rays, like ¹⁴C), or it decays slowly enough that some still remains since the time of the Sun's formation. Such a radioactive decay system is known as a geochronometer. A list of long-lived geochronometers along with their half-lives is given in the table. An age is determined by measuring the ratio of radiogenic daughter over radioactive parent isotope in a mineral. This usually records the time since the atoms in the mineral became isolated as a result of initial crystallization or cooling. Ideally, the host mineral should crystallize with a very high ratio of parent element over the element to which the parent decays, so most of the latter element in the mineral will now be radiogenic. The mineral should also be inert, so the atoms of the decay system remain isolated from outside influences. Many parent and daughter elements are chemically reactive, so parent and daughter would be easily separated in rocks if they were not contained within a chemically unreactive (inert) mineral that protects them from outside influences. These conditions are most easily satisfied for the potassium-argon (K-Ar) and uranium-lead (U-Pb) systems, which are widely used for determining precise ages. The other systems are still very useful as isotopic tracers that provide clues to the origin and history of the rock reservoirs from which their magmas were derived.

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