DISCLAIMER: This article is being kept online for historical purposes. Though accurate at last review, it is no longer being updated. The page may contain broken links or outdated information.
Davis, Donald W. Department of Geology, University of Toronto, Toronto, Canada.
- Potassium-argon (K-Ar) system
- Uranium-lead (U-Pb) system
- Recent innovations and achievements
- Future developments in instrumentation
- Related Primary Literature
- Additional Reading
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.
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information