Uranium-uranium dating


Uranium-uranium dating

Uranium-uranium dating is a radiometric dating technique utilizing the comparison of two isotopes of uranium (U) in a sample: 234U and 238U. 234U/238U dating is one of several radiometric dating techniques exploiting the uranium radioactive decay series, in which 238U undergoes 14 alpha and beta decay events while decaying to the stable isotope 206Pb. Other dating techniques using this decay series include uranium-thorium (using 230Th/238U) and uranium-lead dating.

238U, with a half-life of about 4.5 billion years, decays to 234U through emission of an alpha particle to an isotope of thorium (234Th), which is comparatively unstable with a half-life of just 24 days. 234Th then decays through beta particle emission to an isotope of protactinium, 234Pa. 234Pa decays with a half-life of 6.7 hours, again through emission of a beta particle, to 234U. This isotope has a half-life of about 245,000 years. The next decay product, 230Th, has a half-life of about 75,000 years and is used for the related 230Th/238U technique. Although analytically simpler than 230Th/238U dating, in practice 234U/238U dating is almost never used as unlike 230Th/238U dating it requires prior knowledge of the 234U/238U ratio at the time the material under study was formed. For those materials (principally marine carbonates) for which the initial ratio is known, 230Th/238U remains a superior technique. This restricts the application of 234U/238U to extremely rare cases where the initial 234U/238U is well-constrained and the sample is also beyond the ca. 450,000 year upper limit of the 230Th/238U technique.

Unlike other radiometric dating techniques, those using the uranium decay series (except for those using the stable final isotopes 206Pb and 207Pb) compare the ratios of two radioactive unstable isotopes. This complicates calculations as both the parent and daughter isotopes decay over time into other isotopes.

In theory, the 234U/238U technique can be useful in dating samples between about 10,000 and 2 million years Before Present (BP), or up to about eight times the half-life of 234U. As such, it provides a useful bridge in radiometric dating techniques between the ranges of 230Th/238U (accurate up to ca. 450,000 years) and U-Pb dating (accurate up to the age of the solar system, but problematic on samples younger than about 2 million years).

References

* [http://www.webelements.com/webelements/elements/text/Pa/radio.html "Protactinium: Radioisotope Data"] . WebElements. Retrieved July 8, 2005.
*Shakhashiri, Bassam. [http://scifun.chem.wisc.edu/chemweek/uranium/uclock.htm "Chemical of the Week -- Uranium"] . University of Wisconsin-Madison Chemistry Department. Retrieved July 8, 2005.
* [http://www.britannica.com/eb/article?tocId=9074426&query=Uranium-234-Uranium-238%20Dating&ct= "Uranium-234-uranium-238 dating"] . "Encyclopædia Britannica". Retrieved July 8, 2005.


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