Uranium-uranium dating

From Wikipedia, the free encyclopedia

Uranium-uranium dating is a radiometric dating technique utilizing the comparison of two isotopes of uranium (U) in a sample: ²³⁸U and ²³⁴U. 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).

[edit] References

• "Protactinium: Radioisotope Data". WebElements. Retrieved July 8, 2005.
• Shakhashiri, Bassam. "Chemical of the Week -- Uranium". University of Wisconsin-Madison Chemistry Department. Retrieved July 8, 2005.
• "Uranium-234-uranium-238 dating". Encyclopædia Britannica. Retrieved July 8, 2005.