Uranium–thorium dating, also calledthorium-230 dating,uranium-series disequilibrium dating oruranium-series dating, is aradiometric dating technique established in the 1960s which has been used since the 1970s to determine the age ofcalcium carbonate materials such asspeleothem orcoral.^[1][2] Unlike other commonly usedradiometric dating techniques such asrubidium–strontium oruranium–lead dating, the uranium-thorium technique does not measure accumulation of a stable end-memberdecay product. Instead, it calculates an age from the degree to whichsecular equilibrium has been restored between theradioactiveisotopethorium-230 and its radioactive parenturanium-234 within a sample.

Thorium is not soluble in natural water under conditions found at or near the surface of the earth, so materials grown in or from this water do not usually contain thorium.^[3] In contrast,uranium is soluble to some extent in all natural water, so any material thatprecipitates or is grown from such water also contains trace uranium, typically at levels of between a few parts per billion and fewparts per million by weight. As time passes after such material has formed, uranium-234 in the sample with ahalf-life of 245,000 years decays to thorium-230.^[4] Thorium-230 is itself radioactive with a half-life of 75,000 years,^[4] so instead of accumulating indefinitely (as for instance is the case for theuranium–lead system), thorium-230 instead approaches secular equilibrium with its radioactive parent uranium-234. where the number of thorium-230 decays per year within a sample is equal to the number of thorium-230 atoms produced, which also equals the number of uranium-234 decays per year in the same sample. This limit is represented by the point (1,1) in the graph above.

In 1908,John Joly, a professor of geology atTrinity College Dublin, found higherradium contents in deep sediments than in those of the continental shelf, and suspected that detrital sediments scavenged radium out of seawater. Piggot and Urry found in 1942, that radium excess corresponded with an excess of thorium (²³⁰Th, the direct parent of radium). It took another 20 years until the technique was applied to terrestrial carbonates (speleothems andtravertines). In the late 1980s, the method was refined by mass spectrometry, with significant contributions from Larry Edwards.^[5][6] AfterViktor Viktorovich Cherdyntsev's landmark book abouturanium-234 had been translated into English, U-Th dating came to widespread research attention in Western geology.^[7]: 7

U-series dating is a family of methods which can be applied to different materials over different time ranges. Each method is named after the isotopes measured to obtain the date, mostly a daughter and its parent. Eight methods arelisted in the table below.

The²³⁴U/²³⁸U method is based on the fact that²³⁴U is dissolved preferentially over²³⁸U because when a²³⁸U atom decays by emitting analpha ray the daughter atom is displaced from its normal position in the crystal byatomic recoil.^[8]This produces a²³⁴Th atom which quickly becomes a²³⁴U atom. Once the uranium is deposited, the ratio of²³⁴U to²³⁸U goes back down to its secular equilibrium (at which the activities of the two are equal), with the distance from equilibrium decreasing by a factor of 2 every 245,000 years.

A material balance gives, for some unknown constantA, these expressions for activity ratios (assuming that the²³⁰Th starts at zero):

${\displaystyle {}^{234}\text{U}{/}^{238}\text{U}=1+A\times 2^{-t/245000}}$

${\displaystyle {}^{230}\text{Th}{/}^{238}\text{U}=1+\frac{A}{1-75000/245000}\times 2^{-t/245000}-\left(1+\frac{A}{1-75000/245000}\right)\times 2^{-t/75000}}$

We can solve the first equation forA in terms of the unknown age,t:

${\displaystyle A={(}^{234}\text{U}{/}^{238}\text{U}-1)\times 2^{t/245000}}$

Putting this into the second equation gives us an equation to be solved fort:

${\displaystyle {}^{230}\text{Th}{/}^{238}\text{U}=1+\frac{{}^{234}\text{U}{/}^{238}\text{U}-1}{1-75000/245000}-2^{-t/75000}-\frac{{}^{234}\text{U}{/}^{238}\text{U}-1}{1-75000/245000}\times 2^{t/245000-t/75000}}$

Unfortunately there is noclosed-form expression for the age,t, but it is easily found usingequation solving algorithms.

Uranium–thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, theprecision with which one can measure the thorium-230/uranium-234 ratio in a sample, and the accuracy to which one knows the half-lives of thorium-230 and uranium-234. Using this technique to calculate an age, the ratio of uranium-234 to its parent isotope uranium-238 must also be measured.

U-Th dating yields the most accurate results if applied to precipitated calcium carbonate, that is instalagmites, travertines, and lacustrine limestones. Bone and shell are less reliable.Mass spectrometry can achieve a precision of ±1%. Conventional alpha counting's precision is ±5%. Mass spectrometry also uses smaller samples.^[9]

• Radiocarbon dating

The WikibookHistorical Geology has a page on the topic of:U-Pb, Pb-Pb, and fission track dating

• Shakhashiri, Bassam Z."Uranium".Chemical of the Week on scifun.org. University of Wisconsin-Madison Chemistry Department. Archived fromthe original on 14 February 2015. Retrieved24 October 2015.

• Radiometric dating
• Thorium
• Uranium

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