Uranium–thorium dating

Uranium–thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a

thorium-230 and its radioactive parent uranium-234

within a sample.

Background

parts per million by weight. As time passes after such material has formed, uranium-234 in the sample with a half-life of 245,000 years decays to thorium-230.^[3] Thorium-230 is itself radioactive with a half-life of 75,000 years,^[3] so instead of accumulating indefinitely (as for instance is the case for the uranium–lead

system), thorium-230 instead approaches secular equilibrium with its radioactive parent uranium-234. At secular equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of thorium-230 produced, which also equals the number of uranium-234 decays per year in the same sample.

History

In 1908,

travertines). In the late 1980s, the method was refined by mass spectrometry, with significant contributions from Larry Edwards.^[4]^[5] After Viktor Viktorovich Cherdyntsev's landmark book about uranium-234 had been translated into English, U-Th dating came to widespread research attention in Western geology.^[6]

^: 7

Methods

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 are listed in the table below.

U-series dating methods^[6]
Isotope ratio measured	Analytical method	Time range ( ka )	Materials
²³⁰Th/²³⁴U	Alpha spec.; mass spec.	1–350	Carbonates, phosphates, organic matter
²³¹Pa/²³⁵U	Alpha spec.	1–300	Carbonates, phosphates
²³⁴U/²³⁸U	Alpha spec.; mass spec.	100–1,000	Carbonates, phosphates
U-trend	Alpha spec.	10–1,000(?)	Detrital sediment
²²⁶Ra	Alpha spec.	0.5–10	Carbonates
²³⁰Th/²³²Th	Alpha spec.	5–300	Marine sediment
²³¹Pa/²³⁰Th	Alpha spec.	5–300	Marine sediment
⁴He/U	mass spec. (gas)	20–400(?)	Coral

The ²³⁴U/²³⁸U method is based on the fact that ²³⁴U is dissolved preferentially over ²³⁸U because when a ²³⁸U atom decays by emitting an

alpha ray the daughter atom is displaced from its normal position in the crystal by atomic recoil.^[7]

This produces a ²³⁴Th atom which quickly becomes a ²³⁴U atom. Once the uranium is deposited, the ratio of ²³⁴U to ²³⁸U goes back down to its secular equilibrium (at which the radioactivities 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 constant $A$ , these expressions for activity rations (assuming that the ²³⁰Th starts at zero):

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

^{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 for $A$ in terms of the unknown age, $t$ :

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 for $t$ :

^{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 no

equation solving algorithms

.

Dating limits

Uranium–thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the precision 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.^{[citation needed]}

Precision

U-Th dating yields the most accurate results if applied to precipitated calcium carbonate, that is in

stalagmites, 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.^[8]

References

^ Davis, Owen (Spring 2005). "Uranium-Thorium Dating". Biogeography ECOLOGY 438/538. Department of Geosciences, University of Arizona. Retrieved 24 October 2015.
ISSN 2572-4525
.

^
ISBN 978-1-317-87149-1
.

^ "These two geochemists have one of the largest publishing networks in science". Nature Index. 2020-06-26. Retrieved 2023-09-01.

^ "Uranium–Thorium Dating of Speleothems". pubs.geoscienceworld.org. Retrieved 2023-09-01.

^
doi:10.1016/1040-6182(89)90005-0
. (subscription required)

S2CID 129292401
.

S2CID 83851086
.

External links

The Wikibook Historical 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 from the original on 14 February 2015. Retrieved 24 October 2015.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Uranium–thorium_dating&oldid=1187245888"

[1] Davis, Owen (Spring 2005). "Uranium-Thorium Dating". Biogeography ECOLOGY 438/538. Department of Geosciences, University of Arizona. Retrieved 24 October 2015.

[2] ISSN 2572-4525
.

[Aitken2014-3] 
ISBN 978-1-317-87149-1
.

[4] "These two geochemists have one of the largest publishing networks in science". Nature Index. 2020-06-26. Retrieved 2023-09-01.

[5] "Uranium–Thorium Dating of Speleothems". pubs.geoscienceworld.org. Retrieved 2023-09-01.

[Schwarcz-6] 
doi:10.1016/1040-6182(89)90005-0
. (subscription required)

[7] S2CID 129292401
.

[8] S2CID 83851086
.

[3]

[4]

[5]

[6]

[7]

[8]