Uranium–thorium dating

Uranium–thorium dating is a way scientists figure out how old certain things are. It started in the 1960s and has been used since the 1970s. This method helps find the age of materials made mostly of calcium carbonate, like cave formations called speleothem or pieces of coral.

Unlike some other dating methods, such as rubidium–strontium or uranium–lead dating, uranium–thorium dating doesn’t look at a stable end product. Instead, it looks at how much a radioactive material called thorium-230 has changed back to its parent material, uranium-234, inside the sample.

This technique uses the idea of secular equilibrium and relies on measuring radioactive isotopes. It’s a special kind of radiometric dating that gives scientists important clues about the past.

Background

Thorium does not dissolve in water near the Earth's surface, so materials formed from this water usually do not contain thorium. However, uranium can dissolve in water, so materials formed from such water often contain small amounts of uranium, usually between a few parts per billion and a few parts per million by weight. Over time, uranium-234, which has a half-life of 245,000 years, changes into thorium-230. Thorium-230 is also radioactive, with a half-life of 75,000 years. Unlike some other systems, thorium-230 does not build up forever. Instead, it reaches a balance with uranium-234, where the amount of thorium-230 breaking down each year matches the amount being created from uranium-234.

History

In 1908, John Joly, a geology professor at Trinity College Dublin, noticed that deep sediments had more radium than sediments closer to land. He thought that sediments were taking radium from seawater. Later, in 1942, Piggot and Urry discovered that extra radium was linked to extra thorium (²³⁰Th). It wasn't until the 1960s that scientists started using this method to date rocks like speleothems and travertines. By the late 1980s, improvements in measuring tools helped make the method even better, thanks to work by Larry Edwards. When a major book about uranium-234 was translated into English, more scientists in the West began using uranium-thorium dating.

Methods

U-series dating is a group of methods used to find the age of different materials. Each method is named after special parts of atoms, called isotopes, that scientists measure. There are eight methods shown in the table below.

The ²³⁴U/²³⁸U method works because ²³⁴U gets dissolved more easily than ²³⁸U. When a ²³⁸U atom breaks apart by sending out an alpha ray, it moves from its usual place in a crystal. This creates a ²³⁴Th atom, which quickly changes into a ²³⁴U atom. After the uranium is placed somewhere, the amount of ²³⁴U compared to ²³⁸U goes back to a balanced state. This balance gets closer every 245,000 years.

Dating limits

Uranium–thorium dating can help figure out how old something is, but it works best for items that are not older than about 500,000 years. This limit depends on how long it takes for thorium-230 to change over time and how exactly we can measure the amounts of thorium-230 and uranium-234 in a sample. To use this method, we also need to check the amounts of uranium-234 compared to another type of uranium, called uranium-238.

Precision

U-Th dating works best when used on materials like stalagmites and certain types of rock. It is less reliable when used on bone or shell. With a tool called mass spectrometry, scientists can measure the age very precisely, within about ±1%. Older methods, like alpha counting, are less precise, with about ±5% accuracy. Mass spectrometry also needs smaller pieces of the sample.