Uranium-series (U-series) dating method

Science

Uranium dating is one of the ways of determining the age of ancient objects, even one million years old, by measuring how much of the following are present in them: the amount of radioactive isotopes of uranium, and the amount of other materials into which the radioactive isotopes would decompose

Modern Uranium-series methods use decay chains and lasers to allow dating calculations to around 500,000 years.

Uranium-series (U-series) dating is another type of radiometric dating. You will remember from our consideration of C-14 dating that radiometric dating uses the known rate of decay of radioactive isotopes to date an object. Each radioactive isotope has a known, fixed rate of decay.

As its name suggests, uranium-series dating uses the radioactive decay of uranium to calculate an age.

When uranium decays, it goes through a series of decays until it eventually reaches a stable isotope. So, for example, uranium 238 will decay to uranium 234, which will decay to thorium 230. Thorium will then decay to another isotope, radium, which will in turn decay to radon and so on down the chain until it becomes a stable lead isotope. This is called a decay chain. The first isotope is called a “parent isotope”. The isotopes that occur along the decay chain are called “daughter isotopes”.

Natural uranium consists of two parent isotopes. These are uranium 238 (99.28% in abundance) and uranium 235 (only 0.72%). These two parent isotopes have different decay chains. From the first decay chain, we are interested in the decay of uranium 234 (U-234) to thorium (Th-230).

Remember the cave popcorn

U-series dating was principally used for dating the formation of stalagmitic calcite – like our “cave popcorn”. And it is probably simplest to first explain the dating principles of this method from this perspective. Stalagmites grow because of the formation of calcite crystals from ground water. As the water flows through – say a crack in a cave roof – it leaves behind deposits of the calcite crystals, which build up over time to form different shapes, such as stalactites and stalagmites. These mineral deposits commonly found in cave environments are called speleothems.

The water that carries these calcite crystals also contains traces of the naturally occurring uranium, because uranium is soluble – it is able to be dissolved in water. However, thorium (the daughter isotope) is not soluble, so it is not present in the water. This means that while the water that is creating the speleothem is also depositing traces of uranium in the calcite, it is not depositing thorium. Which in turn means that any thorium in the speleothem has been formed by the gradual decay of uranium to thorium (U-234 to Th-230). The thorium is growing inside the speleothem.

Thorium itself is radioactive and begins its own process of decay in the chain. Eventually the rate that the thorium is decaying will become equal to the rate that the uranium is producing it. Until that state of equilibrium is reached, measurement of the ratio between U-234 and Th-230 allows us to calculate the time that has passed since crystal formation began. Th-230 has a half-life of 75,700 years, allowing dating up to around 500,000 years ago.

This same principle can be used to date corals, as again, the presence of thorium in the corals will be the result of uranium decay – not because the thorium has been deposited there by the sea water.

Some assumptions

So what’s the challenge? Well, here are our assumptions. We assume that at crystal formation the thorium content is zero. We also assume that over the thousands of years, uranium and thorium have not been moved into or out of the material we are now testing. This is called a closed system assumption.

The closed system assumption is particularly relevant to applying U-series dating to human fossils, as bones and teeth do exchange uranium with the environment. This is unlike speleothem, that usually remain closed to any subsequent migration after they have been formed. Fossils can contain hundreds of times more uranium than modern bones, due to exposure to ground water. When a bone is buried in sediment, it acts a bit like a sponge for uranium. Uranium can migrate into the bone (a phenomenon known as ‘incorporation’ or ‘uptake’). Uranium can also move out of the bone (leaching).

This has an effect on our process. If uranium (the parent isotope) has been leached from a bone – we may face a situation where there is more thorium (daughter isotope) than uranium. When this happens – well, we can’t actually calculate a U-series age. One of the main challenges of U-series dating of fossil bones is identifying samples that haven’t experienced uranium leaching.

So… what are we actually dating?

U-series analysis of fossils dates the moment when uranium migrates into the bones, not the moment of the death of an organism. It is possible that the uranium entered the bone a long time after the death of the organism (called a ‘delayed uptake’). This means that any U-series age that is calculated will always provide a minimum age possible for the bone. The age could be similar to the age of the death of the fossil – if the uptake occurred right after the death of the organism. In the case of a delayed uptake, the fossil will be older than the calculated U-series date.

We do attempt to reconstruct the uptake of uranium into a fossil sample. We use a model to do this (called a diffusion-adsorption, or DA model), which predicts the distribution of uranium across a bone or tooth enamel section. This adds a margin of error that is difficult to calculate.

In the early days of the U-series dating method, samples were required to be dissolved for analysis. Modern techniques for U-series dating use laser ablation sampling combined with inductively coupled plasma mass spectrometry analysis (ICP-MS). This method allows us to project a laser onto the flat surface of a sample and atomise the material in a tiny circle that is hardly visible to the naked eye. This gives us high-resolution measurements with minimum sample destruction. Ages are then calculated by comparing the measured isotopes with those of a standard. A standard is a reference sample of known U-series age.

Uranium-lead Dating

Uranium-lead dating is based on the measurement of the first and the last member of the uranium series. Uranium-lead dating is one of the oldest radiometric dating methods. Radiation Dosimetry

Radiometric dating (or radioactive dating) is any technique used to date organic and also inorganic materials from a process involving radioactive decay. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.

The radioactive decay law states that the probability per unit time that a nucleus will decay is a constant, independent of time. This constant is called the decay constant and is denoted by λ, “lambda”. This constant probability may vary greatly between different types of nuclei, leading to the many different observed decay rates. The radioactive decay of certain number of atoms (mass) is exponential in time.

Radioactive decay law: N = N0.e-λt

One of the oldest radiometric dating methods is uranium-lead dating. The age of the earth’s crust can be estimated from the ratio between the amounts of uranium-238 and lead-206 found in geological specimens. The long half-life of the isotope uranium-238 (4.51×109 years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating, including uranium–thorium dating and uranium–uranium dating.

Uranium-lead dating is based on the measurement of the first and the last member of the uranium series, which is one of three classical radioactive series beginning with naturally occurring uranium-238. This radioactive decay chain consists of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. In case of uranium series, the stable nucleus is lead-206. The assumption made is that all the lead-206 nuclei found in the specimen today were originally uranium-238 nuclei.  That means at the crust’s formation the specimen contained no lead-206 nuclei. If no other lead isotopes are found in the specimen, this is a reasonable assumption. Under this condition, the age of the sample can be calculated by assuming exponential decay of uranium-238. That is:

uranium-lead method - age of the Earth

Uranium-lead dating method is usually performed on the mineral zircon. Zircons from Jack Hills in Western Australia, have yielded U-Pb ages up to 4.404 billion years, interpreted to be the age of crystallization, making them the oldest minerals so far dated on Earth.

Age of the Earth – Uranium-lead Dating

The age of the Earth is about 4.54 billion years. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

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