Uranium-Lead Dating

May 2, 2024

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Uranium-Lead Dating

1. Introduction

The mineral incorporates uranium and thorium atoms into its crystal structure but strongly rejects lead. Therefore, one can assume that the entire lead content of the zircon is radiogenic. The rate at which the reaction occurs is different for each concordant isotope: the lower the atomic mass, the greater the rate. Stepwise leaching from the high-temperature part of the system will extract the lead lost from the high-temperature reaction, and heating at very high temperatures overlapped with the common lead method will allow the dating of the intermediate or lower temperature events.

The current understanding of the Earth’s history is built upon the integration of various geological and biological data. The age of the Earth has been a topic of debate among scientists because the techniques used for dating the Earth using uranium (U), which has radioactive isotopes, has changed. Uranium-lead dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils particularly outside the range of radiocarbon. It was used to date zircon crystals in Western Australia, which were formed 4.4 billion years ago. The dating method is usually performed on the mineral zircon.

2. Principles of Uranium-Lead Dating

An innovative principle in the uranium-lead method is to assess the degree of discordance between the two decay schemes. This has commonly been a grand assumption in understanding geological time. A date to an event can usually be assessed by its position in the stratigraphically or a complex of geologic events by spatial juxtaposition. However, with increasing need for precision in assigning age to rock units, it is fundamental to understand the few assumptions underlying the concept of relative age.

In general, with the inability to provide using the uranium-lead method an absolute age of a sample, the best that can be done is to provide a relative age. These mineral relative ages have been employed globally to decipher earth history in terms of geologic events and to understand the complex geological history of a rock unit or structure. This has been a fundamental assumption of the Geologic Time Scale since its inception and a fundamental tool of historical geology.

Another method is to make age measurements on several samples from the same rock unit. This technique helps identify post-formation geologic disturbances because different minerals respond differently to heating and chemical changes. It also is much more common for one to obtain lead ages that are too old, rather than too young. This dissonance is often used to align two separate rock formations chronometrically. A detailed more scientific discussion about this can be found at the start of Harland et al.

Uranium-lead is one of the oldest and most refined of the radiometric dating schemes. It can be used over an age range of about 1 million years to over 4.5 billion years. Precision is in the 0.1-1 percent range. The method is usually applied to zircon. This mineral incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead. Therefore, one can assume that the entire lead content of the zircon is radiogenic. One of the advantages of uranium-lead dating is the two separate, chemically identical chronometers and is accepted as the most accurate means of giving an age to the earth.

3. Applications of Uranium-Lead Dating

By counting lead and uranium isotopes in a zircon, the time since it crystallized can be determined. If the line is a good fit and the half-life is reliable, the age will be accurate. This can be used to deduce past events and processes. Metamorphism can heat the crystal, causing lead to become mobile. Uranium-lead dating can be used to find the age of a uranium-containing mineral. If the mineral has lost some of the daughter element, it will give an inaccurate date. With sufficient lead isotope data, the time can still be modeled. Zircon is the ideal mineral for the U-Pb dating method as it excludes lead, so any lead present must be radiogenic. This can be used to infer an age for metamorphism.

Radiometric dating poses a problem for young earth creationists. The Concordia diagram is useful in this respect. Phenocrysts commonly contain a small lead isotope (Pb206), which can cause discordant dates. The Concordia diagram shows that points will always line up on a straight line for any set of parents and daughters.

4. Limitations of Uranium-Lead Dating

Uranium-lead dating can be an absolute dating method given the correct conditions are met, this is often not the case in today’s sedimentary system. Typically what are being dated are the igneous rocks of sedimentary layers, so the age will be the datable age of the igneous rock not the sedimentary layer. Usually any of the lead isotopes can be utilized and the system is not closed so mineral and/or loss can occur. If the mineral has lost atoms of the element since it was formed the trendline between the remains of the mineral and the solid lead will deviate from the concordia, reducing the accuracy of the date. These fluctuations and deviations of data can make the age of sedimentary rocks a relatively inexact process, as illustrated with the ‘???’ at the start of the diagram, as we are often unsure in these cases whether the age is the radiometric age or the arguable ‘date’ of mineral formation.

This method has several limitations. Radiometric dating is often only done on the mineral when it has formed so age of a mineral gives the youngest possible age for the rock it is in. Time consuming analysis of certain isotopes’ half-life can help bypass this problem in some cases. Minerals usually only record the last time they were in their liquid state and this requires some form of rock to rock recrystallisation to reset the mineral’s radiometric clock. This usually results in an age that is too old. In addition to this, the ratios of parent and daughter isotopes are not the only factors that can affect the age in a radioisotope dating system, so the analysis involved is a complex one and the aim is to get an accurate date. Any loss of the pristine crystal structure will cause an increase in the original dose of daughter isotopes we measure misrepresenting the age. Finally, if a rock melts it can reset its radiometric clock and this in some cases can occur to lead, releasing the isotopes into the environment and allowing them to return to a closed system.

Strontium isotopes are introduced. These break down at a certain rate into strontium isotopes, and the analysis used to date the rock is based on this decay. The method works best on micas, tektites, and meteorites and can date rocks of any age. For an igneous rock, the time during which the mineral forming that rock has been liquid will add to the radiometric age so we would have to examine the mineral content of the rock to calculate an accurate age.

5. Conclusion

(U-Th)/He dating is a more recent isotopic technique and is primarily used as a complement to other systems. The most important ‘proof of concept’ for this method came in 2003 with the results for zircons from the Bishop Tuff in California (Sanidine ~670ka). The U-Pb system has a much shorter half-life than other parent-daughter isotopes used for isotopic dating, therefore it is possible to relate the measured Pb isotopic compositions in a mineral which has lost all its U and Th since it formed. The simplest case requires a wide spread geologically common U and Th source existing for the host mineral. With the minimum age calculated above for the Tuff at the sample site, these have the potential to be an extremely useful tool in constraining the history of volcanic eruptions in various settings around the world. It will also be possible to apply (U-Th)/He zircon dating to metamorphic zircon in an attempt to further understand the time-temperature history of zircon and the isotopic systematics of lead loss.
The differing methods also have unique potential in the field of Tectonics. Due to the slower exhumation rate required in order to reset the isotopic systems of lead, this method is generally applied to relatively young geomorphic features and sections of the Earth’s crust (U-Pb also has implications for the movement of lead and U and Th between earth materials). On the other hand, (U-Th)/He has the potential to date very recent tectonic events due to the high diffusivity of the Helium atom at low temperatures (closure temperature for the U-Th/He system ~65°C). This is because the partitioning of Helium into a mineral is a result of alpha decay from U and Th in the lattice, thus the survival of Helium in the mineral depends on the retention of its parent isotopes.