Uranium-lead dating

The uranium - lead dating is an absolute dating method, in which the radioactive decay series of uranium are exploited to date samples. With this method, for example, underground rock or meteorites are dated. The age of the earth adopted today by 4.55 billion years ago was first determined by Clair Cameron Patterson with the uranium - lead dating. The age of the solar system was by means of this dating method, applied to the probably oldest incurred in our solar system minerals, calcium - aluminum-rich inclusions in meteorites, at 4.567 billion years determined. For the oldest incurred on earth minerals, zircons that have been found in rocks in Australia, we determined an age of up to 4.404 billion years.

Basics

There are two decay chains, each of which begins with uranium isotopes and will continue for several intermediate steps in lead isotopes:

• Uranium - radium series: uranium 238U → ... → lead 206Pb (half-life: 4.5 billion years )
• Uranium - actinium series: uranium 235U → ... → lead 207Pb (half-life: 704 million years )

The various unstable decay products in this series are much shorter than the respective uranium isotope at the beginning of the series. For the age determination therefore only the half-lives of uranium isotopes play an essential role. After the decay law is valid:

This can be calculated from the measurement of lead isotope ratios and the Pb / U ratio, the age in three different ways to calculate. The first two methods result directly from the conversion of the respective decay law:

From two decay laws together can also be simply the third equation for the age derived in the ratios of isotopes occur no different elements, but only ratios of isotopes of an element, respectively:

From this equation, the age can be determined by iterative numerical or graphical methods. In principle must ensure only the ratio of lead isotopes 207Pb: 206Pb are measured, assuming that the current natural uranium isotope ratio is homogeneous on earth. This one took a long time, the value should be in 235U: 137.88 are: 238U = 1. However, new measurements quantify it on 235U: 238U = 1: 137.818 ± 0.045 ( 2σ ), it may be slightly different also depending on locality. Since isotopic ratios of an element can be determined much more accurately than the ratio of different elements, this method is very accurate. A prerequisite of this method is that the current uranium isotope ratio is known in the sample. In particular, the application to meteorites dating assumes a homogeneity of uranium isotopes in the solar nebula, as long as one uses the terrestrial uranium isotope ratio for the calculation of age. However, the required uranium isotope ratios are also determined, in principle, in each individual sample so that this assumption is not absolutely necessary or can even be checked directly. A meaningful way to check whether the respective necessary conditions are given for a concrete sample, is the application of Konkordia diagram.

Another advantage of this method is that the decay constants of uranium are known with an accuracy in tenths of a percent, while the decay constants used for the dating of other radioactive elements usually are only known with an accuracy in the percent range.

Concordia diagram

The Concordia diagram provides a way to verify the reliability of the measured U -Pb age. Plotting the measured 206Pb/238U-Verhältnis and the 207Pb/235U ratio of a measured sample in the Concordia diagram a, the data point in the ideal case should be referred to the Concordia curve. This is approximately the case in crystals, which have only a single stage of history behind it, so have no disturbance in the uranium-lead isotopic system know more after their crystallization. If the measured isotope ratios on the Concordia, that is a single stage to accept history and age can be regarded as very reliable.

A disturbance of the U -Pb isotopic system by a later than the datable event can be a metamorphosis of the rock or lead loss due to diffusion, for example. Once the sample has been disturbed, so the data point is next to that is, it is discordant. Also, do not radiogenic lead components, ie lead from sources other than the decay of uranium (eg primordial lead) can cause a deviation from the Concordia, if they have not been sufficiently corrected with the lead isotope measurements. In fact, many rocks have a complex history behind it, which is why a large number of measured in practice uranium-lead ratios turns out to be discordant.

Concordia diagram

Even with discordant uranium-lead measurements but can be reconstructed often the story of a rock. This is the case if the crystals have been around a rock after the initial crystallization of a further singular event, such as a metamorphosis disturbed, thus having a total of one two- story. Then the data points such crystals are in the Concordia diagram on a straight line connecting the Concordia at the times of the first event ( crystallization) and the second event ( metamorphosis ) cuts. Such a straight line is called Discordia. If you measure so several crystals from a rock, which all have the same two-stage history behind it, the Discordia can be fitted to the data points, and thus the points of intersection with the Concordia and their corresponding time points are determined.

A potential problem may be here that if the uranium-lead isotopic system was disturbed by continuous lead loss, the data points can also be approximated in a wide range lie on a straight line and only at small 207Pb/235U-Verhältnissen to the origin of the diagram towards turn. The danger here is that a shape adapted to such data can be precisely misinterpreted as Discordia.

Correction of the non- radiogenic ( primordial ) Lead - share

In addition to a diskordantem data point in the Concordia diagram, which can be caused by non- radiogenic (also called primordial ) lead, and the lead isotope 204Pb is an important indicator of the presence of nichtradiogenem lead in a sample. Namely, there is no natural decay chain to the 204Pb isotope, and so no radiogenic 204Pb, but this is fully Bleisiotop primordial and the frequency, therefore, a direct measure of the proportion of the non- radiogenic lead in the sample.

For samples or mineral separations, where no radiogenic lead represents a non -negligible portion, it must be corrected before the age calculation. This usually happens by the primordial 206Pb and 207Pb abundances are determined with the measured 204Pb frequency and the known isotopic ratios of primordial lead and are subtracted from the corresponding measured abundances of these isotopes. As a result, we obtain the radiogenic 206Pb and 207Pb - frequencies with which then the age can be calculated.

Important in this correction is the knowledge of the isotopic ratios of the primordial lead. These were, for example, by Tatsumoto et al. determined and published in 1973 and later by Goepel et al. (1985) confirmed. The investigations of Goepel et al. also support strongly the assumption that the primordial lead was homogeneous in the protoplanetary disk.

Development of the uranium - lead dating

A dating due to radioactive decay of uranium first hit in 1905 Ernest Rutherford ago. After Bertram B. Boltwood had demonstrated in 1907 lead as the final product of uranium decay, specified for some rocks in 1911 by Arthur Holmes Age of up to 1.64 billion years. However, this age were too high because they were not based on isotope ratios, but on the chemical ratios of uranium and lead. Isotopes were still unknown.

Isotope ratios of lead were measured in 1927 by Francis William Aston. In 1930 Otto Hahn determined the age of the earth with the uranium-lead method to 1.5 to 3 billion years, although he still used chemical isotopes ratios instead of ratios to calculate and made the assumption that no primordial lead in was the observed of him rocks available. From 1937 undertook Alfred Nier measurements of lead isotope ratios with mass spectrometers. He tried to determine the isotopic ratios of primordial lead. The development of the atomic bomb, especially in the context of the Manhattan Project, also led to the development of improved techniques for the determination of isotope ratios and to a better understanding of the uranium decay, leading the development of the uranium-lead dating technique accelerated greatly. 1953 published Clair Cameron Patterson based on lead isotope measurements in a meteorite that until today accepted age of the Earth of 4.55 billion years.