Radiometric dating

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Radiometric dating is a method of age determination . It is based on the knowledge of the decay rates of naturally occurring isotopes , their usual occurrence and the determination of the isotope ratio .

Basics of radiometric dating

All ordinary matter is made up of combinations of chemical elements . Every chemical element has its own atomic number , which indicates the number of protons in the atomic nucleus of the chemical element. In addition to protons, all chemical elements except hydrogen carry a number of neutrons in the atomic nucleus. The elements can occur in different isotopes , whereby the isotopes of an element differ only in the number of neutrons in their nuclei and have the same number of protons and are therefore subordinate to a chemical element with the same atomic number. A certain isotope of a certain element is called a nuclide . Some nuclides are unstable. This means that at a random point in time an atom of such a nuclide is converted into another nuclide, a process known as radioactive decay . This conversion goes hand in hand with the emission of such particles as electrons (so-called beta radiation ) or alpha particles .

While the point in time at which a certain nucleus decays is random, individual atoms in a sufficiently large collection of atoms of a radioactive nuclide decay exponentially at random at a rate that can be described by a parameter known as the half-life and is typical for a nuclide . Half-life is usually expressed in years when it comes to dating methods. When a half-life has passed, about half of the atoms of the nuclide in question have decayed. Many radioactive substances decay from a nuclide to a final, stable decay product in a series of steps called the decay chain . In this case, the reported half-life is usually the dominant (longest) one of the entire chain, not just one step in the chain. Nuclides used for radiometric dating can have half-lives between a few thousand to a few billion years.

In most cases the half-life of a nuclide depends entirely on the nature of its nucleus; it is not affected by temperature , chemical environment, magnetic fields or electric fields . The half-life of each nuclide is assumed to be constant over time. Although radioactive bombardment can accelerate decay, such bombardment usually leaves traces. Therefore, in any material that contains a radioactive nuclide, the ratio of the original nuclide to its decay products changes in a predictable manner as the nuclide decays. This predictability makes it possible to use the relative abundance of related nuclides as a timer that indicates the time that has elapsed from the uptake of the original nuclides in the test material to the present.

The processes by which certain materials are created are often quite selective in terms of the inclusion of certain elements during the creation. Ideally, the material absorbs the original nuclide, but not the decay product. In this case, all decay products found during the investigation must have arisen since the material was created. If a material takes up both the original nuclides and the decay products during its formation, it may be necessary to assume that the initial proportions of the radioactive substance and its decay product are known. The decay product should not be a gas with small molecules that can escape from the material, and it must itself have a long enough half-life to be present in sufficient quantities. In addition, the starting element and the decomposition product should not be generated or reduced in significant quantities by other reactions. The procedures used to isolate and analyze the reaction products should be straightforward and reliable.

When a material that selectively secretes the decay products is heated, any decay products that have accumulated over time are lost through diffusion so that the isotope “clock” is reset to zero. The temperature at which this occurs is called the "lock temperature" and is specific to a particular material.

In contrast to the simple radiometric dating methods, isochronous dating , which can be used for many isotopic decay chains (e.g. the rubidium-strontium decay chain), does not require any knowledge of the original relationships. Likewise, the argon-argon dating method for the potassium-argon decay chain can be used to ensure that no initial 40 Ar was present.

History of the method

Ernest Rutherford came up with the first ideas for using the constant decay rate of a radioactive substance to determine the age of minerals at the beginning of the 20th century . Bertram Boltwood was a pioneer in the application of this method for the absolute age determination of rocks . In 1907, Boltwood was able to date the absolute age of rocks for the first time using the decay of uranium . Its dating for rocks in Sri Lanka indicated an age of 2.2 billion years.

Age equation

Assuming that the radioactive source elements decay into stable end products, the mathematical equation that links radioactive decay to geological time (the age equation) is as follows:

With

Age of the sample
Number of atoms of the decay product in the sample
Number of atoms of the starting isotope in the sample
Decay constant of the starting isotope
natural logarithm

Limits of the methods

Although radiometric dating is principally accurate, its accuracy depends heavily on the care with which the procedure is carried out. Both the possible interfering effects of an initial contamination of the starting materials and decomposition substances must be taken into account, as well as the effects that could have led to the loss or gain of such isotopes since the sample was created. In addition, the measurement in a mass spectrometer is disturbed by other nuclides with the same mass number. Corrections may have to be made by determining the isotope ratio of elements that overlap with the isotope sought.

Mass spectrometers are subject to influences and inaccuracies. Among these, the quality of the vacuum should be mentioned in particular . In an insufficient vacuum, gaseous atoms can react with the ionized atoms to be measured. The resolution of the receiver is also a factor, but modern devices are much better here than their predecessors.

The precision of the measurements is improved if they are repeated on different samples taken from the same rock body at different locations. Alternatively, the approach exists when several different minerals from the same sample can be dated and it can be assumed that they were formed in the same event. In that case, the age measurements of the minerals should form an isochron . Finally, correlation between different isotopic dating methods may be needed to confirm the age of a sample.

The accuracy of a dating method depends in part on the half-life of the radioactive isotope involved. For example, 14 C has a half-life of less than 6000 years. If an organism has been dead for 60,000 years, there is so little 14 C left that it is impossible to determine its age precisely. On the other hand, the concentration of 14 C drops so steeply that the age of relatively young remains can be determined precisely to within a few decades. The isotope used in uranium-thorium dating has a longer half-life, but other factors make this method more accurate than radiocarbon dating.

Modern dating methods

Radiometric dating can still be performed on samples as small as a billionth of a gram using a mass spectrometer . The mass spectrometer was invented in the 1940s and has been used in radiometric dating since the 1950s . It works by creating a beam of ionized atoms from the sample under investigation. The ions then move through a magnetic field which, depending on their mass and ionization strength, deflects them to sensors called Faraday cups . Upon impact in the cups, the ions create a very weak current that can be measured to measure the rate of impact and the relative concentrations of various atoms in the beams.

The uranium-lead dating is one of the oldest available, and also one of anerkanntesten. It has been refined to such an extent that the error in determining the age of stones about three billion years old is no more than two million years.

Uranium-lead dating is usually done with the mineral zircon (ZrSiO 4 ), but it can also be used for other materials. Zircon incorporates uranium atoms in its crystal lattice in place of zirconium , but not lead . It has a very high blocking temperature, is resistant to mechanical weathering and is chemically inert. Zircon also forms multiple crystal layers during metamorphic events, each of which can represent an isotopic age of the event. These can be determined using a SHRIMP ion microprobe.

One of the great advantages is that each sample provides two timers, one based on the decay of 235 U to 207 Pb with a half-life of approximately 700 million years, the other based on the decay of 238 U to 206 Pb with a half-life of about 4.5 billion years ago. This results in a built-in counter-sample that allows the exact age of the sample to be determined, even if some lead should have been lost.

Two other radiometric methods are used for long-term dating. The K-Ar dating includes electron acceptance or positron decay of 40 K to 40 Ar . 40 K has a half-life of 1.3 billion years, so this method can also be used for the oldest rocks. Radioactive potassium-40 is common in mica , feldspar and hornblende , although the blocking temperature of these materials is quite low: about 125 ° C (mica) to 450 ° C (hornblende).

The rubidium-strontium dating is based on beta decay of 87 Rb to 87 Sr, with a half-life of 50 billion years. This method is used to date ancient igneous and metamorphic rocks. It was also used for moon rocks . The blocking temperatures are so high that they are of no concern. Rubidium-strontium dating is not as precise as the uranium-lead method, with variations of 30 to 50 million years for a 3 billion year old sample.

Dating methods for shorter periods of time

In addition, there are a number of other dating methods that are suitable for shorter periods of time and are used for historical or archaeological research. One of the best known is radiocarbon dating, or the C-14 method.

14 C is a radioactive isotope of carbon with a half-life of 5,730 years (very short compared to the one shown above). In other radiometric dating methods, the initial heavy isotopes were created by explosions of massive stars that scattered material throughout the galaxy, forming planets and other stars. The parent isotopes have since decayed and any isotope with a shorter half-life would have long since disappeared.

14 C is an exception. It is continuously created by the collision of neutrons created by cosmic rays with nitrogen in the upper layers of the atmosphere. This 14 C is found as a trace element in atmospheric carbon dioxide (CO 2 ).

A living being absorbs carbon from carbon dioxide during its life. Plants absorb it through photosynthesis , animals through the consumption of plants and other animals. When a living being dies, it stops taking up new 14 C and the existing isotope decays with the characteristic half-life (5730 years). The amount of 14 C that can still be determined when examining the remains of the living being gives an indication of the time that has passed since its death. The limit of 14 C dating is around 58,000 to 62,000 years ago.

The rate of formation of 14 C appears to be more or less constant, as cross-checks of carbon-14 dating with other dating methods show that the results are consistent. However, local eruptions of volcanoes or other events that release large amounts of carbon dioxide, reduce the local concentration of carbon-14 and thus provide inaccurate results. The release of carbon dioxide into the biosphere as a consequence of industrialization has also reduced the proportion of 14 C by a few percent. In contrast, the 14 C content was increased by aboveground atomic bomb tests carried out in the early 1960s . Likewise, an increase in the solar wind or the earth's magnetic field above the current value could reduce the amount of 14 C produced in the atmosphere . These effects are balanced out when calibrating the radiocarbon time scale. See also the article Radiocarbon Dating Method .

Another relatively short-term dating method is based on the decay of 238 U to 238 Th, a substance with a half-life of about 80,000 years. It is accompanied by a sister process in which 235 U breaks down to 235 Pa , which has a half-life of 34,300 years.

While uranium is water-soluble, thorium and protactinium are water-insoluble, so they selectively precipitate in sediments of the ocean floor where their proportions can be determined. This method spans a few hundred thousand years.

Another method is thermoluminescence dating . This makes use of the fact that natural radiation sources in the environment electrons z. B. stimulate in a piece of pottery.

Lastly, Fission Track Dating should be mentioned . Here, polished slices of a material are viewed to determine the density of the traces left by the spontaneous decay of 238 U-impurities. The uranium content of the sample must be known, but it can be determined by placing a plastic sheet on the polished disc of material and bombarding it with slow neutrons . This causes an induced nuclear fission of 235 U, in contrast to the spontaneous decay of 238 U. The traces of decay caused by this process are held in the plastic film. The uranium content of the material can then be calculated from the number of traces and the neutron flux.

This method is suitable for use over a wide range of geological data. For periods up to a few million years, mica, tektites (fragments of glass from volcanic eruptions), and meteorites are best used. Older materials can be dated with the help of zircon, apatite, titanite, epidote and garnet, which contain variable amounts of uranium. Since the traces of decay are melted again at temperatures above 200 ° C, the technique has both limits and advantages. This technique has potential application to detail the temperature history of deposits.

Large amounts of the otherwise rare 36 Cl were created during the irradiation of seawater due to the detonation of nuclear weapons between 1952 and 1958. 36 Cl only remains in the atmosphere for about a week. Therefore, it can prove the presence of water from the 1950s in the soil and groundwater, 36 Cl is also suitable for dating water that is less than 50 years old. 36 Cl has also been used in other geosciences, for example to date ice and sediments.

Dating with short-lived radionuclides

At the beginning of the solar system, there were some relatively short-lived radionuclides such as 26 Al, 60 Fe, 53 Mn, and 129 I in the solar nebula. These radionuclides - possibly created by the explosion of a supernova - have now disappeared, but their decay products can be detected in very ancient materials such as meteorites. By measuring the decay products of radionuclides that have since disappeared with a mass spectrometer and using isochronous plots, it is possible to determine relative time periods between different events in the early history of the solar system. Dating methods based on these radionuclides, which have since disappeared, can also be calibrated with the uranium-lead method to get absolute age.

Methods of radiometric dating

Notes and individual references

  1. ↑ The rate of decay is not always constant with electron capture, as it is with nuclides such as B. 7 Be, 85 Sr, and 89 Zr occurs. For this type of decay, the rate of decay can be influenced by the local electron density. These isotopes are not used in radiometric dating, however. Read more here .
  2. ↑ Heuel-Fabianek, B. (2017): Natural radioisotopes: the “atomic clock” for determining the absolute age of rocks and archaeological finds. Radiation Protection Practice, 1/2017, pp. 31–42.
  3. ↑ usgs.gov: Geologic Time: Radiometric Time Scale .
  4. ↑ Plastino, Wolfango; Kaihola, Lauri; Bartolomei, Paolo; Bella, Francesco: Cosmic Background Reduction In The Radiocarbon Measurement by Scintillation Spectrometry at the underground Laboratory of Gran Sasso ; In: Radiocarbon , Volume 43, Issue 2, Pages 11-1145 (May 2001), pp. 157-161 (5).

See also

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