Radiometric dating: Encyclopedia II - Radiometric dating - Fundamentals of radiometric dating

Radiometric dating - Fundamentals of radiometric dating

All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing only in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some random point in time, an atom of such a nuclide will be transformed into a different nuclide by the process known as radioactive decay. This transformation is accomplished by the emission of particles such as electrons (known as beta decay) or alpha particles.

While the moment in time at which a particular nucleus decays is random, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life, usually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the substance in question will have decayed. Many radioactive substances decay from one nuclide into a final, stable decay product (or "daughter") through a series of steps known as a decay chain. In this case, usually the half-life reported is the dominant (longest) for the entire chain, rather than just one step in the chain. Nuclides useful for radiometric dating have half-lives ranging from a few thousand to a few billion years.

In most cases, the half-life of a nuclide depends solely on its nuclear properties; it is not affected by temperature, chemical environment, magnetic and electric fields, or any other external factors.¹ The half-life of any nuclide is also believed to be constant through time. Although decay can be accelerated by radioactive bombardment, such bombardment tends to leave evidence of its occurrence. Therefore, in any material containing a radioactive nuclide, the proportion of the original nuclide to its decay product(s) changes in a predictable way as the original nuclide decays. This predictability allows the relative abundances of related nuclides to be used as a clock that measures the time from the incorporation of the original nuclide(s) into a material to the present.

The processes that form specific materials are often conveniently selective as to what elements they incorporate during their formation. In the ideal case, the material will incorporate a parent nuclide and reject the daughter nuclide. In this case, the only daughter nuclides to be found through examination of a sample must have been created since the sample was formed. When a material incorporates both the parent and daughter nuclides at the time of formation, it may be necessary to assume that the initial proportions of a radioactive substance and its daughter are known. The daughter product should not be a small-molecule gas that can leak out of the material, and it must itself have a long enough half-life that it will be present in significant amounts. In addition, the initial element and the decay product should not be produced or depleted in significant amounts by other reactions. The procedures used to isolate and analyze the reaction products must be straightforward and reliable.

If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the "blocking temperature" and is specific to a particular material.

In contrast to the most simple radiometric dating techniques, isochron dating, which can be used for many isotopic decay sequences (e.g. rubidium-strontium decay sequence), does not require knowledge of the initial proportions. Also the argon-argon dating technique can be used for the potassium-argon sequence to ensure that no initial ⁴⁰Ar was present.

Radiometric dating - Limitation of techniques

Although radiometric dating is accurate in principle, the precision is very dependent on the care with which the procedure is performed. The possible confounding effects of initial contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. Additionally, measurement in a mass spectrometer is subject to isotopic interference of other nuclides with the same mass number. Corrections may have to be performed by measuring isotopic ratios of elements which interfere with the target isotope.

Mass spectrometers are liable to interferences and inaccuracies. Primary amongst these is the quality of the vacuum. Poor vacuum permits gaseous atoms to intercept ionised atoms which are meant to be measured. The resolution of the receptor is also a factor, but modern equipment is greatly improved on previous editions.

Precision is enhanced if measurements are taken on different samples taken from the same rock body but at different locations. Alternatively, if several different minerals are able to be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample.

The precision of a method of dating depends in part on the half-life of the radioactive isotope involved. For instance, carbon-14 has a half-life of less than 6000 years. After an organism has been dead for 60,000 years, so little carbon-14 is left in it that accurate dating becomes impossible. On the other hand, the concentration of carbon-14 falls off 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 it more accurate than radiocarbon dating.

Adapted from the Wikipedia article "Fundamentals of radiometric dating", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki