Radiometric Dating Techniques

In general dating techniques are methods of estimating the true age of rocks, paleontologists specimens, archaeological sites, and so on. Relative dating techniques date specimens in relation to each other; for example, stratigraphy is used to establish the succession of fossils. Absolute (or chronometric) techniques give an absolute estimate of the age and fall into two main groups.

The first depends on the existence of something that develops at a seasonally varying rate, as in dendrochronology and varve dating. The other uses some measurable change that occurs at a known rate as in chemical dating, and radiometric (or radioactive) dating. The types of radiometric dating currently available are

  1. Carbon dating
  2. Fission-track dating
  3. Potassium-argon dating
  4. Rubidium-strontium dating
  5. Uranium-lead dating
With the exception of carbon dating, radiometric dating depends for its accuracy on the premise that radioactive decay proceeds at a constant rate and there is no experimental evidence to suggest otherwise. The decay of radioactive elements is the spontaneous transformation of one radioactive nuclide into a daughter nuclide, which may be radioactive or may not, with the emission of one or more particles or photons. The decay of N0 nuclides to give N nuclides after time t is given by:

N = N0exp(-yt),

where y is called the decay constant (also called the disintegration constant). The reciprocal of the decay constant is the mean life. The time required for half the original nuclides to decay (i.e. N = 0.5N0) is called the half-life of the nuclide.

All radiometric dating, is based upon the premise that the radioactive decay rates of given isotopes are constant. There is a great deal of experimental evidence to suggest that decay rates are constant, and no evidence to suggest that they aren't, indeed if the decay rates of elements such as uranium or plutonium had exhibited varying decay rates both the nuclear power industry and nuclear weapons production would be in serious trouble from a production point of view.

Carbon Dating

A method of estimating the ages of archaeological specimens of biological origin. As a result of cosmic radiation a small number of atmospheric nitrogen nuclei are continuously being transformed by neutron bombardment into radioactive nuclei of carbon-14:

147N + n ->146C + P

Some of these radiocarbon atoms find their way into living trees and other plants in the form of carbon dioxide, as a result of photosynthesis. When the tree is cut down photosynthesis stops and the ratio of radiocarbon atoms to stable carbon atoms begins to fall as the radiocarbon decays. The ratio of 14C/12C in the specimen can be measured and enables the time that has elapsed since the tree was cut down to be calculated. The method has been shown to give consistent results for specimens up to some 40,000 years old, though its accuracy depends upon assumptions concerning the past intensity of the cosmic radiation. The technique was developed by Willard F. Libby (1908-80) and his coworkers in 1946-47.

Has the level of background cosmic radiation varied significantly over the last 40,000 years? If the cosmic radiation level had been significantly higher, more carbon-14 would have been produced, and test objects would look younger than they actually were. On the other hand, if the level of cosmic background radiation were less, objects would look older than they actually were because less carbon-14 would have been produced for ingestion by biological entities.

The major source of cosmic radiation that impinges on the earth is the sun. The question now becomes, has the sun shown any significant variation in energy output over the last 40,000 years? Model calculations conclude that the Sun becomes 10 percent brighter every billion years, but such a change is insignificant over a mere 40,000 year timescale (0.0016% increase). Other than that the Sun is a very stable source of energy; its radiative output, called the solar constant, is 137 ergs per square metre per second (ergs/m /sec), or 1.98 calories per square centimeter per minute (cal/cm /min), at the Earth and varies by no more than 0.1 percent.

Fission Track Dating
A method of estimating the age of glass and other mineral objects by observing the tracks made in them by the fission fragments of the uranium nuclei that they contain. By irradiating the objects with neutrons to induce fission and comparing the density and number of tracks before and after irradiation it is possible to estimate the time that as elapsed since the object solidified.

Potassium-Argon Dating
A dating technique for certain rocks that depends on the decay of the radioisotope potassium-40 to argon-40, a process with a half-life of 1.27 x 1010 years. It assumes that all the argon-40 formed in the potassium-bearing mineral accumulates within it and that all the argon present is formed by the decay of potassium-40. The mass of argon-40 and potassium-40 in the sample is estimated and the sample is then dated from the equation:

40Ar = 0.1102 40K(ect-1)

where c is the decay constant and t is the time in years since the mineral cooled to about 300Cwhen the 40Ar became trapped in the crystal lattice. The method id effective for micas, feldspar, and some other minerals.

Rubidium-Strontium Dating
A method of dating geological specimens based on the decay of the radioisotope rubidium-87 in to the stable isotope strontium-87. Natural rubidium contains 27.85% of rubidium-87, which has a half-life of 4.7 x. 1010 years. The ratio 87Rb/87Sr in a specimen gives an estimate of its age (up to several thousand million years).

Uranium-Lead Dating
A group of methods of dating certain rocks that depends on the decay of the radioisotope uranium-235 to lead-207 (half-life 71. x 108 years). One form of uranium-lead dating depends on measuring the ratio of the amount of helium trapped in the rock to the amount of uranium present (since the decay 238U to 206Pb releases eight alpha-particles).

Another method of calculating the age of the rocks is to measure the ratio of radiogenic lead (206Pb, 207Pb) present to nonradiogenic lead (204Pb). These methods give reliable results for ages of the order of ten million to one billion years.