Radiometric dating became a possibility with Becquerel’s discovery in 1896 of natural radioactivity. Rutherford postulated that radioactivity could be used to determine the age of the Earth. His and Soddy’s discovery (1902) of the transmutation of the atom became the basis for understanding exponential decay and the evolution of decay products (“daughter” elements). Age estimates for the Earth that had been determined by rate of heat loss (Lord Kelvin) now had to make allowances for the heat energy associated with radioactive decay. Thus, scholars were able to argue for great antiquity of the rocks on Earth. It was really with the advent of data collection technologies after World War II that the radiometric dating field began to develop with rapidity.
Radiometric dating must be viewed as having two forms: (1) techniques that rely on the decay of an isotope of an element, the production and decay of daughter decay products (radiocarbon dating, potassium-argon, argon-argon, and uranium-lead, uranium series) and (2) the techniques that rely on the crystal damage that is generated by the ionizing radiation generated by the decay of radioactive elements (thermo luminescence, electron spin resonance, and fission track).
All radiometric-dating determinations are a function of a statistical distribution of one or more sets of decay data that must be viewed as a probability result, approximating a particular age with an error attached to it. These errors in age determinations are usually expressed as standard deviations from a mean age value. These standard deviations are probability statements that a determined age actually will fall somewhere within the age distribution defined by the standard deviation. One standard deviation, often termed one sigma, means that 68% of the time, the determined age will fall between the range defined by that standard deviation. In the same manner, two- and three-sigma standard deviations mean the determined age will fall somewhere between the defined age range 95% and 99.6%, respectively. Therefore, it is immediately apparent that it would be a misnomer to suggest that these radiometric dating techniques were methods of absolute dating.
Radiocarbon-14 is the best known of the radio-metric techniques and is in fact an established method that relies only on the decay of an isotope (14C) formed from the outer-atmosphere comic-ray-generated neutron bombardment of Nitrogen-14 (14N) without reference to daughter production. In this respect, it is the most straightforward of the radiometric dating techniques. Living organisms incorporate 14CO2 and maintain an equilibrium until death, at which time the radiocarbon clock begins to tick as the 14C decays exponentially with a rate known as the Libby half-life (5,568 years). A simple ratio measurement of the amount of 14C remaining versus the amount present when the organism left the living biomass yields a radiocarbon age, which can be converted to calendrical years with a dendrochronological curve that corrects for the cosmic ray fluctuations that have taken place in the past. This ratio of original-to-remaining 14C is obtained in one of two ways: (1) The direct decay of the 14C back to 14 yields a beta ray that can be detected in a shielded Geiger counter; and (2) the actual counting of the individual 14C atoms present in a sample compared with the stable 12C in that sample and an accompanying standard. The atom-counting method affords an advantage in some dating situations (for example, shorter counting times, smaller sample sizes, no cosmic ray backgrounds, and the extension of the age range from less than 40,000 years to 70,000 years). Potassium-argon (40K/40Ar) and argon-argon dating (40Ar/39Ar), uranium-lead dating (U/Pb), and uranium series dating are all dependant upon both the exponential decay of a “parent” radioactive isotope and the buildup of one or more “daughter” isotopes, which may or may not themselves be radioactive. The ratio of the “daughter” to the “parent” permits an age estimate to be made. These techniques tend to have an age range orders of magnitude greater than radiocarbon (for example, age of the Earth), because they use half-lives that are very long in comparison to radiocarbon (tU for-40K: 1.28 x 109 yrs; 238U: 4.47 x 109 yrs; 235U: 7.038 x 108 yrs;232Th: 1.41 x 1,010 yrs), though uranium series disequilibrium dating has a dating range of a few days to about 20 million years.
Thermoluminescence (TL), electron spin resonance (ESR), and fission track are dating techniques that rely on the accumulation of radiation damage in materials from the decay of radioactive isotopes. TL and ESR depend on the #945, #946, and #947, decay of 40K, 238U, 235U, and 232Th in the natural environment and the consequent buildup of age information in the form of trapped electrons removed from their valence bands by the ionizing radiation. TL recovers this information by heating the sample or by optically measuring the trapped energy, or optically stimulated luminescence (OSL). ESR identifies radicals that have been formed by ionizing radiation in liquids and solids and electron traps in minerals. ESR measurement is accomplished by applying a microwave frequency to the sample that permits the amount of radiation damage to be quantified. The radiation environment within which a sample was exposed must be known well for both TL and ESR to be effective techniques. Both OSL and ESR techniques do not destroy the signal, as does the TL heating, permitting remeasurement of a sample multiple times. These techniques assume that there is a direct linear relationship between the ionizing radiation flux and the quantity of trapped electrons or radicals and that there are no secondary losses.
Fission track relies on the spontaneous fission of 238U into two heavy elements that travel through the mineral, creating track damage. The number of tracks present is a function of the original 238U concentration and the time that has elapsed since the mineral was formed or last heated. Once the tracks are counted, the mineral is heated to anneal the tracks and irradiated with neutrons to induce spontaneous fission in the 235U. By counting these induced tracks to determine the 235U content, the 238U concentration can be deduced and compared with the original track count to determine the age of the mineral. The method’s success assumes the fission tracks have not been subject to partial annealing.
References:
- Goksu, H. Y. (1991). Scientific dating methods. New York: Kluwer Academic.
- Herz, N., & Garrison, E. (1998). Geological methods for archaeology. Oxford: Oxford University Press.
- Rutter, N. W., & Catto, N. R. (Eds.). (1995). Dating methods for quaternary deposits. St. John’s, Canada: Geological Association of Canada.
