Radiometric dating

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Ale's Stones at Kåseberga, around ten kilometres south east of Ystad, Sweden were dated at 600 AD using the Carbon–14 method on organic material found at the site.[1]

Radiometric dating (often called radioactive dating) is a way to find out how old something is. The method compares the amount of a naturally occurring radioactive isotope and its decay products, in samples. The method uses known decay rates.[2]

It is the main way to learn the age of rocks and other geological features, including the age of the Earth itself. It may be used to date a wide range of natural and man-made materials. Fossils may be dated by taking samples of rocks from above and below the fossil's original position. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Radiometric dating methods are used to establish the geological time scale.[3] Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.

Radioactive decay[change | edit source]

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. Elements exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are naturally unstable. That is, at some point in time, an atom of such a nuclide will spontaneously change into a different nuclide by radioactive decay. The decay may happen by emission of particles (usually electrons (beta decay), positrons or alpha particles) or by spontaneous nuclear fission, and electron capture.

The age equation[change | edit source]

Samarium-neodymium (Sm/Nd) isochron plot of samples.[4] from the Great Dyke, Zimbabwe. The age is calculated from the slope of the isochron (line) and the original composition from the intercept of the isochron with the y-axis.

The mathematical expression that relates radioactive decay to geologic time is:[5][6]

D = D0 + N(eλt − 1)

where

t is age of the sample,
D is number of atoms of the daughter isotope in the sample,
D0 is number of atoms of the daughter isotope in the original composition,
N is number of atoms of the parent isotope in the sample, and
λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope times the natural logarithm of 2.[7]

This equation uses information on the parent and daughter isotopes at the time the material solidified. This is well known for most isotopic systems.[8][9] Plotting an isochron (straight-line graph) is used to solve the age equation graphically. It shows the age of the sample, and the original composition.

Preconditions[change | edit source]

The method works best if neither the parent nuclide nor the daughter product enters or leaves the material after its formation. Anything which changes the relative amounts of the two isotopes (original and daughter) must be noted, and avoided if possible. Contamination from outside, or the loss of isotopes at any time from the rock's original formation, would change the result. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.[10]

Measurements should be taken on samples from different parts of the rock body. This helps to counter the effects of heating and squeezing, which a rock may experience in its long history. Different dating methods may be needed to confirm the age of a sample. For example, a study of the Amitsoq gneisses from western Greenland used five different radiometric dating methods to examine twelve samples and got agreement to within 30 million years on an age of 3,640my.[11][12]

Related pages[change | edit source]

References[change | edit source]

  1. "Ales stenar". The Swedish National Heritage Board. 11 October 2006. http://www.raa.se/cms/extern/en/places_to_visit/our_historical_sites/ales_stenar.html. Retrieved 9 March 2009.
  2. International Union of Pure and Applied Chemistry. Radioactive dating. Compendium of chemical terminology, internet edition. [1]
  3. McRae A. 1998. Radiometric dating and the geological time scale: circular reasoning or reliable tools? Radiometric dating and the geological time scale. TalkOrigins Archive
  4. Oberthür T et al. (2002). "Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt". Precambrian Research 113: 293–306. doi:10.1016/S0301-9268(01)00215-7.
  5. Faure, Gunter (1998). Principles and applications of geochemistry: a comprehensive textbook for geology students (2nd ed.). Englewood Cliffs, New Jersey: Prentice Hall. ISBN 0-02-336450-5. OCLC 37783103.
  6. White, W. M. (2003). "Basics of radioactive isotope geochemistry". Cornell University. http://www.geo.cornell.edu/geology/classes/Geo656/656notes03/656%2003Lecture04.pdf.
  7. "Geologic time: radiometric time scale". United States Geological Survey. 16 June 2001. http://pubs.usgs.gov/gip/geotime/radiometric.html.
  8. Rollinson, Hugh R. (1993). Using geochemical data: evaluation, presentation, interpretation. Harlow: Longman. ISBN 0-582-06701-4. OCLC 27937350.
  9. Stacey, J.S.; J.D. Kramers (June 1975). "Approximation of terrestrial lead isotope evolution by a two-stage model". Earth and Planetary Science Letters 26 (2): 207–221. doi:10.1016/0012-821X(75)90088-6. http://adsabs.harvard.edu/abs/1975E&PSL..26..207S.
  10. Stewart K. et al. (1996). "3-D, 40Ar---39Ar geochronology in the Paraná continental flood basalt province". Earth and Planetary Science Letters 143: 95–109. doi:10.1016/0012-821X(96)00132-X.
  11. Dalrymple, G.B. (1991). The age of the Earth. Stanford University Press. ISBN 0-8047-1569-6.
  12. Dalrymple G.B. 2004. Ancient Earth, ancient skies: the age of the Earth and its cosmic surroundings. Stanford.