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. It is the most used method of geochronology, the main way to learn the age of rocks and other geological features, including the age of the Earth itself.
It is used to date many kinds 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. Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead 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. 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 mathematical expression that relates radioactive decay to geologic time is:
where
This equation uses information on the parent and daughter isotopes at the time the material solidified. This is well known for most isotopic systems. 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.
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.
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.
Gerard López
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Tatsuya Okamoto
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Takashi Fujii
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Go Nakamura
Go Nakamura (born 29 November 1986) is a Japanese football player. He plays for Gifu (reserve team).
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Toru Morino
Toru Morino (born 12 May 1987) is a Japanese football player.
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Noboru Kohara
Noboru Kohara (born 22 July 1983) is a former Japanese football player.
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Sho Kamogawa
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Yusuke Sudo
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Shingo Wada
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Ryota Takahashi
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K–Ar dating
Potassium–Argon dating or K–Ar dating is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar).
Potassium is a common element found in many materials, such as micas, clay, tephra, and evaporites. In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to build up when the rock solidifies (recrystallises). Time since recrystallization is calculated by measuring the ratio of the amount of Ar to the amount of K remaining. The long half-life of K is more than a billion years, so the method is used to calculate the absolute age of samples older than a few thousand years.
Quickly cooled lavas make nearly ideal samples for K–Ar dating. They also preserve a record of the direction and intensity of the local magnetic field at that time. The geomagnetic polarity time scale was calibrated largely using K–Ar dating.
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Ivan Leko (born 7 February 1978) is a Croatian football manager and former player. He has played for Croatia national team.
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Jerko Leko (born 9 April 1980) is a Croatian football player. He plays for Monaco and Croatia national team.
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Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes.
It can be used over an age range of about 1 million years to over 4.5 billion years. Precision is in the 0.1-1 percent range.
The method relies on two separate decay chains, the uranium series from U to Pb, with a half-life of 4.47 billion years and the actinium series from U to Pb, with a half-life of 704 million years.
The existence of two 'parallel' uranium-lead decay routes allows several dating techniques within the overall U-Pb system.
The term 'U-Pb dating' normally implies the coupled use of both decay schemes. However, use of a single decay scheme (usually U to Pb) leads to the U-Pb isochron dating method, analogous to the rubidium-strontium dating method.
Finally, ages can also be determined from the U-Pb system by analysis of Pb isotope ratios alone. This is termed the lead-lead dating method. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium-lead radiometric dating methods, is famous for having used it to obtain one of the earliest accurate estimates of the age of the Earth.
Uranium-lead dating is usually performed on the mineral zircon (ZrSiO), though it can be used on other minerals. Zircon incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Therefore, we can assume that the entire lead content of the zircon is radiogenic. Where this is not the case, a correction must be applied. Uranium-lead dating techniques have also been applied to other minerals such as calcite/aragonite and other carbonate minerals. These minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record.
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Pedro Cea