Isotope geochemistry

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The isotope geochemistry is the study of the distribution, fractionation and the radioactive decay of different isotopes of the chemical elements that make up the Earth and other objects in the solar system are built. Isotope geochemistry is a sub-area of geochemistry and can be divided into two sub-disciplines, the geochemistry of stable isotopes and the geochemistry of radiogenic isotopes . The sub-area of ​​isotope geochemistry, which is primarily devoted to clarifying geological issues, is also called isotope geology .

Stable isotopes

The geochemistry of stable isotopes deals with isotopes that are not subject to radioactive decay (non-radiogenic isotopes). Many chemical elements have multiple stable isotopes. In the case of chemical reactions, including those that take place in the context of biochemical processes, they sometimes behave differently (isotope fractionation). This means that the ratio of different isotopes of the same element differs between different substances ( minerals , waters , organic substances). Isotope ratios allow conclusions to be drawn about the development history of such geomaterials (see also →  Geo-archive ). Important stable isotopes are those of hydrogen , oxygen , carbon , sulfur , calcium , silicon , strontium and iron .

Radiogenic isotopes

The geochemistry of radiogenic isotopes is closely related to geochronology . Their methods are used to determine the age of formation or metamorphosis of minerals, rocks and fossil waters . It makes use of the fact that radioactive isotopes decay over time. As long as the atoms or isotopes can be exchanged between a geological object (e.g. a mineral) and its environment, there is a radioactive isotope x and a stable isotope y in what is known as an “initial isotope ratio”. This is determined by the rate of formation of the radioactive isotope through nuclear fusion in the sun or other stars on the one hand and its decay constant on the other hand, and can change over geological time periods. As soon as the exchange is stopped, because the temperature drops so far that no more significant diffusion takes place, the geochronological clock begins to tick. One speaks of a "closing temperature". The radioactive isotopes present in the mineral decay and there is no longer any replenishment of new radioactive isotopes. The concentration of a radioactive isotope to a stable isotope is thus a measure of the age of the mineral, which is measured from the point in time at which the closing temperature was last undershot. Important dating methods are uranium-lead dating , potassium-argon dating , argon-argon method , rhenium-osmium method, and radiocarbon method .

Investigation methods

The concentration of the isotopes is measured by means of mass spectrometry . For a general overview, not limited to geoscientific applications, see →  Isotope investigations .

literature

  • Hans-Günter Attendorn, Robert NC Bowen: Radioactive and Stable Isotope Geology. Springer-Verlag, 1997, ISBN 978-0-412-75280-3

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