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Samarium-neodymium dating is useful for determining the age relationships of rocks and meteorites, based on radioactive decay of a long-lived samarium (Sm) isotope to a radiogenic neodymium (Nd) isotope. Nd isotope ratios are used to provide information on the source of igneous melts as well as to provide age data. The various reservoirs within the solid earth will have different values of initial 143Nd/144Nd ratios, especially with reference to the mantle. The usefulness of Sm-Nd dating is the fact that these two elements are rare earths. They are thus, theoretically, not particularly susceptible to partitioning during melting of silicate rocks. The fractionation effects of crystallisation of felsic minerals (see above) changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the 143Nd/144Nd ratios with ingrowth of radiogenic 143Nd. The mantle is assumed to have undergone chondritic evolution, and thus deviations in initial 143Nd/144Nd ratios can provide information as to when a particular rock or reservoir was separated from the mantle within the Earth's past. In many cases, Sm-Nd and Rb-Sr isotope data are used together. ==Sm-Nd radiometric dating== Samarium has five naturally occurring isotopes and neodymium has seven. The two elements are joined in a parent-daughter relationship by the alpha-decay of 147Sm to 143Nd with a half life of 1.06 years. 146Sm is an almost-extinct nuclide which decays via alpha emission to produce 142Nd, with a half-life of 1.08 years. 146Sm is itself produced by the decay of 150Gd via alpha-decay with a half-life of 1.79 years. An isochron is calculated normally. As with Rb-Sr and Pb-Pb isotope geochemistry, the initial 143Nd/144Nd ratio of the isotope system provides important information on crustal formation and the isotopic evolution of the solar system. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Samarium-neodymium dating」の詳細全文を読む スポンサード リンク
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