Samarium-neodymium dating
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. Fractional crystallisation of felsic minerals 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.
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 1.06×1011 years. 146Sm is an almost-extinct nuclide which decays via alpha emission to produce 142Nd, with a half-life of 1.08×108 years. 146Sm is itself produced by the decay of 150Gd via alpha-decay with a half-life of 1.79×106 years.
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