Samarium–neodymium dating
radiometric dating method useful for determining ages of rocks and meteorites
Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on the alpha decay of the long-lived samarium isotope (147Sm) to the stable radiogenic neodymium isotope (143Nd). Neodymium isotope ratios together with samarium–neodymium ratios are used to provide information on the age and source of igneous melts. It is sometimes assumed that at the moment when crustal material is formed from the mantle the neodymium isotope ratio depends only on the time when this event occurred, but thereafter it evolves in a way that depends on the new ratio of samarium to neodymium in the crustal material, which will be different from the ratio in the mantle material. Samarium–neodymium dating allows the determination of when the crustal material was formed.
The usefulness of Sm–Nd dating stems from the fact that these two elements are rare earth elements and are thus, theoretically, not particularly susceptible to partitioning during sedimentation and diagenesis. Fractional crystallisation of felsic minerals changes the Sm/Nd ratio of the resultant materials. This, in turn, influences the rate at which the 143Nd/144Nd ratio increases due to production of radiogenic 143Nd.
In many cases, Sm–Nd and Rb–Sr isotope data are used together.
Sm–Nd radiometric dating
Samarium has seven naturally occurring isotopes, and neodymium has seven. The two elements are joined in a parent–daughter relationship by the alpha decay of parent 147Sm to radiogenic daughter 143Nd with a half-life of 1.066(5)×1011 years and by the alpha decay of 146Sm (an almost-extinct radionuclide with a half-life of 9.20(26)×107 years) to produce 142Nd.
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