Samarium–neodymium dating

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 (¹⁴⁷
Sm) to the stable radiogenic neodymium isotope (¹⁴³
Nd). 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 us to determine 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.