Samarium–neodymium dating is a radiometric dating method useful for determining the ages of rocks and meteorites, based on the radioactive decay of a long-lived samarium isotope (147Sm) to a radiogenic neodymium isotope (143Nd). Neodymium isotope ratios together with samarium-neodymium ratios are used to provide information on age information and the 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.