SmNd

SmNd dating is a radiometric dating method based on the beta decay of the radioactive isotope 147Sm to 143Nd, with a half-life of about 106 billion years. Samarium and neodymium are rare earth elements whose geochemical behavior makes the Sm–Nd system useful for tracing mantle and crust differentiation in addition to dating rocks and meteorites. The key observable is the ratio 143Nd/144Nd, measured alongside 147Sm/144Nd, in minerals or whole-rock samples.

Application of the method relies on constructing a Sm–Nd isochron. A plot of 143Nd/144Nd versus 147Sm/144Nd for co-genetic materials yields a straight line whose slope equals e^(λt) − 1, where λ is the decay constant of 147Sm. From the slope, the crystallization or formation age t is obtained, and the intercept provides the initial 143Nd/144Nd ratio at formation. The technique often uses the epsilon notation, εNd(t), to express deviations of a sample’s 143Nd/144Nd from a reference reservoir, such as CHondritic Uniform Reservoir (CHUR). Model ages, such as the depleted mantle model age (TDM), are used to relate Sm–Nd data to planetary differentiation timelines.

Applications and significance include dating crust formation and mantle differentiation on Earth, constraining the timing of early crustal growth, and studying planetary evolution. The Sm–Nd system is complementary to U–Pb and Rb–Sr systems and is widely applied to meteorites and lunar samples. Limitations include potential disturbance by later metamorphism or melting, numerical uncertainties in decay constants and isotopic measurements, and the requirement for samples that record closed-system behavior since formation.