The Sm-Nd system is based on the alpha decay of 147Sm to 143Nd (half-life 106 Gyr). Because both Sm and Nd are light rare earth elements with very similar geochemical behavior, Sm/Nd fractionation during geological processes is small but systematic: partial melting preferentially concentrates the lighter Nd over the slightly heavier Sm in the melt, giving the crust lower Sm/Nd (and therefore lower time-integrated 143Nd/144Nd) than the residual mantle. Epsilon-Nd notation expresses 143Nd/144Nd as deviations from a chondritic reference (CHUR): positive epsilon-Nd indicates a depleted-mantle source; negative epsilon-Nd indicates enriched or crustal source. The Sm-Nd system is resistant to metamorphic resetting and provides robust constraints on crust-mantle differentiation, mantle source heterogeneity, and crustal residence ages.
The Sm-Nd system is the definitive tracer of crust-mantle differentiation because it directly records the fractionation between two elements that are separated only during major silicate melting events. Unlike Rb-Sr (where Rb and Sr have very different chemistry and are easily disturbed), Sm and Nd behave almost identically in all processes except partial melting of silicates.
The epsilon-Nd notation makes the system intuitive. CHUR (Chondritic Uniform Reservoir) represents the undifferentiated bulk Earth composition. Epsilon-Nd = [(143Nd/144Nd-sample)/(143Nd/144Nd-CHUR) - 1] x 10^4. Positive values mean the source has evolved with higher Sm/Nd than CHUR (depleted mantle, from which crust has been extracted). Negative values mean the source has evolved with lower Sm/Nd (continental crust or enriched mantle). The depleted mantle today has epsilon-Nd of approximately +8 to +12; old continental crust ranges from -10 to -40.
Epsilon-Nd vs 87Sr/86Sr plots (mantle arrays) are one of the most powerful discrimination tools in igneous petrology. Depleted mantle plots at high epsilon-Nd, low 87Sr/86Sr; continental crust plots at low epsilon-Nd, high 87Sr/86Sr. Mantle sources enriched by subducted sediment, metasomatism, or recycled oceanic crust plot in characteristic positions on this diagram, enabling identification of mantle source components in ocean island basalts, arc volcanics, and continental magmas.
Nd model ages (T-DM and T-CHUR) calculate when a sample's Nd isotopic composition intersects the depleted mantle or chondritic evolution curves, respectively. These are interpreted as crustal extraction ages -- the time when new continental crust was generated from the mantle. Global compilations of Nd model ages reveal episodic crustal growth, with major peaks at ~2.7, 1.9, and 1.1 Ga corresponding to supercontinent assembly events. This makes the Sm-Nd system a fundamental tool for understanding the growth and evolution of continental crust through Earth history.