Evidence for inherited Sm-Nd isotopes in granitoid zircons

This study presents evidence to show that, in addition to preserving U-Pb isotope systematics, refractory zircons also preserve, at least in part, an inherited Sm-Nd isotope component. The zircons analyzed during this study were taken from the Strontian granitoid (NW Scotland). The inner intrusion of this composite pluton is known from a previous study to contain substantial U-Pb zircon inheritance, whereas the outer part has only minor inheritance. Zircons from the inner intrusion were found to have significantly lower εNd₄₂₅ values than either their host rock, separated apatite or monazite. It is argued that this isotopic disequilibrium is due to the presence of an inherited Sm-Nd isotope component, rather than being due to a post-crystallization disturbance of the zircons. The preservation of inherted Sm-Nd isotopes within refractory zircons implies that they remain closed systems with respect to the diffusion of Sm and Nd (and presumably the other REE) to temperatures in excess of 700°C. The fact that zircons commonly have high Sm/Nd ratios, relative to sialic crustal material, means that the Nd isotopic evolution of inherited zircons will be very different to that of much of the continental crust.