Comparative Sm-Nd isotope behavior of accessory minerals: Reconstructing the Sm-Nd isotope evolution of early Archean rocks

Crustal growth and mantle differentiation through Earth’s history are often traced using two radiogenic isotope systems - 176Lu-176Hf and 147Sm-143Nd. Unlike most post-Archean igneous rocks that show correlated initial Hf and Nd isotopic compositions, many ancient rocks have broadly chondritic zircon initial εHf values but highly variable whole-rock initial εNd values. These features have classically been interpreted as differences in the behavior of the Lu-Hf and Sm-Nd isotope systems during either deep magma ocean crystallization, subduction zone processes, or post-crystallization metamorphism. To clarify the cause of early Archean Hf-Nd isotope relationships, which are essential for understanding early Earth’s evolution, we investigated the in situ U-Th-Pb and Sm-Nd isotope systematics of co-existing titanite, apatite, and allanite—the major Sm-Nd carriers in early Archean felsic rocks—in a representative early Archean (3.5–3.4 Ga) tonalite-trondhjemite-granodiorite (TTG) suite from the Minnesota River Valley (MRV) terrane, northern USA. These rocks exhibit multiple generations of closed-system zircon growth with chondritic initial zircon Hf isotope signatures, and apparent decoupled zircon initial Hf and whole-rock Nd isotopic compositions, and thus serve as an useful test of the role of accessory minerals in controlling the whole rock isotopic signatures.