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

Abstract

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.Our new U-Pb results show that the REE-rich accessory phases were reset during the 2.60–2.58 Ga Sacred Heart Orogeny, representing the accretion of the MRV terrane to the southern margin of the Superior Craton. Additionally, two samples contain apatite that yields significantly younger U-Pb dates of 1.9–1.8 Ga, suggesting that a portion of the MRV basement was thermally overprinted by the ∼1.85 Ga Penokean Orogeny. The ∼2.6 Ga Sm-Nd array recorded by apatite and titanite in these rocks suggest terrane-wide Sm-Nd isotopic re-equilibration in the MRV during the Sacred Heart Orogeny, with no Penokean disturbance of the Sm-Nd isotope systematics. Nevertheless, allanite in one sample has survived the post-crystallization 147Sm-143Nd resetting event, and yielded a Sm-Nd isochron date broadly consistent with the crystallization age of the host rock (∼3.4 Ga), with chondritic initial εNd. Therefore, allanite appears to be an important target in order to obtain the primary Nd isotopic signature of early Archean rocks in ancient terranes. In contrast to the ambiguous whole-rock data, the Nd isotopic compositions of the coexisting apatite, titanite and allanite collectively reconstruct a clear crustal evolution history in which the crust was repeatedly re-melted in a closed-system since formation at 3.5 Ga, initially from a source evolving with broadly chondritic Lu-Hf and Sm-Nd systematics. This suggests a minimally differentiated, or already rehomogenized, mantle at 3.5 Ga. Our study highlights the importance of using REE-rich accessory phases to obtain a clear Nd isotopic record to constrain the history of continent formation on the early Earth.