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Not AvailableThe ancient history of the Earth is marked by pivotal times recorded in Precambrian terrains, providing crucial insight into the evolution of the crust-mantle system and the geodynamic processes operating in the evolving Earth. This research focuses on the Zimbabwe craton, an important ancient terrain where fundamental concepts of Archean tectonics were first defined and that remains largely uncharacterized by high-precision isotope studies. We integrate in-situ LA-ICPMS zircon U-Pb ages and Hf isotope compositions of magmatic samples and river sands to investigate the Paleo- and Meso-Neoarchean (∼3.6–3.3 Ga and ∼2.9–2.7 Ga) crustal evolution of the craton. Zircon U-Pb data from gneiss complexes and granites within the central part of the craton, which encompasses the Sebakwe proto-craton that experienced tectonic stability temporarily between 3.3 and 3.0 Ga ago, define a main magmatic event at 3.6–3.5 Ga and minor components at ∼3.3 Ga, ∼2.9 Ga, and 2.7 Ga. Outside of the central part of the craton within the Odzi-Mutare area the magmatic record is dominated by zircon U-Pb ages between 2.9 and 2.8 Ga. A similar age distribution is recorded by detrital zircon from modern sediments with age clusters in the Paleoarchean at 3.6–3.5 Ga and 3.4–3.3 Ga, with the latter being more prominent, and in the Meso-Neoarchean at 2.9–2.8 Ga and 2.6 Ga. The Paleoarchean magmatic components yield Hf isotope compositions within 4 epsilon units of CHUR (ƐHf(3.6-3.3Ga) + 2 to –4), similar to the isotope compositions of detrital zircons (ƐHf(3.6-3.3Ga) + 2 to –6). Meso-Neoarchean magmatic components yield Hf isotope compositions with ƐHf(i) between CHUR and +2 at ∼2.9–2.8 Ga, while detrital zircons range from ƐHf(i) + 4 to –9 at ∼2.9–2.6 Ga. Data from the Zimbabwe craton indicate crust formation in the Paleoarchean (3.6–3.3 Ga) that involved juvenile magma additions and a possible derivation from a ∼3.8 Ga chondritic source and craton-wide crust forming episodes between ∼2.9 and 2.6 Ga that involved crustal reworking in various degrees along with new additions of juvenile material. The Hf isotope record of the Zimbabwe craton aligns with the global isotope record—with chondritic to sub-chondritic Hf isotope compositions at ∼3.7–3.6 Ga, the appearance of juvenile additions after ∼3.5 Ga, and mixing trends after ∼3.0 Ga—supporting formation of a long-lived depleted mantle reservoir after ∼3.6 Ga. 

The Hf and Nd isotope records from Earth’s earliest history are essential for understanding the processes of crustmantle evolution in the early Earth. These records, however, have been largely obtained from bulk-rock samples that have often been compromised by post-crystallization processes. Complexities resulting from open-system behavior and mixing of different components are reflected in heterogeneous Eo-Paleoarchean bulk-rock Hf and Nd isotope records. In this study, we integrate in-situ Sm-Nd isotope analyses of REE-rich minerals with U-Pb titanite geochronology to assess the reliability of the bulk-rock isotope record in ~3.6 Ga gneisses from the northern S˜ao Francisco Craton (NSFC). Our results demonstrate that Nd isotope compositions of minerals controlling the elemental budget in rocks provide a more robust record of the early Earth. The NSFC mineral phases preserving primary signatures have chondritic ƐNd(i) (0.0 to +0.3) at their crystallization ages (~3.6 Ga). In contrast, bulk-rock Hf and Nd isotope compositions exhibit extreme heterogeneity (i.e. ƐNd(i) +4 to −12 ƐHf(i) +8 to −14) reflecting post-crystallization disturbances at 2.5–2.0 Ga. When mineral phases in these rocks have Sm- Nd isochron ages that agree with their U-Pb crystallization ages, their initial Nd isotope compositions are consistent with the zircon Hf isotope data. These findings align with global zircon Hf isotope data, which indicate a chondritic mantle reservoir until ~3.8 Ga, with evidence of significant mantle depletion and crustal differentiation emerging in the isotope record thereafter. This study underscores the importance of mineral-scale isotope investigations in refining interpretations of early crust-mantle evolution and the mechanisms affecting bulk-rock isotope records in Precambrian terranes. 

Precambrian terrains preserving rocks older than 3.5 Ga contain an essential record of the crustal evolution of the primitive Earth. In this study, we investigated Eo-Paleoarchean rocks from the northern S˜ao Francisco Craton (NSFC) and the Borborema Province in northeastern Brazil to contribute to a more complete global isotopic record of this pivotal time in Earth’s history. Zircon U-Pb ages along with zircon Hf isotope compositions were obtained for migmatitic gneiss complexes in both terrains. Zircon U-Pb data from the NSFC yield well-defined populations with 207Pb/206Pb ages from 3.61 to 3.59 Ga and younger components at ~3.5 and ~3.4 Ga. Similarly, the Borborema Province gneiss yields a main zircon age population of 3.58 Ga and a younger ~3.5 Ga age component. The ~3.6 Ga zircon components yield consistently sub-chondritic Hf isotopic compositions with initial εHf between −1.9 and −3.1 for the NSFC and of εHf −0.5 for the Borborema Province. Gneisses from northeastern Brazil record a main crust forming period at 3.65–3.60 Ga with sub-chondritic Hf isotope compositions that indicate derivation from melting of a ~3.8 Ga source of broadly chondritic isotope composition, similar to that of many Eo-Paleoarchean gneisses worldwide. This Hf isotope record supports the existence of broadly chondritic mantle reservoir in the Eoarchean with development of depleted mantle and the appearance of evolved crust later in the Paleoarchean.