Tracing primary signatures in Eo-Paleoarchean terranes: Mineral-scale constraints on disturbed bulk-rock isotope records

• Heterogeneous Archean bulk-rock isotope records reflect isotope disturbances. • In-situ Hf-Nd isotope records support a ∼chondritic mantle reservoir through 3.8 Ga. • The Hf-Nd isotope records of minerals do not support significant mantle depletion pre 3.8 Ga. • Chemical behavior of allanite and titanite control isotope composition at bulk-rock scale. The Hf and Nd isotope records from Earth’s earliest history are essential for understanding the processes of crust-mantle 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 São 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 Ɛ Nd(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. Ɛ Nd(i) +4 to −12 Ɛ Hf(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.