Evolution of the early to late Archean mantle from Hf-Nd-Ce isotope systematics in basalts and komatiites from the Pilbara Craton

Inferences on the early evolution of the Earth's mantle can be deduced from long-lived radiogenic isotope Lu-176-Hf-176 and systems such as Sm-147-Nd-143, for which both parent and daughter elements largely remain immobile at low metamorphic grades. However, it remains ambiguous when and to what extent mantle-crust differentiation processes had started in the Archean. For a better understanding of Archean mantle-crust evolution, we determined the initial Lu-176-Hf-176, Sm-147-Nd-143, and, in a new approach, the La-138-Ce-138 isotope compositions of a suite of Archean mafic-ultramafic rock samples from the 3.53-2.83 Ga old Pilbara Craton and 2.78-2.63 Ga old Fortescue Group in NW Australia. These rocks represent one of the best-preserved Archean successions worldwide and contain mafic-ultramafic rocks that were erupted during repeated and long-lived pulses of volcanism throughout much of the Archean. Mantlederived mafic-ultramafic rock samples were collected from six major stratigraphic groups of the Pilbara Craton and the overlying Fortescue Group in order to characterize the parental mantle source regions of the lavas and to reconstruct the temporal evolution of the ambient mantle beneath this piece of cratonic lithosphere. In addition, we analyzed contemporaneous TTG-like igneous suites and interbedded sediments in order to reconstruct the lithospheric evolution of the Pilbara Craton. The Hf-Nd-Ce isotope data imply the onset of mantle-crust differentiation in the Pilbara Craton as early as similar to 4.2 Ga, well prior to any of the preserved stratigraphy. Within error, coupled Ce-Nd-Hf isotope arrays all intersect chondritic values, implying that the Earth is of broadly chondritic composition, also for the La-138-Ce-138 isotope system. Mafic rocks usually yield strongly coupled epsilon Hf-(i), epsilon Nd-(i) and epsilon Ce-(i) values that form a mixing line between an evolving depleted upper mantle composition and the primitive mantle value (epsilon Hf-(i) ca. 0.0 to + 3.2, epsilon Nd-(i) ca. +0.2 to +1.7 and epsilon Ce-(i) ca. +0.3 to -0.1). As all Paleoarchean samples lack co-variations between Nb/Th with epsilon Hf-(i) or epsilon Nd-(i), contamination with an enriched crust is unlikely to explain this mixing trend. The most primitive mantle-like mafic samples show elevated Gd-N/Yb-N ratios (2.2-1.4), implying the involvement of a deep-rooted, near-primitive, upwelling mantle that was progressively mixed into the depleted upper mantle. In contrast to the mafic rocks, most, but not all komatiites are decoupled in their initial Hf-Nd-Ce isotope compositions, by having extremely radiogenic epsilon Hf-(i) values at only moderately high epsilon Nd-(i) and low epsilon Ce-(i) values. This decoupling is best explained by the assimilation of mantle domains that underwent early melt depletion in the garnet stability field and evolved at high Lu-176/Hf-176 ratios but at moderate Sm-147/Nd-143 and La-138/Ce-138 ratios over time. The disappearance of rocks with decoupled Hf-Nd isotope compositions after similar to 3.2 Ga is likely linked to decreasing mantle temperatures that were no longer able to melt such refractory mantle domains. Collectively, our new data for mafic rocks from the Pilbara Craton confirm the presence of long-term depleted mantle domains in the early Archean that are not sampled by the zircon Hf isotope record in the Pilbara Craton. (C) 2020 The Authors. Published by Elsevier B.V