Significance Geological processes like mantle convection or plate tectonics are an essential factor controlling Earth’s habitability. Our study provides insights into timescales of convective homogenization of Earth’s early mantle, employing the novel tool of high-precision 182 W isotope measurements to rocks from the Pilbara Craton in Australia, that span an age range from 3.5 billion years to 2.7 billion years. Previous 182 W studies mostly covered snapshots through geologic time, so the long-term 182 W evolution of the mantle has been ambiguous. Together with sophisticated trace element approaches, we can now provide an improved insight into such timescales, arguing for local preservation of primordial geochemical heterogeneities within Earth’s mantle as late as around 3.0 billion years, the putative onset of widespread plate tectonics on Earth.
Inferences on the early evolution of the Earth's mantle can be deduced from long-lived radiogenic isotope systems such as 176Lu-176Hf and 147Sm-143Nd, 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 176Lu-176Hf, 147Sm-143Nd, and, in a new approach, the 138La-138Ce 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. Mantle-derived 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 ∼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 138La-138Ce isotope system. Mafic rocks usually yield strongly coupled εHf(i), εNd(i) and εCe(i) values that form a mixing line between an evolving depleted upper mantle composition and the primitive mantle value (εHf(i) ca. 0.0 to + 3.2, εNd(i) ca. +0.2 to +1.7 and εCe(i) ca. +0.3 to -0.1). As all Paleoarchean samples lack co-variations between Nb/Th with εHf(i) or εNd(i), contamination with an enriched crust is unlikely to explain this mixing trend. The most primitive mantle-like mafic samples show elevated GdN/YbN 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 εHf(i) values at only moderately high εNd(i) and low εCe(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 176Lu/176Hf ratios but at moderate 147Sm/143Nd and 138La/138Ce ratios over time. The disappearance of rocks with decoupled Hf-Nd isotope compositions after ∼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.
Evidence of modern-style plate tectonics is preserved in the continental rock record as orogens and rifts; these orogens represent regions of mountain building resulting from compression between converging plates. Recognition of orogens in the ancient rock record can help identify when plate tectonics began on Earth. Evidence of Paleoproterozoic collisional orogeny is widely accepted. The development, however, of Archean collisional orogens is highly controversial, as is the operation of plate tectonics in general. We review the tectonic evolution of three well-studied Archean terranes—the Pilbara craton of Western Australia, the Barberton granite-greenstone terrane of South Africa, and the Superior Province of Canada—in terms of their geological development and evidence for Archean collisional and accretionary platetectonic processes in the context of secular evolution of the planet. The Pilbara craton preserves geological, geochemical, and geochronological evidence for continental rifting at 3.2 Ga, development of an oceanic-arc subduction complex at 3.12 Ga, and terrane accretion at 3.07 Ga. The Barberton granite-greenstone terrane of the Kaapvaal craton provides thermobarometric evidence for subduction-related high-pressure–low-temperature metamorphism juxtaposed against medium-pressure–high-temperature metamorphism associated with exhumation of high-grade rocks via orogenic collapse, which together are interpreted to represent a paired metamorphic belt. The Superior Province in the Canadian Shield records widespread accretionary and collisional assembly at ca. 2.7 Ga. This evidence argues for "modern-style" plate tectonics on Earth since at least 3.2 Ga.
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