<p>One of the fundamental tenets of geochemistry is that the Earth&#8217;s crust has been extracted from the mantle creating a crustal reservoir enriched&#8212;and a mantle depleted&#8212;in incompatible elements. The Hf-Nd isotope record has long been used to help understand the timing of this process. Increasingly, however, it has become apparent that these two isotope records do not agree for Earth&#8217;s oldest rocks. Hf isotopes of zircon from juvenile, nominally mantle-derived rocks throughout the Eoarchean have broadly chondritic initial isotope compositions and indicate large-scale development of the depleted mantle reservoir started no earlier than ~ 3.8 Ga. In contrast, the long-lived Sm-Nd isotope record shows large variation in Nd isotope compositions. Most notably, Paleo- and Eoarchean terranes with chondritic initial Hf isotope compositions have significantly radiogenic Nd isotope compositions indicative of the development of a widespread depleted mantle reservoir very early in Earth&#8217;s history which, by extension, requires extraction of significant volumes of enriched crust. These two isotope systems, therefore, indicate two fundamentally different scenarios for the early Earth and has been called the Hf-Nd paradox. However, an important unresolved question remains: Do these records represent primary isotopic signatures or have they been altered by subsequent thermomagmatic processes? We have been able to provide clarity in the Hf isotope record by analyzing zircon from Eo- and Paleoarchean magmatic rocks by determining its U-Pb crystallization age and linking this to its corresponding Hf isotope composition. We can do this unambiguously&#8212;even in complex polymetamorphic gneisses&#8212;with the laser ablation split stream (LASS) technique whereby we determine U-Pb age and Hf isotope composition simultaneously in a single zircon volume. The existing Nd isotope data, in contrast, are all from bulk-rock analyses. These analyses are potentially problematic in old, polymetamorphic rocks because of the inability to link the measured isotopic composition to a specific age. In addition, the REE budget in these rocks is hosted by accessory phases that can be easily mobilized during later metamorphic and magmatic events. We can now use the LASS approach in REE rich phases (e.g., monazite, titanite, allanite, apatite) to determine U-Pb age and Nd isotope composition in a single analytical volume. New Nd isotope data from the Acasta Gneiss Complex (Fisher et al., EPSL, 2020) show that REE-rich accessory phases are not in isotopic equilibrium with their bulk rock compositions and clearly demonstrate mobilization after initial magmatic crystallization. This post-magmatic open-system behavior may well explain the disagreement in the Hf-Nd isotope record in high-grade polymetamorphic terranes like Acasta. In less complicated, lower-grade rocks, such as in the Pilbara terrane, these REE-rich phases yield consistent U-Pb and Sm-Nd age and isotope compositions indicating that the Nd isotope system in these rocks has remained closed since formation. Of particular note, in the Pilbara samples, the Hf and Nd isotope systems have consistent, broadly chondritic, initial Hf and Nd isotope compositions. In these less-complicated samples, where the Sm-Nd isotope system has remained closed, the Hf and Nd isotope systems agree and there is no Hf-Nd paradox.</p>
The East Pilbara Craton is a classic example of a dome-andkeel granite-greenstone terrain. As these rocks are among the oldest on Earth, their origin is fundamentally important for understanding how the earliest continents formed. One theory is that dome-and-keel structures are the result of a buoyancy-driven crustal overturn process which predated mobile-lid tectonics on Earth. We address this problem with five well-preserved metarhyolitic rocks collected from the Warrawoona Greenstone Belt in the East Pilbara Craton. These rocks contain both zircon and garnet. We use zircon U-Pb dates to determine the crystallization age of these rhyolites, and Hf isotopes to understand melt sources. We use garnet Lu-Hf and Sm-Nd dates to help understand the timing of the deformation associated with the formation of these dome-and-keel structures. Furthermore, we integrate the microstructures in these rocks with garnet Lu-Hf and Sm-Nd ages to date the formation of the dome-and-keel structures in the East Pilbara. Zircon in these 5 samples have U-Pb ages between 3.45 and 3.46 Ga and have relatively uniform ε Hf(i) between +0.2 and +0.8, indicating these rocks were derived from a reservoir with a time integrated chondritic Lu/Hf. Garnet Lu-Hf ages of two samples record garnet growth at 3.42 Ga, which we interpret as recording a cryptic metamorphic event early in the evolution of the Pilbara Craton. The other three samples have garnet Lu-Hf ages between 3.33 and 3.29 Ga, which coincide with intrusion of granitic rocks throughout the Pilbara Craton. Using microstructures in these rocks, we suggest this younger garnet growth event overlapped with the beginning of dome-associated deformation of the Warrawoona Greenstone Belt. This relationship also constrains the beginning of domeand-keel formation in the East Pilbara at ~3.32 Ga. All samples have systematically younger garnet Sm-Nd ages, between 3.35 and 3.22 Ga. These younger Sm-Nd ages record protracted dome formation associated with continued granitic magmatism in the East Pilbara until cooling of the system at ~3.22 Ga. Ultimately, our data indicate that the East Pilbara dome-and-keel structures formed at ~3.32 Ga, and there is a close relationship between the 3.3-3.2 Ga granitic magmatism and dome formation.
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