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1. U-Pb and Lu-Hf isotopic evolution of ∼3.6 Ga remnants in NE Brazil – Implications on Eo-Paleoarchean global crustal evolution

   https://doi.org/10.1016/j.chemgeo.2025.122646  

   Botero, Manuela; Vervoort, Jeffrey D; Meira, Vinicius T; Martins_de_Sousa, Daniel F; Santos, Ticiano JS (April 2025, Chemical Geology)

   NA (Ed.)

   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. 

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2. Growth and evolution of Neoarchean−Paleoproterozoic crust in the NW Wyoming Province: Evidence from zircon U-Pb age and Lu-Hf isotopes of the Montana metasedimentary terrane

   https://doi.org/10.1130/B37160.1  

   Chen, Tsai-Wei; Vervoort, Jeffrey D; Baldwin, Julia A (October 2023, Geological Society of America Bulletin)

   The Montana metasedimentary terrane in the northern Wyoming Province provides valuable insight into crustal formation and reworking processes along the cratonic margin and offers a unique opportunity to decipher the complex Neoarchean−Paleoproterozoic terrane assembly in southwestern Laurentia. We report new zircon U-Pb dates and Hf isotopes from seven metaigneous samples in the northwestern Montana metasedimentary terrane. The internal textures of zircon in this study are complex; some lack inherited cores and metamorphic overgrowths, while others exhibit core-rim relationships. Based on the cathodoluminescence (CL) features, we interpret these grains to be magmatic populations. These data demonstrate discrete igneous pulses at 2.7 Ga, 2.4 Ga, and 1.7 Ga, which indicate significant crustal formation intervals in the Montana metasedimentary terrane. Zircons at 2.7 Ga have positive εHf values (+2.4 to +0.9) that indicate a depleted mantle source. Most 2.4 Ga and 1.7 Ga samples have negative εHf values (−1.6 to −15.5), which indicate significant contributions from preexisting crust. Two 1.7 Ga samples, however, have near-chondritic εHf values (+0.4 to +0.3) that indicate larger juvenile contributions. The time-integrated Hf isotope trend suggests that the Paleoproterozoic zircons were produced from a mixture of older crust and juvenile mantle inputs. Additionally, the isotopic age fingerprint of the Montana metasedimentary terrane suggests that it differs from northern-bounding terranes. Viewed more broadly, the 2.7 Ga and 1.7 Ga age peaks that the Montana metasedimentary terrane shares with the global zircon age spectrum suggest that the drivers of these events in the Montana metasedimentary terrane were common throughout the Earth and may be associated with the assembly of supercontinents Kenorland and Nuna. 

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3. Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

   https://doi.org/10.1029/2022JB026265  

   Banik, Tenley J.; Coble, Matthew A.; Miller, Calvin F.; Fisher, Christopher M.; Jordan, Brennan T.; Vervoort, Jeffrey D.; Carley, Tamara L. (June 2023, Journal of Geophysical Research: Solid Earth)

   Abstract Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ¹⁸O∼+2 to +4‰ and indicate involvement of a low‐¹⁸O component in their source magmas. Hrafnsfjörður zircon Hf (mean sampleε_Hf∼ +15.3–16.0) and whole rock Hf and Nd (ε_Hf = +14.5 to +15;ε_Nd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sampleε_Hf∼ +11.8–13; whole rockε_Hf = +12.8 to +15.1;ε_Nd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (^208/204Pb = 37.95–37.96;^206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (^208/204Pb = 38.34–38.48;^206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions ofε_Hf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle. 

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4. An Eoarchean continental nucleus for the Fennoscandian Shield and a link to the North Atlantic craton

   https://doi.org/10.1130/G51658.1  

   Petersson, Andreas; Waight, Tod; Kemp, Anthony IS; Whitehouse, Martin J; Valley, John W (December 2023, Geology)

   Abstract Enabling the build-up of continental crust is a vital step in the stabilization of cratonic lithosphere. However, these initial crustal nuclei are commonly either destroyed by recycling or buried by younger rocks. In the Fennoscandian Shield, the oldest rocks are ca. 3.5 Ga, but ca. 3.7 Ga inherited and detrital zircons suggest the presence of an older, unexposed crustal substrate. We present U-Pb, O, and Hf isotope data from detrital zircons of three major Finnish rivers as well as zircon O and Hf isotope data from previously dated rocks of the Archean Suomujärvi and Pudasjärvi complexes, central Finland. Combined, these data indicate a previously unidentified ca. 3.75 Ga crustal nucleus in the Fennoscandian Shield. This adds to the growing number of Eoarchean nuclei recognized in Archean terranes around the globe, highlighting the importance of such nuclei in enabling the growth of continental crust. The isotope signatures of the Fennoscandian nucleus correlate with equivalent-aged rocks in Greenland, consistent with a common Eoarchean evolution for Fennoscandia and the North Atlantic craton. 

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5. Zircon geochemistry from early evolved terranes records coeval stagnant- and mobile-lid tectonic regimes

   https://doi.org/10.1073/pnas.2405378121  

   Mixon, Emily E; Bauer, Ann M; Blum, Tyler B; Valley, John W; Rizo, Hanika; O’Neil, Jonathan; Kitajima, Kouki (September 2024, Proceedings of the National Academy of Sciences)

   Determining the mechanisms by which the earliest continental crust was generated and reworked is important for constraining the evolution of Earth’s geodynamic, surface, and atmospheric conditions. However, the details of early plate tectonic settings often remain obscured by the intervening ~4 Ga of crustal recycling. Covariations of U, Nb, Sc, and Yb in zircon have been shown to faithfully reflect Phanerozoic whole-rock-based plate-tectonic discriminators and are therefore useful in distinguishing zircons crystallized in ridge, plume, and arc-like environments, both in the present and in deep time. However, application of these proxies to deciphering tectonic settings on the early Earth has thus far been limited to select portions of the detrital zircon record. Here, we present in situ trace-element and oxygen isotope compositions for magmatic zircons from crystalline crustal rocks of the Acasta Gneiss Complex and the Saglek-Hebron Complex, Canada. Integrated with information from whole-rock geochemistry and zircon U-Pb, Hf, and O isotopes, our zircon U-Nb-Sc-Yb results reveal that melting of hydrated basalt was not restricted to a single tectonomagmatic process during the Archean but was operative during the reworking of Hadean protocrust and the generation of juvenile crust within two cratons, as early as 3.9 Ga. We observe zircon trace-element compositions indicative of hydrous melting in settings that otherwise host seemingly differing whole-rock geochemistry, zircon Hf, and zircon O isotopes, suggesting contemporaneous operation of stagnant-lid (oceanic plateau) and mobile-lid (arc-like) regimes in the early Archean. 

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