English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

doi: 10.7185/geochemlet.2015 The tectonic regime of the early Earth has proven enigmatic due to a scarcity of preserved continental crust, yet how early continents were generated is key to deciphering Earth’s evolution. Here we show that a compilation of data from 4.3 to 3.4 Ga igneous and detrital zircons records a secular shift to higher 176Hf/177Hf after ~3.8–3.6 Ga. This globally evident shift indicates that continental crust formation before ~3.8–3.6 Ga largely occurred by internal reworking of long-lived mafic protocrust, whereas later continental crust formation involved extensive input of relatively juvenile magmas, which were produced from rapid remelting of oceanic lithosphere. We propose that this secular shift in the global hafnium isotope record reflects a gradual yet widespread transition from stagnant-lid to mobile-lid tectonics on the early Earth. Received 2 December 2019 | Accepted 25 March 2020 | Published 16 April 2020 1. Department of Geoscience, University of Wisconsin-Madison, Madison, WI, USA 2. Department of Geology and Geophysics, Yale University, New Haven, CT, USA 3. Department of Geosciences, The Pennsylvania State University, State College, PA, USA 4. Earth and Planets Laboratory, Carnegie Institution for Science, DC, USA 5. Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada * Corresponding author (email: annie.bauer@wisc.edu, jreimink@psu.edu) The two corresponding authors contributed equally to this work. Introduction On the modern Earth, continental crust is mostly generated at convergent plate margins through subduction-driven mantle melting. Due to the limited extent of the early rock record, it remains unclear whether a similar process was operative throughout Earth history or if there were major changes in the dominant mechanisms of continental crust production over time. On the basis of geological data and theoretical models, some workers have proposed that the earliest continental crust formed by remelting of long-lived mafic crust, either in local oceanic plateaus or in a global stagnant lid (Kemp et al., 2010; Reimink et al., 2014; van Kranendonk et al., 2015). Determining whether protocrustal residence time varied regionally will shed light on the tectonic processes responsible for early crust production. Specifically, a uniformitarian view of continental crust production mechanisms is predicated on little change in the residence time of protocrust, whereas a non-uniformitarian view permits major changes that, in turn, would imply a transition in Earth’s tectonic regime. A stagnant lid can only recycle mafic protocrust through volcanic burial, while a “mobile-lid” regime, with active subduction—possibly in a different style than modern plate tectonics—can rapidly recycle mafic crust and transport surface-derived volatiles into the mantle. Long-lived radiogenic isotope systems are powerful tracers of crustal evolution as they track crustal residence times and changes in magma source. Zircon is particularly useful because it preserves the original Hf isotope composition of the magma from which it is derived and, through the U-Pb decay system, precise information on the time of magma crystallisation. Crust has a lower Lu/Hf than residual mantle and therefore accumulates less radiogenic 176Hf relative to the mantle, which is reflected in the resulting lower εHf signature. Arrays of decreasing εHf values with time suggest repeated reworking of pre-existing crust. Alternatively, shifts in Hf isotopic compositions reflect contributions from other sources such as the mantle, which often coincide with changes in crust production mechanisms. The Hadean Earth may have been dominated by a ubiquitous mafic protocrustal lid (Kamber, 2007; Shirey et al., 2008), but this view is based on only a few locales and therefore its global applicability has remained speculative. An extensive dataset is now available to assess the nature of the sources of Hadean and Archean magmas on a global scale. We compile the global Hf isotope record of 4.3–3.0 Ga zircons (including Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2020) 14, 1-6 | doi: 10.7185/geochemlet.2015 2 magmatic, xenocrystic, and detrital zircons; references in Methods, see Supplementary Information) of eight different Archean cratons. We compare the general εHf-time evolution reflected in that dataset with a comparable dataset for the 4.02−3.4 Ga Acasta Gneiss Complex (AGC) of the Slave craton of Canada, a locale where zircon Hf isotope data is directly linked to parental whole rock geochemistry spanning 600 Myr of crust evolution. This comparison is necessitated by the nature of the extant data from other cratons, which are largely comprised of detrital zircons with only indirect links to their parental igneous rocks. The Global Hf Isotope Dataset The relatively well-characterised meta-igneous rocks of the AGC represent the earliest felsic rock record on Earth. We have previously summarised several decades of research in the AGC, linking the zircon and whole rock geochemical datasets, and have formulated a tectonic model for these rocks based on those datasets (Supplementary Information; Reimink et al., 2019a). The zircons in 4.0-3.75 Ga AGC rocks show a trend towards decreasing εHf values with time. In combination with whole rock compositional data, we interpret the εHf trend to be a result of crustal magma generation by repeated, closed system melting of an ancient, long-lived mafic protocrustal nucleus at <~30 km depth, possibly in an oceanic plateau-like environment. AGC magmas produced after ~3.6 Ga (Fig. 1) show a distinct break in the trend to more negative εHf values, instead displaying a broad range from juvenile values (+0.3) to those indicating contamination of magmas by older crust (-9.6). These Hf isotope data, along with other geochemical and isotopic data for these rocks, are interpreted to reflect a transition to a mobile-lid system, which brought young, surface-derived mafic crust to depths of melting beneath the evolving plateau. Partial melting of this mafic crust produced a pulse of felsic magmas with juvenile εHf values that mixed with older extant crust in a tectonic setting broadly analogous to modern subduction systems. Continuation of this process formed the more extensive Slave craton basement east of the AGC (Reimink et al., 2019b). In many cratons, detrital zircons found in Archean sedimentary sequences deposited on top of older crustal nuclei are the only extant records of evolved crust older than 3.6 Ga (Fig. 1). Analysis of these detrital suites are poorly mixed, thus preserving a high degree of regional variation, indicating limited sediment transport distance and restricted provenance (e.g., Sircombe et al., 2001). This in turn suggests that the preserved continental detrital zircon suites represent erosion of several distinct, isolated terranes rather than of just one or two formerly contiguous terranes. Our compilation includes zircons of magmatic and detrital origins, likely providing a more complete global view of Archean processes. The Slave craton can be used as a comparative framework to interpret the Hf isotope signatures of the global zircon archive. Notably, detrital zircons from the Slave craton sedimentary cover sequence exhibit similar εHf-time trends as zircons recovered from the meta-igneous rocks of the Slave craton. This validates the use of detrital zircon suites for determining the geochemical character of their source rocks. Detrital and xenocrystic zircons (Fig. 1) from the Yilgarn, Singhbhum, Pilbara, Rae, W. Superior, Limpopo, and Wyoming cratons span the age range of from 4.3-3.0 Ga. Initial εHf values of the 4.3-3.8 Ga zircon record are mostly negative and generally decrease with time. In contrast, between ~3.8-3.6 Ga, εHf shifts to higher values in each terrane, requiring input from a juvenile source. This results in a bimodal distribution in εHf probability density when zircon εHf values are binned by age (Fig. 2). These changes are also statistically significant, as shown by change-point analysis and moving averages (Supplementary Information). Occurring in every terrane and closely matching the Hf isotope evolution trends of the Slave craton, we suggest that this isotopic shift represents a global transition from crustal differentiation processes dominated by in situ reworking of older crust to those characterised by significant input of more juvenile crust. The similarity of Hf isotope trends between the Slave record and the worldwide Hadean-Eoarchean detrital zircon record suggests that the processes recorded in the Slave craton were operating globally (Figs. 1, 2). The consistently negative εHf values of Hadean detrital zircons indicate reworking of older mafic material (176Lu/177Hf 0.022-0.015; Blichert-Toft and Albarède, 2008; Kemp et al., 2010), which we propose reflects episodic shallow-level melting of an extensive and persistent stagnant mafic lid (Fig. 3). Mafic crustal reworking may have been focused in areas where the mafic protocrust was locally thickened via plume-like eruptions in a ‘heat-pipe’ regime (Moore and Webb, 2013), where heat is transported out of the mantle by volcanism from a restricted source area through an unbroken, stagnant lithosphere. Later in the Eoarchean (~3.8−3.6 Ga), the widespread shift to higher εHf values likely occurred in response to a mobile-lid tectonic regime that melted surface-derived juvenile mantle (Fig. 3). Rocks in SW Greenland-Labrador and northern Quebec have near-zero zircon εHf at 3.7−3.9 Ga (e.g., Næraa et al., 2012; O’Neil et al., 2013). No pre-3.94 Ga zircons have been found at either location, suggesting that felsic crust production in these terranes only began after ~3.9 Ga coinciding with the localised onset of the mobile-lid regime. Indeed, a subduction environment has been suggested to explain the lithologies and geochemistry of the Isua belt at ~3.8 Ga

Hafnium isotopes in zircons document the gradual onset of mobile-lid tectonics