Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

Abstract

The nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

Secular Evolution of Continents and the Earth System

“…consistent with the interpretation that the earliest stable cratons formed through reworking of long-lived Hadean and early Eoarchean (>3.80 Ga) crust by the addition of isotopically juvenile, mantle-derived melts (Drabon et al, 2022;Mulder et al, 2021).…”

“…The sub‐chondritic endmember compositions of these arrays lie along an evolutionary array consistent with continued reworking of the pre‐3.8 Ga crust sampled by older zircons (Guitreau et al., 2019; Kemp et al., 2010; Kirkland et al., 2021; Mulder et al., 2021; Naeraa et al., 2012; OʼNeil et al., 2013), whereas the chondritic or supra‐chondritic endmember requires interaction with a younger and more isotopically juvenile source. Together with the preservation of Hadean crustal Pb and 142 Nd isotopic signatures in early Archean mafic rocks (e.g., Kamber, 2015; O'Neil et al., 2008), the Hf isotopic record of early Earth zircons, representing felsic rocks, is consistent with the interpretation that the earliest stable cratons formed through reworking of long‐lived Hadean and early Eoarchean (>3.80 Ga) crust by the addition of isotopically juvenile, mantle‐derived melts (Drabon et al., 2022; Mulder et al., 2021).…”

A persistent Hadean–Eoarchean protocrust in the western Yilgarn Craton, Western Australia

“…4,000–3,800 Ma source pattern seen globally (Figure 3c) suggesting earlier protocrust formation soon after planetary accretion, which is consistent with various distinct early geodynamic states (e.g. Drabon et al, 2022; Smithies et al, 2021). The significant offset from other evolution trends towards higher 176 Lu/ 177 Hf (Figure 3c) is most likely a reflection of different formation mechanisms for these two zircon populations, perhaps via some direct or indirect impact melt origin (e.g.…”

“…[Colour figure can be viewed at wileyonlinelibrary.com] The trends of the most evolved zircon grains in Australia and Canada (Figure 3c) that are dictated by zircons from Jack Hills and Acasta, respectively, appear distinct to the broad c. 4,000-3,800 Ma source pattern seen globally (Figure 3c) suggesting earlier protocrust formation soon after planetary accretion, which is consistent with various distinct early geodynamic states (e.g. Drabon et al, 2022;Smithies et al, 2021). The significant offset from other evolution trends towards higher 176 Lu/ 177 Hf (Figure 3c) is most likely a reflection of different formation mechanisms for these two zircon populations, perhaps via some direct or indirect impact melt origin (e.g.…”

Data Quality in Geochemical Elemental and Isotopic Analysis

“…Geochemical studies also helped in solving many of the unresolved questions related to the Earth's earliest crust and crustal processes in the Precambrian era. For example, very recently some trace-element proxies of the tectonic-magmatic settings indicated evidence of Plate Tectonics in a 3.8-billion-year-old zircon crystal [3]. As a result, the data produced today in geochemical laboratories and the quality of analytical data have become more important for both basic and applied geochemical studies, and also for the decisions related to the environment and ecology.…”