Source: AGU Advances
Zircons with ages up to 4.4 Ga indicate that crust was formed early in Earth’s history. However, the significance of this early crust with respect to Earth’s differentiation is debated. The early Earth record is skewed by zircons from Yilgarn Craton in Australia (see figure) and Drabon et al.  make a welcome addition reporting on zircons in a 3.31-Ga sandstone formation of the Barbeton Greenstone Belt, South Africa, that contains zircons up to 4.15 Ga in age. The authors report high U/Nb ratios of zircons, <3.6 Ga in age, similar to zircons from a subduction-type tectonic setting. Zircon older than 3.6 Ga show little U/Nb fractionation (see figure). Additionally, the variation in Hf-isotopic compositions through time indicates a change of origin for BGS zircons around 3.6 Ga from a protolith extracted from a primordial mantle to one that included more juvenile material. The concomitant change in isotope and trace element chemistry is strong evidence for the transition from stagnant lid tectonics to plate tectonics “as we know it” at 3.6 Ga.
Citation: Drabon, N., Byerly, B., Byerly, G., Wooden, J., Wiedenbeck, M., Valley, J., et al. . Destabilization of long-lived Hadean protocrust and the onset of pervasive hydrous melting at 3.8 Ga. AGU Advances, 3, e2021AV000520. https://doi.org/10.1029/2021AV000520
—Vincent Salters, Editor, AGU Advances