Expected results

The rate of formation of the continental crust and the time of the onset of subduction and recycling of the lithosphere is an unsolved problem in the history of the Earth of paramount importance. Our collaborations with Profs. Nadja Drabon and Annie Bauer have contributed to understanding an important geochemical change 3.7 billion years ago (3.7 Ga, Figures 3). The step-changes seen at ca 3.7 Ga in εHfT, U/Nb, and Sc/Yb correlate to εHfT patterns in nine other Archean terranes and indicate an influx of juvenile magmas into Hadean protocrust. These events were previously proposed to mark the unidirectional transition from "stagnant-lid" tectonics to subduction. However, Hf model ages in Eo-Archean zircons, coupled with extremely low initial amounts of radiogenic 87Sr in olivine-hosted melt inclusions from Barberton komatiites with fractionated Ce/Pb, Nb/U and Nb/Th suggest active subduction and massive production of continental crust in Hadean Eon. New geodynamic modeling reconсiles these apparently contradicting geochemical observations.

The synergy of the new geochemical data and new modeling suggests that the intensity of subduction in Hadean and early Archean was oscillatory rather than unidirectional.

After an initial period of active subduction and crust formation at ca 4.4-4.1 Ga, there was a period of subdued subduction, the reworking of crust, and little magma from the mantle from ca 4.1-3.8 Ga. The step changes seen at 3.7 Ga mark the resurgence, rather than onset, of subduction with mantle-derived magma and continental crust. This result could be a real breakthrough in understanding early Earth’s evolution if substantiated by further studies. We will continue to test this model as part of the ongoing MEET Project.

Updated on 15 March 2023