Early Archaean onset of volatile cycling at subduction zones

Abstract

The subduction of pelagic sediments and altered oceanic crust modulates the global cycle of volatile elements. Sulfate and carbonate fluids released when one plate descends beneath another modify the redox state of the mantle, and generate the return of water and reactive gases to the atmosphere and hydrosphere via arc volcanism, affecting planetary habitability over geologic timescales. However, the timing of the onset of subduction remains uncertain, hindering our understanding of how deep geochemical cycles operated on the early Earth. Here we measure sulfur and neodymium isotope data on Eoarchaean mantle-derived rocks of the Innuksuac Complex in northern Québec, Canada, with petrological characteristics of arc magmas. These rocks exhibit anomalous sulfur isotopic compositions originally produced by photochemical reactions in the atmosphere more than 3.8 Gyr ago. Combined sulfur and neodymium isotope data suggest that these signatures were transferred to the Innuksuac mantle through devolatilization and partial melting of terrigenous sediments derived from a Hadean (4.3–4.4 Gyr ago) continental source, providing a record of an early continental margin subduction environment. This result pushes back direct evidence of a subduction-driven volatile cycle to the onset of the terrestrial rock record, approximately 1 Gyr earlier than previously inferred from diamond inclusions. Combined sulfur and neodymium isotopes suggest that volatile cycling at subduction zones began 3.8 Gyr ago or earlier, according to a study of Eoarchaean mantle-derived rocks with arc-lava characteristics.