Oxidation of Archean upper mantle caused by crustal recycling

The redox evolution of Archean upper mantle impacted mantle melting and the nature of chemical equilibrium between mantle, ocean and atmosphere of the early Earth. Yet, the origin of these variations in redox remain controversial. Here we show that a global compilation of similar to 3.8-2.5 Ga basalts can be subdivided into group B-1, showing modern mid-ocean ridge basalt-like features ((Nb/La)(PM) >= 0.75), and B-2, which are similar to contemporary island arc-related basalts ((Nb/La)(PM) < 0.75). Our V-Ti redox proxy indicates a more reducing upper mantle, and the results of both ambient and modified mantle obtained from B-1 and B-2 samples, respectively, exhibit a similar to 1.0 log unit increase in their temporal evolution for most cratons. Increases in mantle oxygen fugacity are coincident with the changes in basalt Th/Nb ratios and Nd isotope ratios, indicating that crustal recycling played a crucial role, and this likely occurred either via plate subduction or lithospheric drips.

Automated summary

This study reveals the significance of redox evolution in the early Earth's mantle, as well as the relationship between chemical characteristics in basalts and crustal recycling.