Oxidized Hadean magmas and Archean mobile-lid tectonics revealed by Jack Hills zircon

Abstract

Mobile-lid tectonics is a first-order feature characterizing the modern Earth, yet its origins remain enigmatic due to a scarcity of ancient terrestrial materials. Detrital zircons provide the most complete archive of Earth’s early crust and preserve the only record extending beyond ~4.0 Ga. Here, we combine U XANES oxybarometry with U-Pb and trace element analysis to investigate the igneous cores and metamorphic rims of Hadean–Archean zircon from the Jack Hills, Australia. Igneous cores record consistent, moderately oxidized magma conditions (FMQ-1 to +1), challenging notions of a highly reduced early Earth and supporting models that evoke efficient mantle convection throughout the Hadean. In contrast, redox states and trace element contents show that metamorphic rims record i) high-ƒO 2 and high-intermediate T/P conditions (FMQ+1.6 to +2.5; >600 °C/GPa) and ii) low-ƒO 2 and generally lower T/P conditions (FMQ-0.2 to +0.5; <500 °C/GPa). While only high T/P conditions are observed in Hadean zircon rims, Archean rims (~3.35 Ga) preserve both the low and high T/P signatures, a pattern typical of large-scale plate underthrusting. These findings imply Earth’s mantle had near-modern redox states by 4.15 Ga and that mobile-lid tectonics was active by the early Archean, at the latest.