2026-03-03 カリフォルニア工科大学（Caltech）
A variety of ancient zircon crystals. Formed in the earliest era of Earth’s history, these zircons contain a record of what the planet was like at that time.Credit: Shane K. Houchin
＜関連情報＞
- https://www.caltech.edu/about/news/ancient-zircon-crystals-provide-a-window-into-early-earth-history
- https://www.pnas.org/doi/10.1073/pnas.2525466123
ジャックヒルズのジルコンが明らかにした冥王代酸化マグマと始生代可動蓋テクトニクス Oxidized Hadean magmas and Archean mobile-lid tectonics revealed by Jack Hills zircon
Shane K. Houchin, François L. H. Tissot, Mauricio Ibañez-Mejia, +3 , and Antonio Lanzirott
Proceedings of the National Academy of Sciences Published:March 2, 2026
Significance
Mobile-lid tectonics, the lateral movement and collision of tectonic plates, is unique to Earth. Yet, when this fundamental process began remains uncertain. Using oxidation state and trace element analysis of ancient zircon crystals from the Jack Hills, Australia, we show that magmas during the Hadean and Archean were relatively oxidized and find evidence for collisional tectonics ~3.35 billion years ago. This evidence is consistent with efficient mantle convection and crustal recycling on the early Earth and suggests that mobile-lid processes were already in operation by the Archean. These results provide constraints on the timing and nature of Earth’s geodynamic evolution, with implications for the coevolution of the lithosphere and atmosphere and the emergence of habitable conditions on our planet.
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-ƒO2 and high-intermediate T/P conditions (FMQ+1.6 to +2.5; >600 °C/GPa) and ii) low-ƒO2 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.
