Andrea Mundl-Petermeier, Sebastian Viehmann, Jonas Tusch, Michael Bau, Carsten Münker

Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth1. Applied to ancient marine chemical sediments such as banded iron formations (BIFs), the short-lived 182Hf-182W isotope system can serve as key instrument to decipher Earth’s geodynamic evolution. High-precision 182W isotope data of the 2.7 Ga old BIF from the Temagami Greenstone Belt, NE Canada, reveal distinct 182W differences in alternating Si-rich (7.9 ppm enrichment) and Fe-rich (5.3 ppm enrichment) bands reflecting variable flux of W from continental and hydrothermal mantle sources into ambient seawater, respectively. Greater 182W excesses in Si-rich layers relative to associated shales (5.9 ppm enrichment), representing regional upper continental crust composition, suggest that the Si-rich bands record the global rather than the local seawater 182W signature. The distinct intra-band differences highlight the potential of 182W isotope signatures in BIFs to simultaneously track the evolution of crust and upper mantle through deep time.