Earth's geodynamic evolution constrained by W in Archean seawater.

Radiogenic isotope systems are important geochemical tools to unravel geodynamic processes on Earth. Applied to ancient marine chemical sediments such as banded iron formations, the short-lived Hf-W isotope system can serve as key instrument to decipher Earth's geodynamic evolution. Here we show high-precision W isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct W 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 W 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 W signature. The distinct intra-band differences highlight the potential of W isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time.