Earth's geodynamic evolution constrained by 182W in Archean seawater

Banded iron formations, precipitates of Precambrian seawater, record global W-182 isotope signatures derived from continental weathering and hydrothermal mantle fluxes into ancient oceans, tracking Earth's geodynamic evolution through deep time. 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-182-W-182 isotope system can serve as key instrument to decipher Earth's geodynamic evolution. Here we show high-precision W-182 isotope data of the 2.7 Ga old banded iron formation from the Temagami Greenstone Belt, NE Canada, that reveal distinct W-182 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-182 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-182 signature. The distinct intra-band differences highlight the potential of W-182 isotope signatures in banded iron formations to simultaneously track the evolution of crust and upper mantle through deep time.

Automated summary

Banded iron formations serve as important geological materials to study the geodynamic evolution of the Earth, and the W-182 isotope signatures can track the flux of materials from continental weathering and hydrothermal activity. The study shows that the W-182 isotope content in banded iron formations is related to the differences in Si-rich and Fe-rich bands, highlighting the potential mechanism for crustal and upper mantle evolution.