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

Area of Science:

• Geochemistry
• Isotope Geochemistry
• Earth Science

Background:

• Radiogenic isotope systems are crucial for understanding Earth's geodynamic processes.
• The short-lived hafnium-182-tungsten-182 (¹⁸²Hf-¹⁸²W) isotope system is a key tool for deciphering early Earth evolution.
• Banded iron formations (BIFs) are ancient marine chemical sediments that preserve valuable geochemical records.

Purpose of the Study:

• To investigate the application of the ¹⁸²Hf-¹⁸²W isotope system in ancient BIFs.
• To decipher geodynamic processes by analyzing high-precision ¹⁸²W isotope data from a 2.7 Ga BIF.
• To understand the sources of tungsten (W) flux into ancient seawater and track crust-mantle evolution.

Main Methods:

• High-precision ¹⁸²W isotope analysis.
• Geochemical analysis of a 2.7 Ga banded iron formation from the Temagami Greenstone Belt, NE Canada.
• Comparison of ¹⁸²W signatures in Si-rich bands, Fe-rich bands, and associated shales.

Main Results:

• Distinct ¹⁸²W enrichments were observed in alternating Si-rich (7.9 ppm) and Fe-rich (5.3 ppm) bands within the BIF.
• Si-rich layers showed greater ¹⁸²W excesses (5.9 ppm) compared to regional upper continental crust (shales).
• These findings indicate variable W flux from continental and hydrothermal mantle sources into seawater.

Conclusions:

• The ¹⁸²W isotope signature in Si-rich BIF bands reflects the global seawater signature, not just local crustal input.
• Intra-band ¹⁸²W differences in BIFs are powerful indicators of simultaneous crust and upper mantle evolution.
• The ¹⁸²Hf-¹⁸²W system in BIFs offers a novel approach to reconstructing deep Earth history.