Tungsten Isotope Composition of Archean Crustal Reservoirs and Implications for Terrestrial μ¹⁸²W Evolution

The evolution of Earth's major geochemical reservoirs over ~4.5 × 10⁹ years remains a matter of intense study. Geochemical tools in the form of short-lived radionuclide isotope ratios (¹⁴²Nd/¹⁴⁴Nd and ¹⁸²W/¹⁸⁴W) have expanded our understanding of the geochemical variability in both the modern and ancient Earth. Here, we present ¹⁴²Nd/¹⁴⁴Nd and ¹⁸²W/¹⁸⁴W data from a suite of rocks from the Slave craton that formed over a 1.1 × 10⁹ year time span in the Archean. The rocks have consistently high ¹⁸²W/¹⁸⁴W, yet ¹⁴²Nd/¹⁴⁴Nd that is lower than bulk mantle and increased over time. The declining variability in ¹⁴²Nd/¹⁴⁴Nd with time likely reflects the homogenization of compositional heterogeneities in the silicate Earth that were initially created by differentiation events that occurred prior to 4.2 Ga. The elevated ¹⁸²W/¹⁸⁴W recorded in the Slave samples help refine models for the broader W-isotope evolution of the silicate Earth. Globally, the Archean mantle that formed continental crust was dominated by ¹⁸²W/¹⁸⁴W elevated by some 10–15 ppm compared to the value for the modern upper mantle. The Slave craton lacks significant volumes of komatiite yet has elevated ¹⁸²W/¹⁸⁴W until 2.9 Ga. This observation, combined with the presence of other komatiite suites that have low ¹⁸²W/¹⁸⁴W, suggests that deep-seated sources contributed low ¹⁸²W/¹⁸⁴W in the Archean Earth. The regional variability in ¹⁸²W/¹⁸⁴W may be explained by invoking chemical and/or isotopic exchange between a well-mixed silicate Earth and the core or a portion of the lower mantle whose W-isotope composition has been influenced by interaction with the core.