Tungsten Isotope Composition of Archean Crustal Reservoirs and Implications for Terrestrial μ<SUP>182</SUP>W Evolution

The evolution of Earth's major geochemical reservoirs over similar to 4.5 x 10(9) years remains a matter of intense study. Geochemical tools in the form of short-lived radionuclide isotope ratios (Nd-142/Nd-144 and W-182/W-184) have expanded our understanding of the geochemical variability in both the modern and ancient Earth. Here, we present Nd-142/Nd-144 and W-182/W-184 data from a suite of rocks from the Slave craton that formed over a 1.1 x 10(9) year time span in the Archean. The rocks have consistently high W-182/W-184, yet(142)Nd/Nd-144 that is lower than bulk mantle and increased over time. The declining variability in(142)Nd/Nd-144 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 W-182/W-184 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 W-182/W-184 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 W-182/W-184 until 2.9 Ga. This observation, combined with the presence of other komatiite suites that have low W-182/W-184, suggests that deep-seated sources contributed low W-182/W-184 in the Archean Earth. The regional variability in W-182/W-184 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.