Geochemistry, Geophysics, Geosystems (Jul 2020)

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

  • Jesse R. Reimink,
  • Andrea Mundl‐Petermeier,
  • Richard W. Carlson,
  • Steven B. Shirey,
  • Richard J. Walker,
  • D. Graham Pearson

DOI
https://doi.org/10.1029/2020GC009155
Journal volume & issue
Vol. 21, no. 7
pp. n/a – n/a

Abstract

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Abstract 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.

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