Abstract
More than a decade after the first high-precision tungsten isotopic measurements were reported, several key observations remain unexplained. Notably, that modern and Archean crust have distinct compositions, which requires the admixture of early formed and distinct geochemical reservoirs within the silicate Earth over its history. The timing and nature in which these reservoirs mixed remains poorly constrained.To investigate this, I have further developed techniques for isotopic analysis of samples containing only trace (often <100 ng/g) quantities of tungsten. Using these techniques, I showed by measurement of samples spanning over 1.8 Ga from four ancient terranes that compositions typical of ancient crust are observed throughout the Archean, and that a striking change in tungsten isotopic compositions is observed in the Palaeoproterozoic.
My new data are consistent with a model where the tungsten isotopic composition of the shallow mantle was perturbed by a globally significant influx of deep, core-influenced lower mantle. Tungsten isotopes therefore act as a vital tool with which to constrain the highly uncertain past exchange between upper and lower mantle.
I have also developed a new double spike technique for measuring mass-dependent tungsten isotopic compositions, which is significantly quicker and more forgiving of residual sample matrix during chemical separation when compared to those currently documented in the literature. I apply this technique first to subduction zones and show that tungsten is a useful tracer of subduction zone conditions, and later to the products of hotspots, to show that the heterogeneity in ocean island basalts could be caused by the variable influence of crustal recycling in the lower mantle.
| Date of Award | 2 Dec 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Tim Elliott (Supervisor) & Christopher D Coath (Supervisor) |