Uranium Isotope Variations Tracing Oceanic Anoxia and Zirconium and Hafnium Isotope Variations Tracing Planetary Differentiation Processes

This dissertation is part of the efforts in developing novel non-traditional isotopic systems and applying them to enhance our understanding of the histories of our Earth and other planetary bodies. Specifically in this thesis I study the uranium isotopic variations in marine sediments to trace the oceanic anoxia in the ancient times, and I also study the zirconium and hafnium isotopic variations in igneous rocks and explore their potentials to trace magmatic differentiation processes. U concentrations and isotopic compositions of a large sample set of old-age carbonates are measured and compared with compiled U datasets of shales and young-age carbonates. Several new modeling results are also presented to reconcile the measurement results of ancient samples with the common understanding of modern U oceanic cycle. This study points out the invalidity of applying some assumptions of modern models into the past time and also calls for more interesting modeling and experimental work (Chapter 2). This dissertation also provides the first ab initio calculation results of the Zr, Hf equilibrium isotopic fractionation factors and they are readily to be used and applied in many interesting questions. Several equilibrium and kinetic modeling results are further presented to explain the documented Zr isotopic variations in igneous rocks and minerals. Our study shows that Zr equilibrium isotopic fractionation is negligible during zircon crystallization and magmatic differentiation, while the diffusion-driven kinetic isotopic fractionation can readily explain the actual observed data (Chapter 3). Finally, this dissertation also presents an ongoing effort to develop high-precision and high-accuracy Hf isotopic measurements in individual zircon grains with low amount of Hf available. The whole analytical procedure is improved and this novel method will be further applied on lunar zircons to provide better constraints on the differentiation age of the Moon (Chapter 4).