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In the late 2000s, an assumption three-quarters of a century old was overthrown when sub-permil variations were discovered in the ratio of "stable" U isotopes. This dissertation is part of the subsequent effort that turned the 238U/235U ratio from a constant value into a crucial variable of Pb-Pb dating and a promising paleoredox tracer.,Herein, the reader will find the analytical details on how to achieve routine high-precision U isotope analysis with double spike technique, while properly estimating the uncertainties via Monte-Carlo simulations (Chapter 2). ,Measurement of many igneous rocks will be used to (i) assess the U isotopic composition of Earth's crust, and (ii) demonstrate the absence of Soret diffusion in Lachlan Fold Belt granitoids. Measurements of seawater samples and data compilation will also be used to (i) show that the ocean is homogeneous with regards to U isotopes, (ii) constrain the extent of modern oceanic anoxia, (iii) confirm that the U oceanic budget is in steady-state, and (iv) validate the assumption that the ratio of U "stable" isotopes tracks the global oceanic redox conditions. Two case studies applying the U proxy on ancient sediments will be presented (Chapter 3).,Analysis of a series of angrite samples will allow the revision of their Pb-Pb ages, thus reducing the discrepancies between absolute Pb-Pb ages and relative Al-Mg ages in early Solar System materials. Finally, the discovery of a large 235U excess in a fine-grained group II CAI, will bring definitive evidence that 247Cm was alive when the Solar System formed, thereby putting to rest a four-decade long controversy. The initial Solar System abundance of Curium-247 derived will be shown to be incompatible with the existence of an actinide-specific r-process site and instead suggests that a single r-process nucleosynthetic environment is relevant to the long-term evolution of the Galaxy, with the last r-process production event occurring about 100 Myr before Solar System formation (Chapter 4).,Of interest to both geo- and cosmochemists, a Pneumatic-Fluoropolymer HPLC system will also be introduced, as a mean of bringing HPLC systems up to the resistance and separation standards of isotope chemistry (Chapter 2).


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