Uranium isotope fractionation

This review focuses on the rapidly growing field of natural 238U/235U variability, largely driven by the technical advances in the measurement of U isotope ratios by mass spectrometry with increasing precision over the last decade. A thorough review on the application of the U-decay series systems within Earth sciences was published in Reviews in Mineralogy and Geochemistry (RiMG) volume 52 in 2003, and will not be discussed further within this review. Instead, this article will first focus on the basic chemical properties of U and the evolution of 238U/235U measurement techniques, before discussing the latest findings and use of this isotopic system to address questions within geochronology, cosmochemistry and Earth sciences. ### Uranium occurrence and properties Uranium constitutes one of the principal long-lived radioactive elements that was formed over the lifetime of the galaxy, then injected into the solar system and Earth when they formed more than 4.5 billion years ago (Ga; Dicke 1969). The discovery of the three naturally occurring radioactive decay chains of U and Th occurred around the start of the twentieth century (Becquerel 1896). The heat production from U decay, together with the decay of Th and K, provides the major radioactive heat source on Earth (e.g., Jaupart and Mareschal 2010). The ultimate decay of U to stable isotopes of Pb also forms the basis of one of the most important geochronometers for dating the Earth and solar system, namely the U–Pb or Pb–Pb dating systems (e.g., Patterson et al. 1955). In nature, U commonly occurs in two oxidation states, U+4 and U+6 (e.g., Langmuir 1978). Intermediate U+5 also occurs naturally, but is generally assumed to be unstable through disproportionation and therefore it is short-lived and uncommon in nature (e.g., Grenthe et al. 1992). Chemical species of U+4 are generally …