UCLA

Measurements of isotope fractionation have long focused on mass dependent fractionations, though mass independent effects have long been recognized. One mechanism of mass independent fractionation is the nuclear field shift. Field shift isotope fractionation was first proposed by Bigeleisen (1996) and Nomura et al. (1996) as a mechanism explain to patterns of redox-driven uranium isotope fractionation. This thesis aims to model the nuclear field shift using ab initio methods in multiple isotope systems, in relevant natural species, as well as to explore potential applications of field shift fractionation measurements for these systems. The main method used is the DFT-PAW method of Schauble (2013), which makes it possible to determine field shifts in solids and complex aqueous species.In the uranium isotope system, 238U/235U is predicted to be ~1-2‰ higher in U(IV) than coexisting U(VI) species at 25ºC. U(V) species are either intermediate or similar to U(IV) species. Fractionations of up to 0.6‰ are predicted between species in the same oxidation state. In the case of plutonium and neptunium, Pu(V) and Pu(IV) species thought to be relevant to natural systems are found to be isotopically similar to each other, whereas Pu(VI) species will be approximately 2‰ lower in 242Pu/239Pu than Pu(V) or Pu(IV), and Pu(III) species will have approximately 3‰ higher 242Pu/239Pu than Pu(IV) or Pu(V). Np(VI) will be 0.6‰ lower in 237Np/235Np than coexisting Np(V) species, whereas Np(IV) and Np(III) will have approximately 1.3‰ higher 237Np/235Np. In the final study, field shift effects in platinum and iridium are studied, using Mössbauer isomer shifts according to the method of Schauble (2023) in to investigate iridium. Predicted field shifts for both 198Pt/194Pt and 193Ir/191Ir are relatively modest, approximately <0.1‰ at ambient temperatures for most species (e.g., mass dependent fractionation effects likely dominate for these elements at low temperatures). Notably, Pt in an Fe-alloy (as a possible analog for the Earth’s core) is predicted to show an equilibrium fractionation of 0.09‰ at 3000 K relative to PtII substituted in an olivine crystal structure (as a speculative analog for the silicate mantle), which could explain observed superchondritic 198Pt/194Pt found in Archean mantle samples.