CAREER: Deciphering the Uranium Isotope Record of Igneous Accessory Phases

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Zircon and other accessory minerals like apatite, titanite, or baddeleyite constitute some of the most complete archives of Earth’s history. These minerals can be precisely dated using the Uranium-Lead radiometric method, and their chemical resistance allows their survival over billion-year timescales. In fact, detrital zircon are the only preserved fragments remaining from Earth’s first 500 Myr, thus making this phase a cornerstone for understanding the events that shaped the Earth and establishing a timeline of its evolution. Recently, variations in the uranium (U) isotopic composition of igneous rocks and accessory phases have garnered significant attention, but the drivers behind this variability remain unknown. If better understood, inter-crystal, inter-mineral and inter-rock U isotope variations in high-temperature systems could become powerful tools for studying magmatic evolution, source, oxidation state and/or differentiation processes. The goals of this CAREER award are to (i) identify the drivers of U isotope fractionation in accessory phases (zircon, titanite, apatite, ± baddeleyite) and igneous melts, and (ii) build the framework needed to ‘read’ the U isotope record of igneous systems. As part of this award, the PI will also develop a series of age-appropriate educational interventions that will bring the research process to students at all grade levels (locally, and internationally) so as to foster and strengthen their sense of identity as future scientists, and increase their sense of belonging and retention in STEM. To identify the drivers of U isotope fractionation in accessory phases and igneous melts, crystals of zircon, titanite, apatite, and baddeleyite from localities encompassing various redox conditions and covering a range of host-rock compositions and ages, will be characterized microtexturally and compositionally (SEM, CL, EBSD, SIMS, EPMA), and the valence (XANES), bonding environment (EXAFS) and isotopic composition (MC-ICPMS) of U will be measured in the very same grains. This approach will allow resolving the drivers of U isotope fractionation in magmatic settings. Exploratory zircon growth experiments done under variable fO2 conditions will directly test the role that zircon crystallization and U oxidation state have on magma U isotope evolution. The resulting concepts will be applied in a case study of a thoroughly characterized collection of detrital zircon crystals documenting one of the most dramatic changes in atmospheric oxygenation conditions on Earth: the Great Oxidation Event (GOE). This last task will test the hypothesis that shifts in the U isotope composition of terrestrial zircon and magmas are linked to the redox evolution of Earth’s hydrosphere and atmosphereThis award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.