Uranium oxidation states in zircon and other accessory phases

Zircon and other U-bearing accessory phases are important time-capsules for studying the evolution of Earth and other planetary bodies as these minerals can record both temporal and compositional information regarding their host rocks. In silicate melts, uranium can occur in either the UIV, UV, or UVI valence state and its redox sensitive nature could, in principle, allow for information on magma oxygen fugacity (ƒO2) to be gleaned from U-bearing phases provided they can incorporate multivalent U during crystallization. Currently, however, little is known regarding the details of how U is speciated in these minerals. In this study, we conducted conventional X-ray absorption near-edge structure (XANES) spectroscopy at the U M4-edge on a set of natural zircon (n = 140), titanite (n = 9), apatite (n = 7), baddeleyite (n = 7), and garnet (n = 2) samples to determine the oxidation state of U in these crystals. We also collected U L3-edge spectra for select zircon samples to investigate the bonding environment of U using extended X-ray absorption fine structure (EXAFS) analysis. The effects of crystallographic orientation and radiation damage on zircon U M4-edge spectra are found to be minimal compared to the magnitude of the peak shifts associated with U oxidation state. We find that titanite and garnet contain only tetravalent U, while zircon, apatite, and baddeleyite can contain U of variable valence. Of these phases, zircon shows the greatest variability, with white-line energy, Ewl (i.e., peak absorbance) covering a range of >2.0 eV between grains: i.e., the entire energy range expected between pure UIV and pure UVI species. Moreover, a correlation is observed between the Ewl of zircon U M4-edge spectra (i.e., relative proportions of UIV, UV, UVI) and the ƒO2 of their host rocks. Our results thus establish U oxidation states in zircon as a powerful new tracer of magma redox. Since XANES is non-destructive and can be performed in situ, this technique can be utilized alongside other microanalytical methods (e.g., LA-ICPMS, SIMS) to further expand the breadth of information that can be extracted from single mineral grains.