Uranium oxidation states in zircon and other accessory phases

Creators: Houchin, Shane K.¹; Tissot, François L.H.¹; Ibañez-Mejia, Mauricio²; Newville, Matthew³; Lanzirotti, Antonio³; Pavia, Frank¹; Rossman, George¹

1. California Institute of Technology
2. University of Arizona
3. University of Chicago

Abstract

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 U^IV, U^V, or U^VI 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, E_wl (i.e., peak absorbance) covering a range of >2.0 eV between grains: i.e., the entire energy range expected between pure U^IV and pure U^VI species. Moreover, a correlation is observed between the E_wl of zircon U M₄-edge spectra (i.e., relative proportions of U^IV, U^V, U^VI) 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.

Copyright and License

Acknowledgement

SKH thank Chi Ma for training and help with the SEM, as well as Maddie Lewis for providing zircons from the HLMC, Jade Star Lackey for providing zircons from the Sierra Nevada plutons and the garnet samples, Claire Bucholz for providing the zircons from the Dariv Igneous Complex and the SPG samples, Barbara Ratschbacher for providing rock samples of LF042-02 (Lassen) and MC18-2,4 (Long Valley Caldera), and Carl Swindle for providing rock samples of the Pacoima Canyon Pegmatite. Thoughtful comments from the associate editor, George Calas, and three anonymous reviewers helped to strengthen the manuscript. This work was supported by NSF CAREER grant EAR-2145780 to FLHT, as well as a Discovery grant and start-up funds to FLHT provided by Caltech. MIM and the Arizona LaserChron Center acknowledge support by NSF award EAR-2050246. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation - Earth Sciences (EAR-1634415).

Funding

This work was supported by NSF CAREER grant EAR-2145780 to FLHT, as well as a Discovery grant and start-up funds to FLHT provided by Caltech. MIM and the Arizona LaserChron Center acknowledge support by NSF award EAR-2050246. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation - Earth Sciences (EAR-1634415).

Contributions

FLHT and MIM conceived and initiated the project. FLHT, MIM and SKH designed the research. SKH prepared and characterized the samples. SKH, FLHT, MIM, MN, AL performed the XAS measurements. SKH and GR performed the Raman measurements. SKH, FLHT, MIM, MN, AL analyzed and interpreted the data. SKH wrote the initial manuscript under FLHT’s guidance, and with input from MIM, MN, AL and GR. FP contributed to the initial beamtime request and the first XAS session.

Data Availability

Data are available through Caltech Data at: https://doi.org/10.22002/36sg9-yhj98.

Files

1-s2.0-S0016703724003879-mmc1.pdf

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes