Uranium Oxidation States in Zircon and Other Accessory
Phases
Houchin SK1, Tissot FLH1,
Ibaņez-Mejia M2, Newville M3,
Lanzirotti A3, Pavia F1,
Rossman GR4
1The Isotoparium, Division of
Geological and Planetary Sciences, Caltech, Pasadena, CA, USA
2Department of Geosciences, University. of
Arizona, Tucson, AZ, USA
3Center
for Advanced Radiation Sources, The University of Chicago, Chicago, IL,
USA
4Division
of Geological & Planetary Sciences, Caltech, Pasadena, CA, USA
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 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.