Annealing zircon with zoned radiation damage distribution and
implications for (U-Th)/He thermochronology
Abstract
Radiation damage in zircon directly impacts diffusion of He from the
crystal lattice and is a key factor in defining the kinetics of the
zircon (U-Th)/He system. Damage accumulates within a crystal as a
function of time and U and Th concentration, but can be thermally
annealed as well. The total level of radiation damage in a zircon
crystal is governed by a thermally-activated, kinetic process, which in
turn influences the interpretation of zircon (U-Th)/He dates for thermal
histories. Several annealing models have been defined for the zircon
system based on measurements in natural crystals; however, few studies
have investigated how multiple levels of radiation damage due to
zonation of actinides within a crystal may influence the annealing
process. Here we use Raman spectroscopy to map the full width half
maximum (FWHM) of then (SiO) band, a proxy for radiation damage, in
zircon crystals from the Lucerne pluton (Maine, USA) with heterogeneous
distributions of U and Th. We compare FWHM maps before and after
annealing these crystals at laboratory times and temperatures. These
maps show that each damage zone within a single zircon acts as an
isolated domain that is dictated by an independent set of annealing
kinetics. Thermally activated annealing decreases radiation damage in
all radiation damage zones; however, the rate of annealing is not
consistent across all zones. We identify specific modes of damage in
probability density plots of all measured FWHM in a crystal that are not
present in pre-annealed imagery, but persist in post-annealing Raman
maps, regardless of laboratory time temperature conditions: FWHM modes
at 2-5 cm, 10-15 cm, and25-30 cm. We attribute these persistent damage
modes to variable annealing kinetics that are partially dependent on the
level of pre-annealing damage, combined with the inability of
high-damage crystals, or zones within crystals, to fully recover their
crystallinity. That is, some damage is permanent. These findings
therefore show that zircon crystals with non-uniform distributions of U
and Th can anneal to create long-lived damage zones at specific damage
levels, which has implications for treating the zircon (U-Th)/He
chronometer as a multi-domain diffusion system.