Abstract
Zircon Raman dating is an emergent thermochronological method. It
exploits the disruption of the zircon lattice due to α-disintegration of
trace amounts of 238U, 235U, 232Th, and their daughters. The radiation
damage broadens the Raman bands and shifts them to lower wavenumbers.
The Raman bandwidths provide a sensitive measure for the accumulated
lattice damage (Nasdala et al., 1995; Nasdala et al., 2001). The
measured bandwidth and the effective uranium concentration define the
Raman age (Härtel et al., 2021). Radiation damage anneals upon heating
and the meaning of the Raman age depends on the zircon’s thermal
history. The Raman age is a formation age if no annealing has taken
place, or a reset age if all pre-existing damage has been annealed by a
geological heating event. In the case of partial annealing, however, the
Raman age is a mixed age with no obvious geological significance. Mixed
ages are difficult to interpret and cannot be distinguished from reset
ages using the standard procedure for annealing detection (Nasdala et
al., 2001). On the other hand, inhomogeneous damage distributions due to
actinide zoning within zircon grains present a problem for zircon Raman
dating. Overlapping signals from more and less damaged zones lead to
asymmetric Raman bands and overestimated bandwidths (Nasdala et al.,
2005). We discuss Raman spectra of zircon from partially annealed
samples and spectra with asymmetric bands. We introduce discrimination
plots based on the 356, 439, and 1008 cm-1 bandwidths that provide a
means for detecting and distinguishing asymmetry and partial annealing.
We discuss examples of zircon Raman dating and present a measurement
protocol.