RESOLVING LARGE MAGNITUDE AND WIDESPREAD ANNEALING OF LUNAR ZIRCON THROUGH CORRELATIVE SIMS, EBSD AND RAMAN SPECTROSCOPY

The trace element and isotope chemistry of lunar zircon places constraints on the solidification and thermal evolution of the lunar surface and interior. To a first order, most lunar zircon grains have persisted in a cool, dry environment, which kinetically inhibits both structural recovery and the migration of chemical tracers. Early lunar volcanism and impact events have generated short-lived, high temperature episodes, and the potential introduction of shock microstructure. While several studies have documented disruption of U-Pb systematics associated with shock deformation, there are few published studies considering the combined structural evolution of lunar zircon including deformation, self-irradiation, and thermal annealing. Each of these processes influence the nanoscale structure of zircon, which controls the retention and/or disruption of primary zircon chemistry in the lunar environment.

We have applied a combination of secondary ion mass spectrometry (SIMS), electron backscatter diffraction (EBSD), and Raman spectroscopy to a suite of lunar zircon grains from four Apollo landing sites and possessing a variety of lithologic contexts: zircon grains within a quartz monzodiorite rock fragments, isolated grains within lunar breccias, and loose grains in the lunar regolith. Raman spectroscopy identifies the local damage state as the primary control on both band broadening and band position, and documents significant annealing. The fractional dose, calculated as the ratio of effective dose and total dose, varies from 0.22 to 0.77. These are the lowest fractional doses reported for lunar zircon, and require that some thermal processes driving annealing are young. Notably, present-day damage states do not correlate with known episodes of thermal/mechanical disturbance. These observations have several implications for the study of lunar zircon including: (1) high-T processes may anneal a large percentage of lunar zircon grains, (2) damage states may reflect periods of incomplete annealing, complicating any broad application of Raman-based “radiation damage ages” to study the thermal histories of lunar zircon, and (3) in the absence of shock deformation, the balance (or lack thereof) between radiation damage and annealing will exert the main control on trace element and isotope mobility.