Positioning the zircon fission-track and zircon micro-Raman partial annealing zones

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Zircon is a common constituent of igneous, metamorphic and derived clastic sedimentary rocks. It is a valuable mineral for geological age dating because of U and Th substitutions in its lattice. Their radioactive decay produces Pb, He, fission products and various forms of lattice damage. My first aim is to advance zircon fission-track dating by determining the geolog-ical conditions under which the zircon fission-track clock is reset. This involves locating the zircon fission-track partial annealing zone (ZPAZ). In addition, lattice damage from self-irradiation affects both the etching and annealing properties of the fission tracks. The kinetics of radiation-damage accumulation and repair have been studied in lab experiments. Their extrapolation to geological timescales predicts that the amorphization of zircon is reduced, or suppressed, due to damage annealing at temperatures above ~160 °C. My second aim is therefore to provide geological benchmarks for the zircon radiation-damage annealing zone (ZRAZ). Both aims can be realized by examining the fission-track and radiation-damage records of zircons from the Kontinentale Tiefbohrung, which reached 265 °C at 9.1 km depth. I predict that it traversed the ZRAZ between 3 and 5 km depth and the ZPAZ between 7 and 9 km depth. The results of this investigation could help to evaluate the potential of zircon for the long-term sequestration of high-level nuclear waste and for the exploration of thermogenic gas.