FISSION-TRACK DATING OF ZIRCON - ZIRCONS FROM MONT-BLANC GRANITE (FRENCH-ITALIAN ALPS)

Zircon population in granite rocks are most often heterogeneous with respect to uranium concentrations (which can vary from a few hundred ppm to > 1%) and the typology of the grains (for example, in the type of dominant prism [100] or [110]), making them difficult to date using Fission Track (FT) dating. The Mont Blanc zircons, which vary greatly in both uranium content and zircon type, are typically difficult to date by the Fission Track method. To obtain information about fission track retention characteristics of these minerals, experimental isothermal annealing was performed on grains free of alpha-damage (i.e., annealing of freshly induced tracks). Uranium-poor grains, with a dominant (100) prism, were found to be more stable. A Step Etching Method (SEM) was used to date the zircons. Uranium-poor zircons with a dominant (100) prism yielded higher fission track ages than did uranium-rich ones with a (110) dominant prism. Thus, etching conditions (temperature and etching time) and type of prism chosen ([100] or [110]) must be carefully chosen and accounted for to avoid scattered fission track ages on a zircon population. Fission track and other dates from the Mont Blanc granite show a late thermal history that began after it cooled down to a temperature of 350-degrees-C at 33 Ma. Its cooling history was rather slow, at 6-degrees-C/m.y. from 33-11 Ma. The last cooling below 130-degrees-C occurred at 11 Ma, and the associated uplift phase has been very rapid.