HETEROGENEOUS METAMICTIZATION OF ZIRCON ON A MICROSCALE

The ability of modern Raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 mu m and a volume reso lution of less than 5 mu m(3) opens up new opportunities for correlati ng structural, chemical, and isotopic data on the same micro-area with in a zircon grain. Combining this information we confirm by direct mea surement the inhomogeneity in the degree of crystallinity of complex z ircon grains and the correlation between U-Th concentration, isotopic discordance, and degree of metamictization in micro-areas of these gra ins. Micro-areas with a higher degree of metamictization are more chem ically heterogeneous than neighbouring more crystalline areas and cont ain higher concentrations of Ca, Fe, U, Th, and Y but not always Hf, w hich in some cases shows higher concentration in the less metamict are as. Raman observations that there are no ''radiation-damage halos'' ex tending over distances greater than 1-1.5 mu m preclude alpha and beta radiation as well as fission recoil as viable causes for amorphizatio n, and confirm. that alpha recoil is the most important mechanism caus ing zircons to disorder into the metamict state.