HETEROGENEOUS METAMICTIZATION OF ZIRCON ON A MICROSCALE

Abstract

The ability of modern Raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 μm and a volume resolution of less than 5 μm3 opens up new opportunities for correlating structural, chemical, and isotopic data on the same micro-area within a zircon grain. Combining this information we confirm by direct measurement the inhomogeneity in the degree of crystallinity of complex zircon grains and the correlation between U-Th concentration, isotopic discordance, and degree of metamictization in micro-areas of these grains. Micro-areas with a higher degree of metamictization are more chemically heterogeneous than neighbouring more crystalline areas and contain higher concentrations of Ca, Fe, U, Th, and Y but not always Hf, which in some cases shows higher concentration in the less metamict areas. Raman observations that there are no ?radiation-damage halos? extending over distances greater than 1-1.5 μm preclude alpha and beta radiation as well as fission recoil as viable causes for amorphization, and confirm that alpha recoil is the most important mechanism causing zircons to disorder into the metamict state.