RADIATION DAMAGE IN ZIRCON FROM A JACK HILLS GRANITOID, WESTERN AUSTRALIA
Ion microprobe studies on granitoid zircons from the Jack Hills have reported discordant ages for some grains, particularly in grains that show complex zoning. We have investigated the zoning in zircons from a nearby pluton using electron microprobe analysis and transmission electron microscopy (TEM). Zircons (~20 grains, sample W61), were selected from a porphyritic granite in the Jack Hills region, Western Australia. The age of the W61 zircons is ~3.3 Ga, but >300 Ma younger ages were reported in the complexly zoned portion of a crystal (SHRIMP, Pidgeon and Wilde, 1989).
Back-scattered electron images show well developed zoning in all grains. Bright zones are nearly pure zircon with 1.2-1.9 wt% of Hf, while dark zones contain U, Th, and Pb, up to 94000, 10000 and 1100 ppm, respectively, and a variety of impurities: Al up to 1.0 wt%, Ca up to 0.55 wt%, Fe up to 10 wt%, Y up to 0.50 wt%, Ce up to 1.2 wt%, and other rare earth elements in the minor amounts. The calculated damage by alpha radiation in dark zones is up to 1.1x1017 (alpha-decay events/mg), which is equivalent to 4.9 dpa (displacement per atom) as calculated using the SRIM (stopping power and range of ions in matter) code. Most of the dark zone has experienced from 1.0-3.0 dpa of calculated damage, which is much higher than the critical amorphization dose of synthetic zircon, 0.1-0.6 dpa, in the range of 0-800 K, as determined by ion irradiation. High-resolution TEM images reveal that amorphous domains developed at the nanoscale, but crystalline zircon is still present and the orientation of the remnant crystallites is nearly the same as the original grain. In the portion that contains the largest amount of impurities, electron diffraction shows a ring pattern of the zircon structure, and randomly oriented zircon nanocrystals are observed in this area. The TEM results suggest intensive thermal annealing has occurred in these zircons. The pluton containing sample W61 is a possible source for some of the younger detrital zircons in the Jack Hills metaconglomerate, however such high radiation damage is not seen in samples with concordant U-Pb ages, suggesting destruction of highly damaged zircons during erosion or rejection in sample selection prior to geochronology.