Recrystallization and New Growth of Radiation-Damaged Zircon

English Abstract:

Nearly metamict zircon crystals from the Saranac Prospect near Bancroft, Ontario were ground to a fine powder for powder XRD measurements, HR-TEM imaging and analyses, and CA-TIMS analyses. Aliquots of the ground zircon powder were annealed in situ using a Pt furnace during which time simultaneous powder diffraction data were acquired starting at 25 C, at elevated temperature (from 500 C to 1400 C) at selected time intervals. The powder XRD results indicate that below ~900 C the recrystallization of zircon is incomplete, even after 36 hours. At 1150 C the zircon powder shows significant recrystallization in less than one hour. Before annealing the zircon powder consisted of clear, transparent to brown, translucent, complexly zoned fragments. After heating at 900 C the majority of material had transformed to white opaque microcrystalline fragments. The clear fragments are thought to be preexisting original minimally radiation damaged crystalline zircon, the brown complexly zoned fragments are likely preexisting extremely metamict zircon, and the white opaque fragment newly recrystallized zircon and ZrO2. At 1150 C all that remained after heating were dominantly white opaque fragments and extremely rare clear fragments. A variety of fragment types from the unannealed, 900 C, and 1150 C anneals were chemically abraded at 190 C for 12 hours. After chemical abrasion, all unannealed material, nearly all material from the 900 C anneal, and all white opaque microcrystalline material from the 1150 C anneal, dissolved. Only the rare residual clear, transparent fragments from the 1150 C anneal remained and yielded concordant U-Pb ID-TIMS dates of 1064 Ma confirming the hypothesis that low-U closed system domains are preserved through annealing up to 1150 C and can be extracted via chemical abrasion from even dominantly metamict zircon crystals. By contrast, newly formed post-annealing recrystallized zircon appear to be quite soluble during chemical abrasion process.