Lead oxide nanospheres in seismically deformed zircon grains

The presence of Pb oxide nanospheres in zircon from mylonites of the Ivrea-Verbano Zone (IVZ), Northern Italy is described for the first time. Isotopic dating of zircon reveals detrital cores with ages scattered from 960 to 320 Ma, and metamorphic rims with a mean age of 280 ± 4 Ma. Zircon crystals, derived from samples that contain pseudotachylite formed during seismic events, have planar fractures (PFs) and planar deformation bands (PDBs). The PDBs are associated with straight dislocation arrays in glide configuration. Detrital zircon cores have mottled diffraction contrast in TEM bright field images, indicative of irradiation damage and/or annealing in radiation damaged zircon. Lead oxide nanospheres up to 9 nm in diameter, as recognized in TEM, occur in detrital cores but not in metamorphic rims of the zircon. No relationship was observed between the nanospheres and PDBs or PFs. This is the first report of lead oxide nanospheres in zircon from a Phanerozoic metamorphic rock, as well as being the first observed case of such nanoinclusions being composed of Pb oxide, rather than native Pb. The formation of the nanospheres is attributed to high-temperature metamorphism, with subsequent seismic activity having little effect on distribution or preservation.