Microscopic effects of shock metamorphism in zircons from the Haughton impact structure, Canada
A.C. Singleton, G.R. Osinski, S.R. Shieh, 2015. "Microscopic effects of shock metamorphism in zircons from the Haughton impact structure, Canada", Large Meteorite Impacts and Planetary Evolution V, Gordon R. Osinski, David A. Kring
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The shock metamorphism of rocks and minerals is an important consequence of hypervelocity impact events. Much remains to be understood regarding effects in accessory geochronology minerals, such as the effect of very high shock conditions on zircon (ZrSiO4). This study explores the effects of shock metamorphism on zircon in crystalline, quartzofeldspathic basement rocks from the ~23–39-m.y.-old, 23-km-diameter Haughton impact structure on Devon Island, Arctic Canada. This is a valuable location for this study because the structure is very well preserved and contains materials covering a wide range of shock levels. A petrographic survey of 255 zircon grains revealed a variety of microfeatures, including fracturing, planar features, and granular texture, as well as a microporosity texture in highly shocked (level 6, 55–60 GPa) zircon grains. This survey showed that general trends exist in the proportions of grains from each shock level that display certain microscopic features, including a decrease in fractures and appearance of granular textures related to increasing shock pressures. Raman spectroscopy data from 22 zircon grains showed evidence of radiation damage, impurities, the presence of reidite, and recrystallized grains. The overall pattern with increasing shock level is that of metamict, low-crystallinity zircon in basement gneiss outside the impact structure, increased crystallinity and preservation of reidite in an intermediate-shock-level sample, and highly crystalline, annealed zircon (no reidite) in highly shocked samples otherwise composed entirely of glass. The sequence of structural and phase changes observed in this study is consistent with the findings of previous work, and our results expand the body of knowledge about the series of shock features that can be applied to other similar structures, both on Earth and other planetary bodies.