Geocheminar spring-2014

We investigate the presence of epitaxial overgrowth rims and 'reset' zircon from highly shocked terrestrial impactites to constrain the occurrence of such phenomena in impact environments and explore the use of (U-Th)/He dating of zircon to evaluate this geochronometer in identifying impact ages. Our results show that (U-Th)/He ages of zircon, isolated from the shocked target of the Morokweng impact structure, can accurately date an impact event and provides another tool with which to determine impact ages when no dateable melt sheet exists, an alternative to problematic interpretations of commonly used apparent 40Ar/39Ar plateau ages. No evidence of epitaxial overgrowth rims and/or zircon 'reset' in U-Pb was observed and suggests that even shocked zircon has remarkably slow Pb diffusion, possibly explaining the lack of 'reset' grains in terrestrial impactites. We also present a zircon (U-Th)/He age for the ~100 km-diameter Popigai crater, Siberia, Russia, of 33.9±1.3 Ma. This is significantly younger than a 35.7±0.2 Ma age previously reported (Bottomley et al., 1997) and is, within error, synchronous with the Eocene-Oligocene boundary mass extinction at 33.7±0.5 Ma (Prothero, 1994). Dramatic cooling observed at this interval is presumably due to gas fluxes and 'impact winter' type conditions consistent with modeled scenarios for the Chicxulub impact (Pope et al., 1994) and is similar to estimates for Popigai (Kring, 2003), providing the first evidence of an impact induced mass extinction in the Cenozoic.