(U-Th)/He dating of terrestrial impact structures: The Manicouagan example

The accurate dating of meteorite impact structures on Earth has proven to be challenging. Melt sheets are amenable to high-precision dating by the U-Pb and 40Ar/39Ar methods, but many impact events do not produce them, or they are not preserved. In cases where high-temperature shock metamorphism of the target materials has occurred without widespread melting, these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact. However, the (U-Th)/He chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites. We report (U-Th)/He dates for accessory minerals from the Manicouagan impact structure in Quebec, Canada. Nine zircons from a melt sheet sample yield a weighted mean age of 213.2 +/- 5.4 Ma (2SE), indistinguishable from the published 214 +/- 1 Ma (2s) U-Pb zircon age for the impact. In contrast, five apatites from this sample yield dates between 205.9 +/- 6.5 and 162.0 +/- 5.3 Ma (2 sigma), indicating variable postimpact helium loss due to low-temperature thermal disturbance. Preimpact titanite crystals from a shocked meta-anorthosite sample yield two dates consistent with the impact age, at 212 +/- 27 and 214 +/- 13 Ma (2 sigma), and two younger dates of 189.6 +/- 6.9 and 192.2 +/- 9.8 Ma (2 sigma), suggestive of postimpact helium loss. These results indicate that (U-Th)/He chronometry is a suitable method for dating impact events, although interpretation of the results requires recognition of possible He-4 loss related to reheating subsequent to impact.