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

Abstract

[1] 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 40 Ar/ 39 Ar 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 (2s), 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 (2s), and two younger dates of 189.6 ± 6.9 and 192.2 ± 9.8 Ma (2s), 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 4 He loss related to reheating subsequent to impact.

Impact thermochronology and the age of Haughton impact structure, Canada

“…Raw and alpha ejection‐corrected dates are reported in Table S1 in the supporting information, with 2σ errors based on propagated analytical uncertainties only. Further details on NG 3 L procedures are available elsewhere [ van Soest et al ., ].…”

Dynamic interaction between impact melt and fragmented basement at Manicouagan: The suevite connection

“…Based on the U‐Pb (Hodych and Dunning ) and (U‐Th)/He (Van Soest et al. ) dating of zircons from the impact melt body, formation ages of 214 ± 1 Ma and 213.2 ± 5.4 Ma have been obtained, respectively. Titanites from central uplift anorthosites have been dated at 208.9 + 5.1 Ma (Biren et al.…”

(U‐Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073

“…Additional details of the (U‐Th)/He analytical procedures can be found in Van Soest et al. (). (U‐Th)/He dates were calculated iteratively from blank‐corrected 4 He, 232 Th, and 238 U concentrations.…”