Diachroneity of the Clearwater West and Clearwater East impact structures indicated by the (U-Th)/He dating method

The (U-Th)/He method has been applied to constrain the formation ages of the Clearwater West and East impact structures of Quebec, Canada. Zircons were separated from impact melt samples derived from a surface exposure at Clearwater West (32 km diameter), and from a drill core at Clearwater East (26 km diameter). The (U-Th)/He results indicate ages of 280±27 Ma (2σ, n=7) for Clearwater West, and 450±56 Ma (2σ, n=8) for Clearwater East. Our (U-Th)/He date for Clearwater West supports the findings of previous Rb-Sr (266±15 Ma; 2σ) and 40Ar/39Ar (280±4 Ma and 283.8±2.2 Ma, 2σ) impact melt studies. Our (U-Th)/He date for Clearwater East also overlaps with previously published 40Ar/39Ar dating results, which yielded U-shaped spectra, with 'maximum' and 'best-estimate' dates of ∼ 460-470 Ma. Our results support the contention, previously based solely on 40Ar/39Ar data, that the Clearwater West and East impact structures do not comprise an impact doublet that formed coevally from a binary asteroid pair.