Anders Scherstén

The formation age of the large (∼50 km) Carswell impact structure, Canada, has been a matter of debate ever since its discovery five decades ago, with proposed ages ranging from Mesoproterozoic to Early Cretaceous. Here, we present new 40Ar-39Ar data for aliquots of euhedral adularia, separated from vesicles in an impact melt rock from the central uplift of the structure. The analyses of the adularia yielded a statistically robust Early Ordovician crystallization age of 481.5 ± 0.8 Ma (2σ, mean square of weighted deviates = 1.06, P = 0.30). The most plausible explanation for the formation of vesicle-filling adularia is through low-temperature mineral precipitation during residual hydrothermal circulation that followed the impact, as no other known major intrusive, extrusive, or thermal events have occurred in the Carswell region in the Phanerozoic. The new age of the Carswell impact structure overlaps within uncertainty with the most precise Ar-Ar ages proposed for the L-chondrite parent body breakup event, but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered. Thus, either the Carswell impact represents a separate, unrelated impact event, or the dynamic evolution of the L-chondrite parent body breakup is more complicated than presently understood, and Carswell represents one of the earliest and largest known impacts of this event on Earth.