(U-Th)/He, U/Pb, and radiation damage dating of the Rochechouart-Chassenon impact structure, France

It has been hypothesized that the ~25 km Rochechouart-Chassenon impact structure (RCIS) in the NW Massif Central, France, was formed during a Late Triassic (ca. 214 Ma) terrestrial impact event that produced a catena of several large craters. Testing this hypothesis, and assessing its possible impacts on biological evolution, requires both accurate and precise dating of candidate impact structures. Like many of these structures, the age of the RCIS is controversial because geochronological datasets yield contradictory results, even when a single isotopic system is used; for example, the two most recent 40Ar/39Ar studies of RCIS yielded statistically inconsistent dates of 201 ± 2 Ma (2σ) and 214 ± 8 Ma (2σ). While the precision offered by various geochronometers used to date impact structures varies significantly, a fair way to assess the confidence scientists might have in the accuracy of an impact age is to establish whether or not multiple chronometers yield statistically indistinguishable ages when applied to that structure. With that in mind, I have applied the (U-Th)/He, U/Pb, and radiation damage chronometers to zircons separated from two different RCIS impactites. My best estimate of the zircon (U-Th)/He age of the impact event is 191.6 ± 9.1 Ma at the 95% confidence level. U/Pb zircon dates suggest that most zircons in the RCIS target rocks were not completely reset during impact, but a subset (n = 8) of zircons appear to have crystallized from the impact melt or to have been completely reset; these zircons indicate a U/Pb impact age of 202.6 ± 5.8 Ma (95% confidence level). Zircon radiation damage dates are highly variable, indicating that the RCIS event resulted only in partial annealing of pre-impact zircon in the country rock, but a small sub-population of zircons yielded a mean date of 211 ± 13 Ma (95% confidence level). These results – all statistically indistinguishable from the previously published 40Ar/39Ar date of 201 ± 2 Ma – collectively argue that the impact age was near the presently agreed upon Triassic-Jurassic boundary. This age raises the possibility that seismite and tsunamite deposits found in the present-day British Isles may be related to the RCIS.