A new high‐precision40Ar/39Ar age for the Rochechouart impact structure: At least 5 Ma older than the Triassic–Jurassic boundary

Benjamin E. Cohen,Sarah L. Simpson,Martin R. Lee,Darren F. Mark

doi:10.1111/maps.12880

Abstract

AbstractThe Rochechourt impact structure in south‐central France, with maximum diameter of 40–50 km, has previously been dated to within 1% uncertainty of the Triassic–Jurassic boundary, at which time ~30% of global genera became extinct. To evaluate the temporal relationship between the impact and the Triassic–Jurassic boundary at high precision, we have re‐examined the structure's age using multicollectorARGUS‐V40Ar/39Ar mass spectrometry. Results from four aliquots of impact melt are highly reproducible, and yield an age of 206.92 ± 0.20/0.32 Ma (2σ, full analytical/external uncertainties). Thus, the Rochechouart impact structure predates the Triassic–Jurassic boundary by 5.6 ± 0.4 Ma and so is not temporally linked to the mass extinction. Rochechouart has formerly been proposed to be part of a multiple impact event, but when compared with new ages from the other purported “paired” structures, the results provide no evidence for synchronous impacts in the Late Triassic. The widespread Central Atlantic Magmatic Province flood basalts remain the most likely cause of the Triassic–Jurassic mass extinction.

Highlights

Accurate and precise ages for impact structures are crucial to evaluate the geological effects of these sudden cataclysmic events (Jourdan et al 2009a, 2012)
The Schmieder et al (2010) Rochechouart age of 203 Æ 2 Ma, with a 2r uncertainty of Æ1% (Table 1), lacks the precision necessary to evaluate if the impact event predates, postdates, or is synchronous with the Triassic–Jurassic boundary (Lambert 2010; Jourdan et al 2012)
The fine grain size of K-feldspar in our Babaudus impact melt sample indicates that this material cooled and crystallized rapidly after the impact event

Summary

Introduction

Accurate and precise ages for impact structures are crucial to evaluate the geological effects of these sudden cataclysmic events (Jourdan et al 2009a, 2012). The existing most precise age for the Rochechouart impact event is 203 Æ 2 Ma (2r; Schmieder et al 2010), recalculated using the decay constants and Fish Canyon sanidine standard age from Renne et al (2011) (Table 1). This age of 203 Æ 2 Ma overlaps with the Triassic–Jurassic boundary age of 201.33 Æ 0.27 Ma (2r full external uncertainties; Schoene et al 2010; Gradstein et al 2012). The Schmieder et al (2010) Rochechouart age of 203 Æ 2 Ma, with a 2r uncertainty of Æ1% (Table 1), lacks the precision necessary to evaluate if the impact event predates, postdates, or is synchronous with the Triassic–Jurassic boundary (Lambert 2010; Jourdan et al 2012)

Methods

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Discussion

Conclusion