^1,2Benjamin E. Cohen,^1,3Darren F. Mark,²Martin R. Lee,²Sarah L. Simpson
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12880]
¹NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre, East Kilbride, UK
²School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
³Department of Earth and Environmental Sciences, University of St Andrews, St Andrews, UK
Published by arrangement with John Wiley & Sons

The 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 multicollector ARGUS-V 40Ar/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.

^1,2,3M. van Ginneken, ⁴J. Gattacceca, ⁴P. Rochette, ⁴C. Sonzogni, ⁴A. Alexandre, ⁴V. Vidal, ¹M.J. Genge
Geochmica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.008]
¹IARC, Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
²Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 2AZ, UK
³Laboratoire G-Time, Université Libre de Bruxelles, Av. F.D. Roosevelt 50, 1050 Brussels, Belgium1
⁴CNRS/Aix-Marseille Université, IRD, Collège de France, CEREGE UM34, Aix-en-Provence, France
Copyright Elsevier

High-precision oxygen isotopic compositions of eighteen large cosmic spherules (>500 µm diameter) from the Atacama Desert, Chile, were determined using IR-laser fluorination – Isotope Ratio Mass spectrometry. The four discrete isotopic groups defined in a previous study on cosmic spherules from the Transantarctic Mountains (Suavet et al., 2010) were identified, confirming their global distribution. Approximately 50% of the studied cosmic spherules are related to carbonaceous chondrites, 38% to ordinary chondrites and 12% to unknown parent bodies. Approximately 90% of barred olivine (BO) cosmic spherules show oxygen isotopic compositions suggesting they are related to carbonaceous chondrites. Similarly, ∼90% porphyritic olivine (Po) cosmic spherules are related to ordinary chondrites and none can be unambiguously related to carbonaceous chondrites. Other textures are related to all potential parent bodies. The data suggests that the textures of cosmic spherules are mainly controlled by the nature of the precursor rather than by the atmospheric entry parameters. We propose that the Po texture may essentially be formed from a coarse-grained precursor having an ordinary chondritic mineralogy and chemistry. Coarse-grained precursors related to carbonaceous chondrites (i.e. chondrules) are likely to either survive atmospheric entry heating or form V-type cosmic spherules. Due to the limited number of submicron nucleation sites after total melting, ordinary chondrite-related coarse-grained precursors that suffer higher peak temperatures will preferentially form cryptocrystalline (Cc) textures instead of BO textures. Conversely, the BO textures would be mostly related to the fine-grained matrices of carbonaceous chondrites due to the wide range of melting temperatures of their constituent mineral phases, allowing the preservation of submicron nucleation sites. Independently of the nature of the precursors, increasing peak temperatures form glassy textures.