¹E. Quirico, ²H. Yabuta, ¹P. Beck, ¹L. Bonal, ^3,5A. Bardyn, ^3,4L.R. Nittler, ³C.M.O’D. Alexander
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.05.046]
¹Université Grenoble Alpes, CNRS, Institut de Planétologie et Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble F-38041, France
²Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Japan
³Earth and Planets Laboratory, Carnegie Institution for Science, 5241 Broad Branch Road, N.W., Washington, DC 20015, USA
⁴School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁵Brin Mathematics Research Center, The University of Maryland, 4146 CSIC Bldg. #406, 8169 Paint Branch Drive, College Park, MD 20742-3289, USA
Copyright Elsevier

A significant population of primitive carbonaceous chondrites experienced short-duration heating, which is usually attributed to either impact or solar heating. Shock recovery experiments performed on carbonaceous chondrites have successfully reproduce the typical evolution in the petrographies and mineralogical compositions of natural samples. However, only few studies focused on the chemical and structural transformations of insoluble organic matter (IOM). We report here on shock recovery experiments conducted on two chondrites: Murchison (CM2) and Elephant Moraine EET 90628 (L3.0). Experiments on Murchison show carbonization and oxidation of IOM at all shock intensities (5–50 GPa) and a pronounced structural evolution at 40 GPa associated with complete dehydroxylation of serpentines, as well as formation of olivine and amorphous silicates. The δD value of Murchison IOM (initial δD = 1636 ± 529 ‰) evolves significantly, with the rapid disappearance of isotopic hot spots and a bulk δD of −79 ‰ at 40 GPa. At 40 GPa, the extent of dehydroxylation of serpentines is consistent with stage III heated chondrites, but the structural characteristics of the IOM resembles material from stage II meteorites, i.e. a slight modification of the IOM in a matrix dominated by serpentines.
These experiments only partially reproduce the characteristics of natural samples, and they show that the IOM evolution in short-duration heated C2 chondrites is essentially controlled by the post-shock cooling episode, which lasts from hours to years, compared to < ∼1 µs for the shock peak pressure. The high pressure conditions in the shock do not catalyze the carbonization process and the maturation of IOM. In contrast, the IOM evolution in heated C2 chondrites is better simulated by conventional heating experiments under controlled redox conditions over durations of hours. Shock recovery experiments, however, could be interesting to assess the effect of hypervelocity impacts by small impactors on the surface of airless bodies. Experiments performed on EET 90628 show a structural evolution consistent with natural objects. In particular, the co-evolution of the width and ratio of the peak intensities of the D-band (FWHM-D and ID/IG, respectively) in the Raman spectra of the IOM from the shocked samples is consistent with those measured on type 3 ordinary and carbonaceous chondrites. An interesting finding is that the G-band width and position parameters (FWHM-G and ωG) do not correlate with the shock intensity, just as these parameters do not correlate with the intensity of thermal metamorphism in the case of type 3 chondrites. This lack of correlation is not observed on Earth in the case of coals and kerogens that experienced a progressive thermal history.

¹Daniela Guerrero, ¹Wolf Uwe Reimold, ¹Natalia Hauser, ²Gavin Kenny, ²Martin Whitehouse, ³Philippe Lambert
Geochimica et Cosmochimica (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.05.049]
¹Postgraduate Program in Geology, Institute of Geosciences, University of Brasília, 70910-900, Brasília, DF, Brazil
²Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
³CIRIR – Centre International de Recherche et de Restitution sur les Impacts et sur Rochechouart 87600 Rochechouart, France
Copyright Elsevier

The >23 km diameter, ∼207 Ma old Rochechouart impact structure is located in the NW part of the Paleozoic basement of the French Massif Central. Despite significant erosion, this impact structure preserves a heterogeneous suite of impactites, and the transition between the crater floor and the basement. Recent textural and geochronologic studies of U-Pb on zircon from impactites and basement lithologies of this structure have shown a wide variety of shock deformation textures and age distributions. In this study, we present a comprehensive analysis combining detailed textural characterization (CL, BSE, EBSD) and U-Pb geochronological analyses at different spatial resolutions (SIMS and LA-ICP-MS) of zircon from two melt-bearing breccias (suevites) from Chassenon and Videix, and one impact melt rock (IMR) from Babaudus. The analyzed crystals display a variety of shock deformation textures. Identification of FRIGN zircon and grains with high proportions of reidite in the Videix suevite indicates that these types of shock deformation are more widespread than previously reported. In the Chassenon suevite, U-Pb age resetting increases with shock intensity, whereas in the Videix suevite, higher U and/or Th contents also appear to control resetting. In the Babaudus IMR, the similar ages for shocked granular zircons and some unshocked grains suggest that additional factors, beyond shock deformation and zircon composition, influence age resetting. The SIMS analyses yielded more reliable results after common Pb correction. The best estimate of the impact age obtained from this study is 203 ± 4 Ma (2σ, MSWD = 3.4, probability = 0.065) for SIMS analyses of two granular grains from the Babaudus IMR and one granular crystal from the Videix suevite. Zircons with younger (191 ± 4 Ma, post-impact) ages show similar characteristics to those close to the widely accepted age for the impact at 207 Ma, highlighting the challenge of distinguishing between grains and separating ages related to the impact from possible post-impact events (e.g., hydrothermal alteration). Finally, the geochronological results for the Videix and the Chassenon suevites show a clear correlation with provenance results for granitic and gneissic target lithologies, respectively. In contrast, the Babaudus IMR has an age distribution comparable with other impact melt rocks from Montoume and Recoudert but cannot be related to an identified target lithology.