¹Axel Wittmann, ²Philippe Lambert
Earth and Planetary Science Letters 674, 119748 Link to Article [https://doi.org/10.1016/j.epsl.2025.119748]
¹Eyring Materials Center, Arizona State University, 1001 S. McAllister Ave., Tempe, AZ 85287-8301, USA
²CIRIR ‒ Centre International de Recherche et de Restitution sur les Impacts et sur Rochechouart, 2-4 Faubourg du Puy du Moulin, Rochechouart 87600, France
Copyright Elsevier

The 205 Ma Rochechouart impact structure in southern France exhibits variable levels of erosion that mask its original diameter for which estimates vary between 10 and 35 km. Exclusively at Chassenon, the size-sorted, “ash-like” impactoclastite deposit occurs as the last preserved material directly produced by the impact. To test whether impactoclastite is indeed a fallback deposit from the impact plume, we studied 18 Rochechouart impactite samples, of which 15 are dike-like intercalations of impactoclastite in suevite from Chassenon. We found 63 impact spherules in 13 samples from Chassenon, down to a drill core depth of 27.65 m. These spherules are impact melt droplets that record suspended flight. Of these spherules, 30 % crystallized Ni-bearing spinel, 11 % contain small NiO particles, and one includes a ∼140 nm Pt-Os-Ru-Ir-Rh-Pd nugget; these are impactor components, confirming formation in close proximity to the point of impact. The exclusive occurrence of impact spherule-bearing impactoclastite associated with suevite at the Chassenon location suggests special formation conditions that we link to the collapse of the Rochechouart central peak, which induced the down-thrusting of the ∼3 km2 “Chassenon slab”. Resulting fissures in suevite were filled with debris that fell back from the ejecta plume one hour to ca. 1 day after the impact. This interpretation negates the deposition of the Chassenon suevite from marine resurgence immediately following the impact. Instead, we invoke a “debris-inhalation” process that injected impactoclastite dikes due to brief vacuum conditions generated in the sub-crater floor during collapse of the Chassenon slab.

¹Cuiping Wang, ²Haolan Tang, ³ Miao, ² Yu, ¹ He, ^1,4 Liu, ²Fang Huang, ⁵Frederic Moynier, Jingao Liu
Earth and Planetary Science Letters 674, 119747 Link to Article [https://doi.org/10.1016/j.epsl.2025.119747]
¹State Key Laboratory of Geological Processes and Mineral Resources, and Frontiers Science Center for Deep-time Digital Earth, China University of Geosciences, Beijing 100083, China
²National Key Laboratory of Deep Space Exploration/State Key Laboratory of Lithospheric and Environmental Coevolution, University of Science and Technology of China, Hefei 230026, China
³Institution of Meteorites and Planetary Materials Research, Key Laboratory of Planetary Geological Evolution, Guilin University of Technology, Guilin 541006, China
⁴Key Laboratory of Earth and Planetary Physics, Chinese Academy of Sciences, CNRS, Beijing, China
⁵Université Paris cité, Institut de Physique du Globe de Paris, Paris 75005, France
Copyright Elsevier

Primitive differentiated meteorites serve as key messengers to reveal the formation and evolution of planetesimals in the early solar system. Ureilites, a group of achondritic meteorites, are interpreted as remnants of a disrupted asteroid’s residual mantle, yet the accretion location of their parent body remains uncertain. Here we report that ureilites exhibit distinct Mg and Si isotopic compositions, characterized by heavy Mg isotope (δ26Mg = -0.22 ‰ ± 0.01) and light Si isotope (δ30Si =-0.50 ‰ ± 0.02) compositions relative to ordinary and carbonaceous chondrites (δ26MgOC&CC:0.27 ‰ ± 0.01, δ30SiOC&CC:0.44 ‰ ± 0.01). Following an assessment of pressure and redox conditions on Si isotopic fractionation between silicate and metal, we propose that the subchondritic δ30Si signature of ureilites reflects the accretion of the ureilite parent body (UPB) occurred in an extremely reduced environment. The suprachondritic δ²⁶Mg signatures are attributed to evaporation processes from the UPB precursors during early accretionary stages. To constrain the precursors of the UPB, we conducted numerical simulations of Si-Mg isotopic variations in chondritic planetesimals under early nebular conditions, incorporating vapor loss. Results indicate that the UPB precursors possessed a Si isotope composition similar to enstatite chondrites. Collectively, we conclude that the UPB accreted proximal to the reservoirs of enstatite chondrites in the inner solar system under reduced conditions, and the UPB’s precursors had experienced silicon and magnesium loss via magma ocean evaporation.