^1,2Philip R. Heck et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13129]
¹Department of Science and Education, Robert A. Pritzker Center for Meteoritics and Polar Studies, The Field Museum of Natural History, Chicago, Illinois, USA
²Chicago Center for Cosmochemistry and Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
Published by Arrangement with John Wiley & Sons

We present He and Ne isotopes of individual presolar graphite grains from a low‐density separate from Orgueil. Two grain mounts were analyzed with the same techniques but in a different sequence: The first one was measured with NanoSIMS followed by noble gas mass spectrometry, and the second one in reverse order. No grain contained 4He and only one grain on the second mount contained 3He. On the first mount, the grains had been extensively sputtered with NanoSIMS ion beams prior to noble gas analysis; we found only one grain out of 15 with presolar 22Ne above detection limit. In contrast, we found presolar 22Ne in six out of seven grains on the second mount that was not exposed to an ion beam prior to noble gas analysis. All 22 grains on the two mounts were imaged with scanning electron microscopy (SEM) and/or Auger microscopy. We present evidence that this contrasting observation is most likely due to e‐beam–induced heating of the generally smaller grains on the first mount during SEM and Auger imaging, and not primarily due to the NanoSIMS analysis. If thermal contact of the grains to the substrate is absent, such that heat can only be dissipated via radiation, then the smaller, sputter‐eroded grains are heated to higher temperatures such that noble gases can diffuse out. We discuss possible gas loss mechanisms and suggest solutions to reduce heating during e‐beam analyses by minimizing voltages, beam currents, and dwell times. We also found small amounts of 21Ne in five grains. Using isotope data we determined that the dominant sources of most grains are core‐collapse supernovae, congruent with earlier studies of low‐density presolar graphite from Murchison. Only two of the grains are most likely from AGB stars, and two others have an ambiguous origin.

¹Clément Ganino,^2,3Guy Libourel,⁴Akiko M. Nakamura,⁵Suzanne Jacomet,⁶Olivier Tottereau,²Patrick Michel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13131]
¹Université Côte d’Azur, OCA, CNRS, Géoazur, Sophia‐Antipolis, Valbonne, France
²Université Côte d’Azur, OCA, CNRS, Lagrange, Boulevard de l’Observatoire, Nice Cedex 4, France
³Hawai’i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai’i at MānoaHonolulu, Hawai’i, USA
⁴Graduate School of Science, Kobe University, Kobe, Japan
⁵MINES ParisTech, PSL—Research University, CEMEF—Centre de mise en forme des matériaux, CNRS, UMR 7635Sophia‐Antipolis, France
⁶CRHEA, CNRS UPR 10Sophia Antipolis, France
Published by Arrangement with John Wiley & Sons

Hypervelocity impacts are common in the solar system, in particular during its early phases when primitive bodies of contrasted composition collided. Whether these objects are chemically modified during the impact process, and by what kind of processes, e.g., chemical mixing or gas–liquid–solid fractionation, are still pending questions. To address these issues, a set of impact experiments involving a multielemental doped phonolitic projectile and a metallic target was performed in a 3–7 km s−1 range of impact speeds which are typical of those occurring in the asteroid belt. For each run, both texture and chemistry of the crater and the ejecta population have been characterized. The results show that the melted projectiles largely cover the craters at all speeds, and that melted phonolitic materials are injected into fractures in the crater in the metallic target. Ejecta are generally quenched droplets of silicate impact melt containing metal beads. Some of these beads are extracted from the target, but we propose that some of the Fe metal beads are the result of reduction of FeO. A thin FeO‐SiO2‐rich condensate layer is found at the edge of the crater, suggesting that a limited amount of vapor formed and condensed. LA‐ICP‐MS analyses suggest, however, that within analytical uncertainties, no volatility‐controlled chemical fractionation of trace elements occurred in the ejecta. The main chemical fractionation during impact at such velocities and energies are the result of projectile‐target mixing.