Michael Zolensky¹, Takashi Mikouchi², Kenji Hagiya³, Kazumasa Ohsumi^4,5, Mutsumi Komatsu⁶, Andrew Cheng⁷, Loan Le⁸
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13909]
¹ARES, NASA Johnson Space Center, Houston, Texas 77058, USA
²University Museum, University of Tokyo, Tokyo 113-0033, Japan
³Graduate School of Life Science, Universtiy of Hyogo, Hyogo 678-1297, Japan
⁴Japan Synchrotron Radiation Research Institute (JASRI), Hyogo 679-5198, Japan
⁵Japan and High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
⁶The Graduate University for Advanced Studies, SOKENDAI, Kanagawa 240-0193, Japan
⁷The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
⁸Jacobs JETS, Johnson Space Center, Houston, Texas 77058, USA
Published by arrangement with John Wiley & Sons

We explore impact shock processing of the regolith of parent asteroids of carbonaceous chondrites, which has not been considered a major process for hydrous carbonaceous chondrites. We describe shock-produced minerals and features found in brecciated CI, CM, and CV chondrites, including agglutinates, a glassy melt pod, a shock melt vein, and melted sulfides. We also reexamine cognate clasts present in the Vigarano CV3 chondrites which appear to derive from asteroid ponds and exhibit cross-bedded dish structures.

Tarunika Ramprasad¹, Pierre Haenecour², Kenneth Dominik², Thomas J. Zega^1,2
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13910]
¹Department of Materials Science and Engineering, University of Arizona, 1235 E. James E. Rogers Way, Tucson, Arizona
²Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, Arizona 85721, USA
85721, USA
Published by arrangement with John Wiley & Sons

Compact type A calcium-aluminum-rich inclusions (CTA CAIs) are believed to have experienced partial melting that erased all information on their original nebular condensation. To investigate this question, we report new microstructural data on a CTA CAI, composed primarily of melilite, spinel, and perovskite, in the Northwest Africa 5028 CR2 chondrite. The melilite grains contain low (5–10 mole%) åkermanite contents and are not compositionally zoned. Spinel and perovskite each occur as near endmember compositions MgAl₂O₄ and CaTiO₃ and contain minor V and Al, respectively. A continuous rim composed of melilite, spinel, and perovskite, with minor hibonite grains occur around the CAI. We extracted two regions of interest from the interior CAI and two from the rim using focused ion beam techniques for detailed analysis using transmission electron microscopy. Evidence for thermal processing occurs as a perovskite–spinel–spinel triple junction in an interior section and a spinel inclusion within perovskite in a rim section. Evidence for parent-body alteration occurs in the form of Fe-rich sheet silicates in the rim, and localized amstallite in the interior of the CAI. While previous work suggested that many CTA CAIs experienced thermal processing in the solar nebula, including partial melting, our data show that signatures of primary condensation can be preserved in the form of more refractory phases contained within less refractory minerals, namely melilite and perovskite grains within spinel, and hibonite grains within perovskite, respectively. The inclusion we report on here has a complex history involving gas-phase condensation, nebular thermal processing, and parent-body alteration.

Dilyara M. Kuzina¹, Jérôme Gattacceca², Natalia S. Bezaeva³, Dmitry D. Badyukov³, Pierre Rochette², Yoann Quesnel², François Demory², Daniel Borschneck²
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13906]
¹Institute of Geology and Petroleum Technologies, Kazan Federal University, 4/5 Kremlyovskaya Str, 420008 Kazan, Russia ²CNRS, Aix Marseille Univ, IRD, INRAE, Aix-en-Provence, France
³Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 19 Kosygin Str, 119991 Moscow, Russia
Published by arrangement with John Wiley & Sons

We present a paleomagnetic study of the ~10 km diameter Karla impact structure in Russia. We sampled the target carbonate rocks, and a yet undocumented fragmental melt-bearing lithic breccia layer. This impact breccia, which contains carbonate melt, is enriched in stoichiometric magnetite by a factor of ~15 compared to the target lithologies, and carries a stable natural remanent magnetization. The weak remanent magnetization and the presence of both normal and reverse polarities down to the centimeter scale indicate that the breccia does not carry a thermoremanent magnetization (TRM), but rather a chemical remanent magnetization (CRM). The presence of stoichiometric magnetite and the absence of TRM suggest that the magnetite was formed during relatively low-temperature postimpact hydrothermalism that affected the porous impact breccia layer. During this process, the breccia acquired a CRM. The paleomagnetic direction is compatible with a Cenozoic age for the impact event, but cannot bring more precise constraint on the age because of the stable position of the Eurasian plate over the last 60 Myr. However, the presence of both polarities indicates that mild hydrothermalism took place over a period of time long enough to span at least one reversal of the geomagnetic field, that is, over a time scale of the order of 100 kyr. This confirms that protracted hydrothermal systems associated with impact craters are long lived, even in relatively small craters such as Karla, and are key features of the geologic and environmental effects of impacts on Earth.