¹Oliver Christ,¹Anna Barbaro,²Frank E. Brenker,¹Paolo Nimis,¹Davide Novella,³M. Chiara Domeneghetti,^1,2Fabrizio Nestola
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13907]
¹Department of Geosciences, University of Padova, Via Gradenigo 6, 35131 Padova, Italy
²Geoscience Institute, Goethe-University Frankfurt, Altenhöferallee 1, 60438 Frankfurt, Germany
³Department of Earth and Environmental Sciences, University of Pavia, Via A. Ferrata 1, I-27100 Pavia, Italy
Published by arrangement with John Wiley & Sons

Carbon aggregates from two differently shocked ureilites were analyzed to gain insight into the shock transformation of graphite to diamond in ureilites, which happened when the ureilite parent body (UPB) was most likely destroyed by massive impact events. We present data for carbon aggregates from the highly shocked (U-S6) Northwest Africa (NWA) 6871 and the medium shocked (U-S3) NWA 3140. Both samples contain abundant carbon aggregates which were analyzed by X-ray diffraction and micro-Raman spectroscopy revealing the presence of close associations of (compressed) nanographite, micro- and nanodiamond, as well as Fe-rich phases. Graphite and diamond in NWA 6871 show shock indicators that are absent in NWA 3140. Based on Raman geothermometry on graphite, we calculated mean temperatures of 1368 ± 120 °C and 1370 ± 120 °C for NWA 3140 and NWA 6871, respectively. For comparison, a geothermometer based on the partitioning of Cr between olivine and low-Ca pyroxene was applied on NWA 3140, which yielded a temperature of only 1215 ± 16 °C. The graphite-based temperatures are the highest reported for graphite in ureilites so far and exceed calculated magmatic temperatures for ureilites from silicate- and chromite-based geothermometers. Graphite temperatures fall into the temperature field of catalytic diamond synthesis, which supports the hypothesis of direct transformation from graphite to diamond upon shock. Although the temperatures estimated seem to be independent of the shock degree, they can be ascribed to the shock event that destroyed the UPB.

¹Addi Bischoff,¹Markus Patzek,^2,3Stefan T. M. Peters,^4,5Jean-Alix Barrat,²Tommaso Di Rocco,²Andreas Pack,¹Samuel Ebert,¹Christian A. Jansen,⁶Kryspin Kmieciak
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13905]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, German
²Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstr. 1, D-37077 Göttingen, Germany
³Museum der Natur Hamburg – Mineralogie, LIB, Grindelallee 48, D-20146 Hamburg, Germany
⁴University of Brest, CNRS, IRD, Ifremer, LEMAR, F-29280 Plouzané, France
⁵Institut Universitaire de France, Paris, 75005 France
⁶Olsza 2, 63-100 Śrem, Kraków, Poland
Published by arrangement with John Wiley & Sons

On July 15, 2021, a huge fireball was visible over Poland. After the possible strewn field was calculated, the first and so far only sample, with a mass of 350 g, was discovered 18 days after the fireball event. The Antonin meteorite was found August 3, 2021, on the edge of a forest close to a dirt road near Helenow, a small suburb of the city of Mikstat. The rock is an ordinary chondrite breccia and consists of equilibrated and recrystallized lithologies. The boundaries between different fragments are difficult to detect, and the lithologies are of petrologic type 5 and type 4. The rock is moderately shocked (S4) and contains local impact melt areas and thin shock veins. The low-Ca pyroxene and olivine are equilibrated (Fs20.6 and Fa24.0, respectively), typical of L chondrites. The L chondrite classification is also supported by O isotope data and the results of bulk chemical analysis. The Ti isotope characteristics confirm that Antonin is related to the noncarbonaceous (NC) meteorites. One of the studied thin sections shows an unusual metal–chondrule assemblage, perhaps indicating that the metal in the chondrite is heterogeneously distributed, which is, however, not clearly visible in the element abundances.