¹Bruno Daniel Leite Mendes,¹Agnes Kontny,²Katarzyna Dudzisz,³Franziska D. H. Wilke
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14170]
¹Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany
²Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland
³Helmholtz Centre Potsdam – Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, Potsdam, Germany
Published by arrangement with John Wiley & Sons

Large-scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic anomalies. Despite magnetic anomalies in craters being well-documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post-impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact-melt bearing lithologies. We report new chemical, rock-, and mineral-magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO-18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low-cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post-impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral-to-reducing post-impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak-ring.

^1,2Daisuke Nakashima,^3,4Takaaki Noguchi,^2,5Takayuki Ushikubo,^6,7Makoto Kimura,²Noriko Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.011]
¹Department of Earth Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
³Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁴Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
⁵Kochi Institute for Core Sample Research, JAMSTEC, Monobe-otsu 200, Nankoku, Kochi 783-8502, Japan
⁶Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
⁷National Institute of Polar Research, Tokyo 190-8518, Japan
Copyright Elsevier

Oxygen isotope ratios and elemental compositions of porphyritic chondrules and olivine and pyroxene fragments in the Asuka-881020 CH chondrite were analyzed. The oxygen isotope ratios inside individual porphyritic chondrules are homogeneous within the uncertainty, except for relict grains of olivine and low-Ca pyroxene that have distinct oxygen isotope ratios. The average oxygen isotope ratios of the individual chondrules plot along and above the primitive chondrule mineral (PCM) line with Δ17O (=δ17O – 0.52 × δ18O) values from −4.7 ‰ to +4.1 ‰. The olivine and pyroxene fragments, which have Δ17O values ranging from −2.1 ‰ to +3.2 ‰, are likely to be fragments of the porphyritic chondrules.

Unlike the non-porphyritic chondrules in CH and CB chondrites and chondrules in other carbonaceous chondrites, the type I and II chondrules do not show a systematic difference in the Δ17O values. Furthermore, the Δ17O values of the type I chondrules increase from −4.7 ‰ to +4.1 ‰ with increasing Mg# (=molar [MgO]/[MgO + FeO] × 100) from 96 to 99. We argue that the positive Δ17O-Mg# trend is explained by an addition of 16O-poor carbon-rich organics as a reducing agent to the relatively 16O-rich precursor silicate, which is a new environment for chondrule formation. This hypothesis is supported by the two lines of evidence observed in the present study. (1) The chondrules and fragments with higher Δ17O values show larger deviations from the PCM line towards low δ18O, suggesting oxygen isotope mass fractionation between the chondrule melt and CO or CO2. (2) Olivine phenocrysts in the chondrules with high Δ17O values contain Ni-poor Fe-metal particles surrounded by silica-rich glass, which may be reduction products during the chondrule formation. Thus, it is suggested that the porphyritic chondrules in CH and CB chondrites have different origins from chondrules in any other chondrite types, even from the non-porphyritic chondrules in CH and CB chondrites.