^1,2M. Kimura,³R. C. Greenwood,⁴M. Komatsu,¹N. Imae,¹A. Yamaguchi,²R. Sato
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13704]
¹National Institute of Polar Research, Tokyo, 190-8518 Japan
²Ibaraki University, Mito, 310-8512 Japan
³The Open University, Milton Keynes, MK7 6AA UK
⁴SOKENDAI, Hayama, Kanagawa, 240-0193 Japan
Published by arrangement with John Wiley & Sons

Most carbonaceous (C) chondrites are classified into eight major groups: CI, CM, CO, CV, CK, CR, CH, and CB. However, some are ungrouped. We studied two such chondrites, Asuka (A)-9003 and A 09535. The abundance of chondrules and matrix and chondrule sizes in these meteorites are similar to those in ordinary chondrites and unlike any known carbonaceous chondrite group. In contrast, they contain 4–6 vol% of refractory inclusions and have oxygen isotopic compositions within the range of CO and CV chondrites. Therefore, A-9003 and A 09535 are classified as C chondrites. Petrologic subtypes of A-9003 and A 09535 are 3.2. All these features closely resemble those of another ungrouped chondrite, Yamato (Y)-82094, and differ from those of any C chondrites reported by now. A-9003, A 09535, and Y-82094 likely represent a new type of C chondrite. We provisionally call them CA chondrite after Asuka in Antarctica. Our study suggests a wider range of formation conditions for C chondrites than currently recorded by the major C chondrite groups.

¹Jérôme Gattacceca,¹Pierre Rochette,¹Yoann Quesnel,²Sounthone Singsoupho
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13703]
¹CNRS, Aix-Marseille Univ, IRD, INRAE, Aix-en-Provence, France
²Department of Physics, Faculty of Natural Sciences, National University of Laos, Vientiane, Laos
Published by arrangement with John Wiley & Sons

Among Australasian tektites, the so-called Muong Nong tektites stand out for their peculiar layering and blocky aspect. Although the source crater for the Australasian tektites is not known, Muong Nong tektites are generally considered as a relatively proximal ejecta. The mechanism responsible for the formation of the layering has been a matter of debate. In this work, we revisit the paleomagnetism of Muong Nong tektites. They retain a thermoremanent magnetization acquired during cooling below 585 °C in the presence of the ambient geomagnetic field, and carried magnetite in most samples, although at least one sample containing metallic iron was detected. The inclination of the paleomagnetic direction with respect to the layering plane clusters around 18 ± 12°, compatible with the inclination of the geomagnetic field for this latitude at the time of impact. This indicates that the layering of the Muong Nong tektites was subhorizontal while they were cooling below 585 °C. The preferred scenario for the formation of the layering of layered tektite is therefore by horizontal shear in pools or sheets of molten material.

¹O. Pravdivtseva,²M. E. Varela,¹A. Meshik,³A. J. Campbell,²M. Saavedra,⁴D. Topa
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13708]
¹Laboratory for Space Sciences and Physics Department, Washington University, CB1105, Saint Louis, Missouri, 63130 USA
²ICATE (CONICET-UNSJ), Avenida España 1512 sur, San Juan, J5402DSP Argentina
³Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago, Illinois, 60637 USA
⁴Central Research Laboratories, Natural History Museum, Burgring 7, Vienna, 1010 Austria
Published by arrangement with John Wiley & Sons

The petrographic study of the San Juan A2 polished section demonstrated textural and compositional similarities with the Campo del Cielo IAB iron meteorite, with trace element abundances in metal following the pattern of bulk Campo del Cielo. Xenon, neon, and helium isotopic compositions have been measured in radial graphite rims, massive graphite inclusions, fine-grained graphite aggregates, cliftonite, and platy graphite. Two silicate inclusions and two areas of metal were also analyzed. ³He/⁴He versus ⁴He/²¹Ne data for San Juan metal plot next to the values reported for the El Taco fragment of Campo del Cielo, supporting San Juan being a part of the Campo del Cielo meteorite shower. Based on the Ne isotopic composition of its components, and the observed correlation between ¹²⁸Xe and ¹²⁹Xe, the San Juan A fragment of Campo del Cielo was well shielded from the primary galactic cosmic ray high-energy irradiation. Its size allowed the secondary neutrons to be fairly well thermalized, receiving an equivalent (normalized to the research reactor with highly thermalized neutron spectrum) fluence of thermal and epithermal neutrons of 6.6 × 10¹⁷ n cm⁻². Considering 1.8 × 10⁸ years single-stage and constant exposure geometry irradiation history for Campo del Cielo, and assuming the identical neutron flux spectra for the research reactor and Campo del Cielo, the average thermal equivalent neutron flux for San Juan is about 1.2 × 10² n cm⁻² s⁻¹. Xe isotopic composition in the radial graphite rims and platy graphite shows evidence of live ¹²⁹I in San Juan and is consistent with a mixture of iodine-derived and tellurium-derived Xe.