¹Nozomi Matsuda,²Naoya Sakamoto,^1,3Shogo Tachibana,^1,4Hisayoshi Yurimoto
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.006]
¹Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
²Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
³UTokyo Organization for Planetary and Space Science (UTOPS), University of Tokyo, Tokyo 113-0033, Japan
⁴ISAS/JAXA, Sagamihara, Kanagawa, 252-210, Japan
Copyright Elsevier

Due to their common occurrence in various types of chondrites, igneous rims formed on pre-existing chondrules throughout chondrule-forming regions of the solar nebula. Although the peak temperatures are thought to reach similar values to those achieved during chondrule formation events, the heating duration in chondrule rim formation has not been well defined. We determined the two-dimensional chemical and oxygen isotopic distributions in an igneous rim of a chondrule within the Northwest Africa 3118 CV3_oxA chondrite with sub-micrometer resolution using secondary ion mass spectrometry and scanning electron microscopy. The igneous rim experienced aqueous alteration on the CV parent body. The aqueous alteration resulted in precipitation of the secondary FeO-rich olivine (Fa_40―49) and slightly disturbed the Fe-Mg distribution in the MgO-rich olivine phenocrysts (Fa_11―22) at about a 1 µm scale. However, no oxygen isotopic disturbances were observed at a scale greater than 100 nm. The MgO-rich olivine, a primary phase of igneous rim formation, has δ¹⁷O = ―6 ± 3‰ and δ¹⁸O = ―1 ± 4 ‰, and some grains contain extreme ¹⁶O-rich areas (δ¹⁷O,□δ¹⁸O = ˜―30‰) nearly 10 µm across. We detected oxygen isotopic migration of approximately 1 µm at the boundaries of the extreme ¹⁶O-rich areas. Using oxygen self-diffusivity in olivine, the heating time of the igneous rim formation could have continued from several hours to several days at near liquidus temperatures (˜2000 K) in the solar nebula suggesting that the rim formed by a similar flash heating event that formed the chondrules.

^1,2Alan E.Rubin,^1,3,4YeLia
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.009]
¹Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, CA, 90095-1567, USA
²Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME, 04217, USA
³Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210034, China
⁴Chinese Academy of Sciences Center for Excellence in Comparative Planetology, China
Copyright Elsevier

Primitive CO3.00–3.1 chondrites contain ˜2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H₂ and reacts with SiO₂ to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr₂O₃ in a few R4-6 chondrites. Magnetite grains with high Cr₂O₃ behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ˜4 vol.% magnetite with substantial Cr₂O₃ (up to ˜15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.

¹Addi Bischoff et al. (>10)
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.007]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, D-48149, Münster, Germany
Copyright Elsevier

On July 10, 2018 at 21:29 UT extended areas of South-Western Germany were illuminated by a very bright bolide. This fireball was recorded by instruments of the European Fireball Network (EN). The records enabled complex and precise description of this event including the prediction of the impact area. So far six meteorites totaling about 1.23 kg have been found in the predicted location for a given mass during dedicated searches. The first piece of about 12 g was recovered on July 24 close to the village of Renchen (Baden-Württemberg) followed by the largest fragment of 955 g on July 31 about five km north-west of Renchen.
Renchen is a moderately-shocked (S4) breccia consisting of abundant highly recrystallized rock fragments as well as impact melt rock clasts. The texture, the large grain size of plagioclase, and the homogeneous compositions of olivine (˜Fa26) and pyroxene (˜Fs22) clearly indicate that Renchen is composed of metamorphosed rock fragments (L5-6). An L-group (and ordinary chondrite) heritage is consistent with the data on the model abundance of metal, the density, the magnetic susceptibility as well as on O-, Ti-, and Cr-isotope characteristics. Renchen does not contain solar wind implanted noble gases and is a fragmental breccia. An unusually large mm-sized merrillite-apatite aggregate shows trace element characteristics like other phosphates from ordinary chondrites.
Data on the bulk chemistry, IR-spectroscopy, cosmogenic nuclides, and organic components also indicate similarities to other metamorphosed L chondrites. Noble gas studies reveal that the meteorite has a cosmic ray exposure (CRE) age of 42 Ma and that most of the cosmogenic gases were produced in a meteoroid with a radius of at max. 20 cm based on the radionuclide 26Al and 10-150 cm based on cosmogenic 22Ne/21Ne. K-Ar and U/Th-He gas retention ages are both in the range ˜3.0 to 3.2 Ga. Both systems do not show evidence for a complete reset 470 Ma ago, and may instead have recorded the same resetting event 3.0 Ga ago.