A coordinated study of the petrology, mineral chemistry, and bulk chemical and isotopic composition of the five ungrouped carbonaceous chondrites Coolidge, Loongana 001, Los Vientos (LoV) 051, Northwest Africa (NWA) 033, and NWA 13400 reveals that these meteorites have a similar set of properties that distinguishes them from the other carbonaceous chondrite groups and allows definition of the new Loongana (CL) group of carbonaceous chondrites. The characteristics of the investigated samples include: (1) Fe-Ni metal abundances considerably higher than for CV chondrites, but similar to CR chondrites; (2) Chondrule size-frequency distributions similar to CV chondrites, but dissimilar to CR chondrites; (3) Mean CAI abundances of ~1.4 vol%, i.e., lower than in CV but much higher than in CR chondrites; (4) Very low amounts of inter-chondrule matrix (17-21 vol%), the lowest among the main carbonaceous chondrite groups (CI, CY, CM, CO, CV, CR, CK); (5) Nearly equilibrated olivine with mean fayalite (Fa) values of olivine between 12.5 mol% (Loongana 001) and 14.7 mol% (NWA 13400) as a metamorphic effect; (6) Lower Al 2 O 3 and higher MgO and Cr 2 O 3 concentrations in inter-chondrule matrix compared to matrix in CV, CK, and CR chondrites; (7) Considerable bulk depletion of volatile lithophile elements (Mn, Na, K, Rb, Cs) and chalkophile elements (Zn, Se, Te, Pb, Tl) compared to all other main carbonaceous chondrite groups; (8) Bulk O isotope compositions plotting along the CCAM line (∆ 17 O -3.96 to -5.47‰), partly overlapping with the CV and CK chondrite field but including samples that are more 16 O-rich; (9) Unique positions in the є 54 Cr-є 50 Ti isotope plot, with є 54 Cr values similar to CV and є 50 Ti values similar to CR chondrites.Taking all observations together, it can be concluded that CL chondrites are unrelated to CR chondrites but may be genetically related to CV and CK chondrites. All CL chondrites studied here are of petrologic type 3.9 to 4, indicating that they have been thermally overprinted on the parent body.The diagnostic features of CL chondrites detailed here should provide a basis for identifying CL members of lower petrologic types. Such samples would be important in determining the pristine state of these meteorites and their components.

No abstract

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

No abstract

We report NanoSIMS Si and Mg–Al isotopic data (and C, N, and Ti isotopic data, when available) for 85 submicron- to micron-sized presolar SiC grains from the CM2 Murchison meteorite, including 60 mainstream (MS), 8 AB1, 8 X, 7 AB2, and 2 Y grains. The MS and Y grain data demonstrate that (1) C and N contamination mainly appears as surface contamination, and sufficient presputtering is needed to expose a clean grain surface for obtaining intrinsic C and N signals, and (2) Mg and Al contamination appears as adjacent grains and rims, and high-resolution imaging and the choice of small regions of interest during data reduction together are effective in suppressing the contamination. Our results strongly indicate that previous studies of presolar SiC grains could have sampled differing degrees of contamination for C, N, Mg, and Al. Compared to the literature data, our new MS and Y grains are in better agreement with carbon star observations for both the C and N isotopic ratios. By comparing our new, tighter distributions of 12C/13C, 14N/15N, and initial 26Al/27Al ratios for MS and Y grains with FRUITY low-mass asymptotic giant branch (AGB) stellar models, we provide more stringent constraints on the occurrence of cool bottom processing and the production of 26Al in N-type carbon stars, which are classical AGB stars.