Solid-state ¹³C NMR characterization of insoluble organic matter from Antarctic CM2 chondrites: Evaluation of the meteoritic alteration level

Chemical structures of the insoluble organic matter (IOM) from the Antarctic CM2 chondrites (Yamato [Y-] 791198, 793321; Belgica, [B-] 7904; Asuka [A-] 881280, 881334) and the Murchison meteorite were analyzed by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. Different types of carbons were characterized, such as aliphatic carbon (Ali-C), aliphatic carbon linked to hetero atom (Hetero-Ali-C), aromatic carbon (Aro-C), carboxyls (COOR), and carbonyls (C=O). The spectra of the IOM from Murchison and Y-791198 showed two major peaks: Ali-C and Aro-C, while the spectra from the other meteorites showed only one major peak of Aro-C. Carbon distribution was determined both by manual integration and deconvolution. For most IOM, the Aro-C was the most abundant (49.8-67.8%) of all carbon types. When the ratios of Ali-C to Aro-C (Ali/Aro) were plotted with the atomic hydrogen to carbon ratio (H/C), a correlation was observed. If we use the H/C as a parameter for the thermal alteration event on the meteorite parent body, this result shows a different extent of thermal alteration. In addition, IOM with a lower Ali/Aro showed a lower ratio of Ali-C to COOR plus C=O (Ali/(COOR + C=O)). This result suggests that the ratio of CO moieties to aliphatic carbon in IOM might reflect chemical oxidation that was involved in hydrothermal alteration.