Nanoscale Infrared Characterization of Dark Clasts and Fine-Grained Rims in CM2 Chondrites: Aguas Zarcas and Jbilet Winselwan

Abstract

Carbonaceous chondrites are among the most primitive meteorites that escaped extreme temperatures and melting in their parent bodies and, as such, offer valuable records of the parent body origins, formation, and evolution. The presence of organic molecules and carbonaceous phases make CM chondrites invaluable as they may have contributed prebiotic material to early Earth. Fine-grained rims (FGRs) and organic-rich dark clasts are particularly interesting features, the origin, formation, and evolution of which are not fully understood. In this study, we aimed to characterize several FGRs and dark clasts in two CM2 chondrites, Aguas Zarcas and Jbilet Winselwan, using backscattered electron images, confocal micro-Raman spectroscopy, and nanoscale near-field infrared imaging and spectroscopy. The nano-FTIR spectra show that the dark clasts and FGRs are chemically heterogeneous at a submicron scale and those of Aguas Zarcas are composed of organics (such as aliphatics, aromatics, and carbonyls) as well as alteration phases (such as phyllosilicates, carbonates, and sulfates). The FGRs are compositionally almost identical and exhibit heterogeneous alteration as well as a lack of fragmentation. The thicknesses of FGRs positively correlate with the enclosed chondrule diameter regardless of the chondrule type. The samples appeared to have experienced minimal brecciation after the chondrules were surrounded by the FGRs. These observations suggest nebular origin for the FGRs. The presence of organics embedded within these FGRs may further indicate that they may have formed in the solar nebula as well. In comparison, Jbilet Winselwan contains relatively less organics and exhibits more thermally metamorphosed mineralogy and matrix textures. These features could be the result of short-duration heating, such as impact heating, which also likely caused shock and dehydration/decomposition of the hydrated phases. © 2021 American Chemical Society.