Spatially and structurally distinct IOM populations in carbonaceous chondrites describe pre-parent body thermal alteration histories and parent body aqueous alteration
Ryan S. Jakubek, Marc D. Fries, Francis M. McCubbin, Devin L. Schrader, Andrew Steele, Jemma Davidson
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 29 January 2026
“We collected Raman images of 78 chondrules and their surrounding matrix from 12 Antarctic meteorite thin sections. We identified three spatially zoned, distinct structural populations of insoluble organic matter (IOM). A majority of IOM is spatially associated with the matrix and is consistent with Raman analysis of matrix and bulk demineralized IOM reported in the literature. We observe an IOM population within most chondrules that is more thermally altered compared to the chondrule’s surrounding matrix. The chondrule IOM population is observed in all chondrite types examined in this work: OC, CO, CV, CM, and CR, and shows a structural dependence on petrologic type, similar to that reported for matrix/bulk IOM, indicating that the chondrule IOM population was present during parent body thermal metamorphism. The structural differences between the chondrule and matrix IOM populations decrease with increasing petrologic type as thermal alteration homogenizes the IOM. Petrologic type 1–2 chondrites show the largest chondrule-matrix IOM structural differences, indicating significant differences between these populations at the time of parent body accretion. These results suggest that IOM material in matrix and chondrule precursors experienced different alteration histories prior to parent body accretion. The chondrule IOM Raman spectra contain features consistent with alteration by flash heating–cooling, possibly implicating the chondrule formation event(s) as an alteration pathway that differentiates it from matrix IOM. We also observe a disordered IOM population referred to as broad IOM. The broad IOM is observed across matrix, chondrules, and clasts, indicating its formation after parent body accretion. In several Raman images of low petrologic type CO meteorites, broad IOM is found co-located with magnetite though the current dataset is not sufficient to prove a statistical correlation. We hypothesize that broad IOM is an aqueous alteration product and propose a few possible formation pathways including oxidation of iron carbide and/or precipitation from a C-O–H bearing fluid.”
