The Effects of Aqueous Alteration on Polycyclic Aromatic Hydrocarbon Content in Carbonaceous Chondrites

The organic inventories of carbonaceous chondrites (CCs) provide insights into the physicochemical environments involved in the formation of the Solar System and the post-accretionary evolution of meteorite parent bodies. Studying changes in these inventories across samples that have experienced varying degrees of aqueous/hydrothermal alteration untangles one aspect of such complex records. Here, the polycyclic aromatic hydrocarbon (PAH) and heterocyclic aromatic compound (HAC) contents of 15 CCs representing CI1, CM1, CM2, C2u, C3u, and CO3 classes were probed with two-step laser mass spectrometry (L2MS) and subjected to a comparative analysis using PCA, a multivariate analysis method. PAHs with mass-to-charge ratios (m/z) of 116 to \u3e 300 were detected, with large PAHs (4-6 rings) dominating the spectra of the most aqueously altered samples like Ivuna (CI1) and NWA 12328 (CM1), while a larger relative fraction of smaller PAHs (3 rings) was observed in relatively unaltered samples like Chwichiya 002 (C3.00u). The same gradual trend was observed across CM2 samples with varying degrees of aqueous alteration with the exception of Jbilet Winselwan (CM2.3-2.5), which has experienced impact shock. Alkylated homologs of C14H10 and C16H10 were detected in all samples, the latter showing a stronger correlation with aqueous alteration. Additionally, oxygen-containing PAHs and thiophenes were detected. These experiments demonstrate that aqueous alteration generally increases the size and alkylation of PAHs in CCs of multiple chemical groups. Thus, secondary alteration significantly alters the PAH contents of CCs, and PAH size distributions in particular may be an additional parameter to consider when evaluating a meteorite\u27s alteration history