Link between Polycyclic Aromatic Hydrocarbon Size and Aqueous Alteration in Carbonaceous Chondrites Revealed by Laser Mass Spectrometry

Abstract

The organic inventories of carbonaceous chondrites (CCs) provide insights into the physicochemical environments involved in the Solar System’s formation and the postaccretionary 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 principal component analysis (PCA), a multivariate analysis method. PAHs with mass-to-charge ratios (m/z) of 128 to >300 were detected, with large PAHs (4–7 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 (3–4 rings) was observed in less altered samples like Chwichiya 002 (C3.00u). The same 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 homologues of C14H10 and C16H10 were detected in all samples, with the latter showing a stronger correlation with aqueous alteration. Additionally, oxygen-containing PAHs and thiophenes were detected. These experiments demonstrate that aqueous alteration induces aromatic condensation in CCs of multiple chemical groups, and many of the 4–7-ring PAHs (>215 m/z) in CCs may be products of aqueous alteration. Thus, PAH size distributions are an additional parameter to consider when evaluating a meteorite’s alteration history.