Organic Molecular Indicators of Aqueous Alteration: Extensive Pyrolysis Survey

Introduction: A variety of spectroscopic techniques have revealed that variations in functional group chemistry of insoluble organic matter (IOM) among the groups and sub-types of chondrites sensitively respond to secondary processes, including aqueous alteration [1] and thermal metamorphism [2]. However, such variations are minimal within the same meteorite group [3], unlike mineralogical studies that have shown intra-group variations due to aqueous alteration [4]. Here we report subtle but systematic differences in the molecular characteristics of IOM from CM, CI, CR, and Tagish Lake chondrites by pyrolysis-gas chromatography coupled with mass spectrometry (pyrolysis-GC-MS). In particular, we focus on the intra-group variations that could reflect multiple factors associated with aqueous alteration. (Ivuna), and Tagish Lake. The IOM was purified by CsF/HF demineralization of each meteorite powders. Roughly 0.5 mg of IOM aliquots were loaded into quartz capillary tubes flash-heated (610°C for 10 s) in a He atmosphere and analyzed by GC-MS. Results and discussion: The common pyrolysates from IOM included alkylated series from one to four ring aromatics as well as hetero-atom (O, S, N) containing aromatic compounds. Total yield of IOM pyrolysates (PYR N) varied among meteorite groups and even within groups. For instance, PYR N for GRO95577 was much less than those for EET92042 and Al Rais, reflecting the different degrees of aqueous alteration among CR chondrites. PYR N for Tagish Lake was the least. An increase in relative abundance in alkylated benzenes with increasing a ratio of two-to one-ring PAHs is seen in CMs in the order: Bells < Kives-vaara < Mighei ~ ALH83100 < Murchison ~ Murray < Cold Bokkeveld. The ranking is roughly similar to that of the degree of aqueous alteration [4]. Correlated increase in the relative abundances of O-and S-containing compounds in CMs could also reflect the progress of aqueous alteration. For the O-and S-containing compounds, however, the order was Murchison < Murray ~ MET01070 < Kivesvaara ~ Mighei ~ ALH83100 < Cold Bokkeveld < Bells, which is closer to the alteration ranking of [5] rather than [4], in that Bells is evaluated as the most altered. These results show that the abundances of individual pyro-lysate moieties from IOM reflect different factors of aqueous alteration. As it is known that Bells was much more heavily brec-ciated than the other CMs, relative abundances of O-and S-containing compounds may be more sensitive to the temperature of aqueous alteration rather than PAHs.