Aromatic moieties in meteoritic macromolecular materials:: Analyses by hydrous pyrolysis and δ13C of individual compounds

Hydrous pyrolysis, supercritical fluid extraction (SFE), gas chromatography-mass-spectrometry (GC-MS) and isotope ratio monitoring-gas chromatography-mass spectrometry (irm-GC-MS) were used to investigate the constitution of macromolecular materials in meteorites. Results from the carbonaceous chondrites Orgueil (CI1) and Cold Bokkeveld (CM2) were compared with those obtained previously from Murchison (CM2). Fragments of meteoritic macromolecular materials were produced by hydrous pyrolysis, extracted by SFE, and identified by GC-MS. The CI1 and CM2 hydrous pyrolysates all contain volatile aromatic compounds, some of which have aliphatic side chains, hydroxyl groups, and thiophene rings attached. The results indicate that the macromolecular materials in these meteorites are qualitatively similar. However, the pyrolysates show significant quantitative differences, with the products of ether linkages and condensed aromatic networks being less abundant in the more aqueously altered meteorites. In addition, the methylnaphthalene maturity parameter negatively correlates with aqueous alteration. These features are interpreted as the result of chemical reactions favored under hydrous conditions. Hence, the extent of aqueous alteration on the meteorite parent body appears to be the most important evolutionary stage in determining the final structure of macromolecular materials in the CI1 and CM2 meteorites. The carbon isotopic compositions of the fragments of macromolecular materials were determined by irm-GC-MS. delta(13)C values for the hydrous pyrolysis products range from -25.5 to -10.2 parts per thousand for Orgueil and -22.9 to +4.0 parts per thousand for Cold Bokkeveld. These values can be compared to the -24.6 to -5.6 parts per thousand range obtained previously for Murchison. The low molecular weight components in each hydrous pyrolysate display shifts to increased (13)C contents with carbon number. This indicates the production of simple organic entities by the preferential cracking of (12)C-(12)C bonds in more complex starting materials. The shifts extend from C(7) to C(8) for Orgueil and Cold Bokkeveld but from C(7) to C(10) for Murchison. Higher molecular weight components for all of the hydrous pyrolysates show a general trend of decreasing (13)C content with carbon number. The higher molecular weight features can be explained by the preferential addition of (12)C during the primary synthesis of the macromolecular materials. In addition, delta(13)C values for the methylnaphthalenes are consistent with the addition of (12)C to the most reactive site on the naphthalene parent molecule providing supporting evidence for synthesis. Hence, the macromolecular materials are composed of organic units created by both synthesis and cracking. Therefore, secondary processing by liquid water on the meteorite parent body exerts a strong control on the final molecular architecture of meteoritic macromolecular materials. Yet, the carbon isotopic compositions of some individual moieties may retain a record of primary synthesis. Copyright (C) 2000 Elsevier Science Ltd.