Aromatic moieties in meteoritic macromolecular materials: Analyses by hydrous pyrolysis and delta C-13 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 chond rites Orgueil (CI1) and Cold Bokkeveld (CM2) were compared with those obtai ned previously from Murchison (CM2). Fragments of meteoritic macromolecular materials were produced by hydrous p yrolysis, extracted by SFE, and identified by GC-MS. The CI1 and CM2 hydrou s pyrolysates all contain volatile aromatic compounds, some of which have a liphatic side chains, hydroxyl groups, and thiophene rings attached. The re sults indicate that the macromolecular materials in these meteorites are qu alitatively similar. However, the pyrolysates show significant quantitative differences, with the products of ether linkages and condensed aromatic ne tworks being less abundant in the more aqueously altered meteorites. In add ition, the methylnaphthalene maturity parameter negatively correlates with aqueous alteration. These features are interpreted as the result of chemica l reactions favored under hydrous conditions. Hence, the extent of aqueous alteration on the meteorite parent body appears to be the most important ev olutionary stage in determining the final structure of macromolecular mater ials in the CI1 and CM2 meteorites. The carbon isotopic compositions of the fragments of macromolecular materia ls were determined by irm-GC-MS. delta(13)C values for the hydrous pyrolysi s 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 compa red to the -24.6 to -5.6 parts per thousand range obtained previously for M urchison. The low molecular weight components in each hydrous pyrolysate di splay shifts to increased C-13 contents with carbon number. This indicates the production of simple organic entities by the preferential cracking of C -12-C-12 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 sho w a general trend of decreasing C-13 content with carbon number. The higher molecular weight features can be explained by the preferential addition of C-12 during the primary synthesis of the macromolecular materials. In addi tion, delta(13)C values for the methylnaphthalenes are consistent with the addition of C-12 to the most reactive site on the naphthalene parent molecu le providing supporting evidence for synthesis. Hence, the macromolecular m aterials are composed of organic units created by both synthesis and cracki ng. Therefore, secondary processing by liquid water on the meteorite parent bod y exerts a strong control on the final molecular architecture of meteoritic macromolecular materials. Yet, the carbon isotopic compositions of some in dividual moieties may retain a record of primary synthesis. Copyright (C) 2 000 Elsevier Science Ltd.