Enrichment of deuterium in insoluble organic matter from primitive meteorites: A solar system origin?

Because of a systematic enrichment in deuterium, the insoluble organic matter (TOM) of the carbonaceous chondrites is considered to have formed in the interstellar medium. However, the D/H ratios in TOM remain much lower than those measured in the organic molecules commonly observed in the dense interstellar medium. In this study, the D/H ratio of different aromatic and aliphatic molecular fragments of TOM from the Orgueil meteorite was measured by GC-irMS (gas chromatography-isotopic ratio mass spectrometry). No correlation was observed between the D/H ratios and structural parameters characterizing the TOM, such as the H / C ratio. However, the 6D of the benzylic, aliphatic and aromatic hydrogen into the TOM can be determined to be 1250 parts per thousand, +550 parts per thousand and + 150 parts per thousand, respectively, relative to SMOW. This indicates that D-enrichment in TOM is correlated with the C-H bond dissociation energy. Such a correlation rules out TOM formation from observed interstellar molecules and suggests instead that the different components of TOM have acquired their D/H ratios by an exchange with a deuterium-rich reservoir after its synthesis. The same process can be invoked to account for the D/H composition of meteoritic water. Findings point to a common process for deuterium enrichment in the solar system. (c) 2006 Elsevier B.V. All rights reserved.