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 (IOM) of the carbonaceous chondrites is considered to have formed in the interstellar medium. However, the D / H ratios in IOM 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 IOM 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 IOM, such as the H / C ratio. However, the δD of the benzylic, aliphatic and aromatic hydrogen into the IOM can be determined to be 1250‰, + 550‰ and + 150‰, respectively, relative to SMOW. This indicates that D-enrichment in IOM is correlated with the C–H bond dissociation energy. Such a correlation rules out IOM formation from observed interstellar molecules and suggests instead that the different components of IOM 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.