Can D/H Isotope Composition of Polycyclic Aromatic Hydrocarbons be Used to Indicate the Origin, Conditions, and Alteration History of Carbonaceous Chondrites?

The stable isotope composition of soluble and insoluble organic compounds in carbonaceous chondrites has been used to assess terrestrial contamination but can also be used determine the provenance of organic molecules in space. Deuterium enrichment in meteoritic organics could be a residual signal of synthetic reactions occurring in the cold interstellar medium or an indicator of hydrothermal parent body reactions. D/H ratios have been measured in grains and bulk samples for a wide range of meteorites; however, these reservoirs are highly variable and may have experienced fractionation during thermal and/or aqueous alteration. Polycyclic aromatic hydrocarbons (PAHs) are stable and abundant in carbonaceous chondrites and their D/H ratio may preserve evidence about the formation environment and the influence of parent-body processes. This study tests hypotheses about the link between deuteration and formation conditions by examining the D/H ratio in a variety of carbonaceous chondrites representing a range of petrologic types. We use enrichment in smaller PAHs (<40 carbons) as an indicator of photo driven deuteration due to unimolecular photodissociation in warm regions of space. Conversely, enrichment that does not trend with PAH size or hydrogen number can be used as an indicator of gas phase ion-molecule reactions in cold environments of space. Enrichment in partially hydrogenated PAHs and heterocyclic aromatic compounds can be related to deuteration reactions in ice grains. Indicators of diagenesis during parent body processing can include D/H differences between partially and fully aromatized PAHs and D/H differences associated with alkylation. Methods were developed to quantitatively extract even the most volatile PAHs without fractionation. We present evidence from a selection of carbonaceous chondrites to examine each of these criteria, source formation regions, and test the extent to which alteration can be gauged by methods using trace derivatives of PAHs.