Deuterium enrichments in chondritic macromolecular material-Implications for the origin and evolution of organics, water and asteroids

Here we report the elemental and isotopic compositions of the insoluble organic material (TOM) isolated from several previously unanalyzed meteorites, as well as the reanalyses of H isotopic compositions of some previously measured samples (Alexander et al., 2007). The IOM in ordinary chondrites (OCs) has very large D enrichments that increase with increasing metamorphism and decreasing H/C, the most extreme delta D value measured being almost 12,000 parts per thousand. We propose that such large isotopic fractionations could be produced in the OC parent bodies through the loss of isotopically very light 112 generated when Fe was oxidized by water at low temperatures (<200 degrees C). We suggest that similar isotopic fractionations were not generated in the IOM of CV and CO chondrites with similar metamorphic grades and IOM H/C ratios because proportionately less water was consumed during metamorphism, and the remaining water buffered the H isotopic composition of the TOM even a H was being lost from it. Hydrogen would also have been generated during the alteration of Cl, CM and CR carbonaceous chondrites. The TOM in these meteorites exhibit a considerable range in isotopic compositions, but all are enriched in D, as well as N-15, relative to terrestrial values. We explore whether these enrichments could also have been produced by the loss of H-2, but conclude that the most isotopically anomalous IOM compositions in meteorites from these groups are probably closest to their primordial values. The less isotopically anomalous IOM has probably been modified by parent body processes. The response of IOM to these processes was complex and varied, presumably reflecting differences in conditions within and between parent bodies. The D enrichments associated with H-2 generation, along with exchange between D-rich IOM and water in the parent bodies, means that it is unlikely that any chondrites retain the primordial H isotopic composition of the water ice that they accreted. The H isotopic compositions of the most water-rich chondrites, the CMs and CIs, are probably the least modified and their compositions (delta D <= -25 parts per thousand) suggest that their water did not form at large radial distances from the Sun where ice is predicted to be very D-rich. Yet models to explain the O isotopic composition of inner Solar System bodies require that large amounts of ice were transported from the outer to the inner Solar System. (C) 2010 Elsevier Ltd. All rights reserved.