MOLECULAR IDENTIFICATION OF THE CARRIER OF THE D-ANOMALY IN INSOLUBLE ORGANIC MATTER OF ORGUEIL METEORITE: REVISING THE ORIGIN OF THE IOM? L. Remusat 1,2,3 , F. Robert 1 , A. Meibom 1 , S. Mostefaoui 1 , O. Delpoux 4 , L. Binet 4 , D. Gourier 4 , S. Derenne 2 , 1 LEME, MNHN-CNRS, Paris, France, 2 LCBOP, CNRS, Paris, France, 3 present address : GPS division, Caltech, Pasadena, USA (remusat@gps.caltech.edu), 4 LCMCP, ENSCP-CNRS, Paris, France. Introduction: Insoluble organic matter (IOM) in primitive carbonaceous meteorites has preserved its composition and its isotopic heterogeneity since the Solar System formed ca. 4.567 billion years ago. IOM is known to be enriched in deuterium, with D/H ratios > 300×10 -6 [1]. It is also characterized by a high degree of isotopic heterogeneity, as demonstrated by the molecular distribution of the D [2] and by the observation of D-rich “hot spots” in NanoSIMS ion microprobe images [3]. Understanding the origin of this heterogeneity represents a fundamental challenge with implications for the origin and distribution of organics in the interstellar medium and in the protoplanetary disk from which our planetary system formed. Moreover, it can have implication on the origin of life in the solar system, as IOM was delivered to the early Earth and may have provided the molecular precursors to life. Molecular characterisation of IOM reveals that it is constituted by rather small aromatic units, highly substituted [4, 5] and linked to each other by short and branched aliphatic linkages [6]. Pyrolysis and ruthenium tetroxide oxidation release the building blocks of IOM which can be isotopically (D/H) analysed by gas chromatography-isotope ratio mass spectrometry. This has led to the identification of distinct D-compositions at the molecular level [2]. Three different types of H were distinguished based on their C-H bond energy, which is related to their position within the macromolecule. These are aromatic, aliphatic and benzylic H (fig.1). This study reveals that, surprisingly, the weakest C-H bond is the most enriched in D. Recently, NanoSIMS imaging of IOM has revealed the occurrence of D-rich hot spots [3]. Regions of some hundreds of nm to 1μm appear to have D/H isotopic ratios ranging from 460 to 720 ×10 -6 (fig. 2A). This is 2 to 3 times the bulk D/H. These areas do not exhibit special chemical characteristics (e.g. C/H or N/C ratios) compared to the bulk, indicating that they do not consist of organic matter significantly different from the bulk IOM itself. Electron paramagnetic resonance (EPR) spectroscopy allows the identification and the quantification of organic radicals in the IOM [7]. This technique reveals that organic radicals exhibit a specific behavior in chondritic IOM. In contrast to terrestrial organic matter, radicals in IOM are heterogeneously distributed. They appear to be concentrated in small areas with higher local concentration than the bulk concentration. This pattern leads to the assumption that organic radicals are implied in the formation of the D-rich hot spots. Furthermore, pulsed EPR has shown that hydrogen in the benzylic bond of organic radicals has a deuterium to hydrogen (D/H) ratio of 1.5±0.5×10 -2 in Orgueil IOM, which is the highest solar system D/H ratio ever reported [8]. Experimental: H, D along with 12 C, 26 CN and 18 O images of Orgueil IOM were acquired at the same time and the same location with the NanoSIMS 50 installed at the Museum National d’Histoire Naturelle. The images were analysed thanks to l’image software, developped by L. Nittler, allowing the determination of ratio images (fig. 2). The instrumental fractionation, determined from a standard charcoal, was corrected for. An upper threshold was applied to the image to erase the hot spots and to determine the isotopic ratio of the organic matter that does not exhibit the isotopic anomaly of the D-rich hot spots (fig. 2B). Results: By combining pulsed EPR data with quantitative image analysis recorded at a high spatial resolution with the NanoSIMS, we are able to prove that the organic radicals can account for the deuterium excess in the IOM D-rich “hot spots”. First, by considering the local radical concentration in radical rich regions and the D/H isotopic ratio of these radicals, it is easy to determine that the D/H of these regions is higher than 500×10 -6 . The only areas where so high D/H values occur are the D-rich hot spots. Second, the surface covered by the D-rich hot spots (15-38%) is broadly consistent with the volume estimation of the radical-rich areas (20%). It must be noted that thermally altered IOM do not exhibit D-rich hot spots, consitent with the thermal instability of the organic radicals. D/H isotopic ratio in IOM can be considered as a mixing between organic radicals and radical free organic matter. Then a simple mass balance equation can be defined : (D/H) bulk = (D/H) radicals ×(H radicals /H total ) + (D/H) radical-free ×(1-H radicals /H total ). (D/H) radicals is given by pulsed EPR, all the other isotopic ratios are determined from NanoSIMS data. Then, this equation offer the determination of H radicals /H total , which can also be determined independantly by EPR. Our estimate from NanoSIMS images lies between 1.65 and 2.9 ×10 -3 in agreement with the EPR estimation (3.2 ±1.2×10 -3 , [7]). This results implies that D-rich hot spots can be attributed entirely to organic radicals. The same mass balance, applied to D-rich hotspots, leads to a concentration (H radicals /H total ) in average 6.5 Lunar and Planetary Science XXXIX (2008) 1399.pdf