Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles

The formation of organic compounds is generally assumed to result from abiotic processes in the Solar System, with the exception of biogenic organics on Earth. Nitrogen-bearing organics are of particular interest, notably for prebiotic perspectives but also for overall comprehension of organic formation in the young solar system and in planetary atmospheres. We have investigated abiotic synthesis of organics upon plasma discharge, with special attention to N isotope fractionation. Organic aerosols were synthesized from N2-CH4 and N2-CO gaseous mixtures using low-pressure plasma discharge experiments, aimed at simulating chemistry occurring in Titan s atmosphere and in the protosolar nebula, respectively. Nitrogen is efficiently incorporated into the synthesized solids, independently of the oxidation degree, of the N2 content of the starting gas mixture, and of the nitrogen speciation in the aerosols. The aerosols are depleted in 15N by 15-25 permil relative to the initial N2 gas, whatever the experimental setup is. Such an isotopic fractionation is attributed to mass-dependent kinetic effect(s). Nitrogen isotope fractionation upon electric discharge cannot account for the large N isotope variations observed among solar system objects and reservoirs. Extreme N isotope signatures in the solar system are more likely the result of self-shielding during N2 photodissociation, exotic effect during photodissociation of N2 and/or low temperature ion-molecule isotope exchange. Kinetic N isotope fractionation may play a significant role in the Titan s atmosphere. We also suggest that the low delta15N values of Archaean organic matter are partly the result of abiotic synthesis of organics that occurred at that time

Origin of isotopic fractionations of nitrogen and noble gases in meteorites and planetary atmospheres

L’azote et les gaz rares présents dans les astéroïdes, les comètes et les atmosphères planétaires sont piégés dans de la matière organique et ont des compositions chimiques qui sont différentes de celle du Soleil, représentatif du gaz primordial à partir duquel les différents objets du système solaire se sont formés il y a 4,5 milliards d’années. Au cours de cette thèse, des synthèses de matière carbonée à partir d’un mélange de gaz ont été réalisées dans un plasma appelé le Nébulotron, afin de mieux comprendre les processus à l’origine des compositions de l’azote et des gaz rares présents dans les météorites. Les caractéristiques de la matière organique ainsi que la composition des gaz rares piégés dans les météorites sont relativement bien reproduites dans les expériences, mais pas celle de l’azote. Ces résultats expérimentaux permettent de proposer des mécanismes clé à l’origine des compositions des éléments volatils présents dans les objets du système solaire.Nitrogen and noble gases present in asteroids, comets or planetary atmospheres are trapped in organic matter and bear a composition that is different from the composition of the Sun, which is representative of the primordial gas from which the different objects in the solar system were formed 4.5 billion years ago. During this thesis, experimental syntheses of organic matter from gas mixtures in a plasma setup called the Nebulotron were performed in order to better understand the processes responsible for this chemical difference between the meteorites and the Sun for nitrogen and noble gases. The characteristics of the organic matter and the signature of the noble gases trapped in meteorites are relatively well reproduced in the experiments, whereas the composition of nitrogen is not. These experimental results give hints about the key mechanisms that are responsible for the variations of the volatile elements composition in the solar system objects

Origine des fractionnements isotopiques de l'azote et des gaz rares dans les météorites et les atmosphères planétaires

Accès restreint aux membres de l'Université de Lorraine jusqu'au 2015-01-31Nitrogen and noble gases present in asteroids, comets or planetary atmospheres are trapped in organic matter and bear a composition that is different from the composition of the Sun, which is representative of the primordial gas from which the different objects in the solar system were formed 4.5 billion years ago. During this thesis, experimental syntheses of organic matter from gas mixtures in a plasma setup called the Nebulotron were performed in order to better understand the processes responsible for this chemical difference between the meteorites and the Sun for nitrogen and noble gases. The characteristics of the organic matter and the signature of the noble gases trapped in meteorites are relatively well reproduced in the experiments, whereas the composition of nitrogen is not. These experimental results give hints about the key mechanisms that are responsible for the variations of the volatile elements composition in the solar system objects.L’azote et les gaz rares présents dans les astéroïdes, les comètes et les atmosphères planétaires sont piégés dans de la matière organique et ont des compositions chimiques qui sont différentes de celle du Soleil, représentatif du gaz primordial à partir duquel les différents objets du système solaire se sont formés il y a 4,5 milliards d’années. Au cours de cette thèse, des synthèses de matière carbonée à partir d’un mélange de gaz ont été réalisées dans un plasma appelé le Nébulotron, afin de mieux comprendre les processus à l’origine des compositions de l’azote et des gaz rares présents dans les météorites. Les caractéristiques de la matière organique ainsi que la composition des gaz rares piégés dans les météorites sont relativement bien reproduites dans les expériences, mais pas celle de l’azote. Ces résultats expérimentaux permettent de proposer des mécanismes clé à l’origine des compositions des éléments volatils présents dans les objets du système solaire

Synthesis of refractory organic matter in the ionized gas phase of the solar nebula

International audienceRefractory organic compounds are ubiquitous in primitive chon-drites and cometary samples, though their origin is poorly understood. Those organic compounds are the main host of primordial noble gases, known as Q-gases, and nitrogen, which isotopic fractionations recorded physicochemical conditions of the solar system formation. Here, we report the characterization of organic compounds synthesized under ionizing conditions in a plasma setup from gas mixtures (H 2 (O)-CON 2-Noble gases) reminiscent of the protosolar nebula composition. Ionization of the gas phase was achieved at temperature up to 1000 K. Synthesized solid compounds share chemical and structural features with chondritic organics, and trapped noble gases reproduce the elemental and isotopic Q-gases patterns. These results suggest that both the formation of chondritic refractory organics and the trapping of Q-gases took place simultaneously in ionized areas of the proto-planetary disk, via photon-and/or electron-driven reactions and processing. Thus synthesis of primitive organics might not have required a cold environment as often assumed, and could have occurred anywhere it is ionized in the disk, including in its warm regions. This scenario also supports N 2 photodissociation as the cause of the large nitrogen isotopic range in the solar system. solar nebula | organics | meteorites | noble gases | nitroge

Processes of noble gas elemental and isotopic fractionations in plasma-produced organic solids: Cosmochemical implications

International audienceThe main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn- type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk

Cometary dust organics analogues: production, composition and scattered light

International audiencePolarimetric observations of cometary comae may be used to infer dust particles properties through experimental simulations. Cometary organic solid materials are poorly known. Here different organic materials found in nature or synthetic are studied. Their light scattering response was correlated to their chemical composition (under heating or not). Some cometary dust analogues were obtained by mixing them with silicates and lifted in the PROGRA2 light scattering experiment

Organic molecular heterogeneities can withstand diagenesis

International audienceReconstructing the original biogeochemistry of organic fossils requires quantifying the extent of the chemical transformations that they underwent during burial-induced maturation processes. Here, we performed laboratory experiments on chemically different organic materials in order to simulate the thermal maturation processes that occur during diagenesis. Starting organic materials were microorganisms and organic aerosols. Scanning transmission X-ray microscopy (STXM) was used to collect X-ray absorption near edge spectroscopy (XANES) data of the organic residues. Results indicate that even after having been submitted to 250 °C and 250 bars for 100 days, the molecular signatures of microorganisms and aerosols remain different in terms of nitrogen-to-carbon atomic ratio and carbon and nitrogen speciation. These observations suggest that burial-induced thermal degradation processes may not completely obliterate the chemical and molecular signatures of organic molecules. In other words, the present study suggests that organic molecular heterogeneities can withstand diagenesis and be recognized in the fossil record

The Paris meteorite, the least altered CM chondrite so far

The Paris chondrite provides an excellent opportunity to study CM chondrules and refractory inclusions in a more pristine state than currently possible from other CMs, and to investigate the earliest stages of aqueous alteration captured within a single CM bulk composition. It was found in the effects of a former colonial mining engineer and may have been an observed fall. The texture, mineralogy, petrography, magnetic properties and chemical and isotopic compositions are consistent with classification as a CM2 chondrite. There are ∼45 vol.% high-temperature components mainly Type I chondrules (with olivine mostly Fa0–2, mean Fa0.9) with granular textures because of low mesostasis abundances. Type II chondrules contain olivine Fa7 to Fa76. These are dominantly of Type IIA, but there are IIAB and IIB chondrules, II(A)B chondrules with minor highly ferroan olivine, and IIA(C) with augite as the only pyroxene. The refractory inclusions in Paris are amoeboid olivine aggregates (AOAs) and fine-grained spinel-rich Ca–Al-rich inclusions (CAIs). The CAI phases formed in the sequence hibonite, perovskite, grossite, spinel, gehlenite, anorthite, diopside/fassaite and forsterite. The most refractory phases are embedded in spinel, which also occurs as massive nodules. Refractory metal nuggets are found in many CAI and refractory platinum group element abundances (PGE) decrease following the observed condensation sequences of their host phases. Mn–Cr isotope measurements of mineral separates from Paris define a regression line with a slope of 53Mn/55Mn = (5.76 ± 0.76) × 106. If we interpret Cr isotopic systematics as dating Paris components, particularly the chondrules, the age is 4566.44 ± 0.66 Myr, which is close to the age of CAI and puts new constraints on the early evolution of the solar system. Eleven individual Paris samples define an O isotope mixing line that passes through CM2 and CO3 falls and indicates that Paris is a very fresh sample, with variation explained by local differences in the extent of alteration. The anhydrous precursor to the CM2s was CO3-like, but the two groups differed in that the CMs accreted a higher proportion of water. Paris has little matrix (∼47%, plus 8% fine grained rims) and is less altered than other CM chondrites. Chondrule silicates (except mesostasis), CAI phases, submicron forsterite and amorphous silicate in the matrix are all well preserved in the freshest domains, and there is abundant metal preserved (metal alteration stage 1 of Palmer and Lauretta (2011)). Metal and sulfide compositions and textures correspond to the least heated or equilibrated CM chondrites, Category A of Kimura et al. (2011). The composition of tochilinite–cronstedtite intergrowths gives a PCP index of ∼2.9. Cronstedtite is more abundant in the more altered zones whereas in normal highly altered CM chondrites, with petrologic subtype 2.6–2.0 based on the S/SiO2 and ∑FeO/SiO2 ratios in PCP or tochilinite–cronstedtite intergrowths (Rubin et al., 2007), cronstedtite is destroyed by alteration. The matrix in fresh zones has CI chondritic volatile element abundances, but interactions between matrix and chondrules occurred during alteration, modifying the volatile element abundances in the altered zones. Paris has higher trapped Ne contents, more primitive organic compounds, and more primitive organic material than other CMs. There are gradational contacts between domains of different degree of alteration, on the scale of ∼1 cm, but also highly altered clasts, suggesting mainly a water-limited style of alteration, with no significant metamorphic reheating