Multiple carriers of Q noble gases in primitive meteorites

The 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. Indirect techniques have revealed that phase Q might be composed of two subphases, one of them associated with sulfide. Here we provide experimental evidence that noble gases trapped within meteoritic sulfides present chemically- and thermally-driven behavior patterns that are similar to Q-gases. We therefore suggest that phase Q is likely composed of two subcomponents: carbonaceous phases and sulfides. In situ decay of iodine at concentrations levels consistent with those reported for meteoritic sulfides can reproduce the 129Xe excess observed for Q-gases relative to fractionated Solar Wind. We suggest that the Q-bearing sulfides formed at high temperature and could have recorded the conditions that prevailed in the chondrule-forming region(s)

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

Condensate evolution in the solar nebula inferred from combined Cr, Ti, and O isotope analyses of amoeboid olivine aggregates

Refractory inclusions in chondritic meteorites, namely amoeboid olivine aggregates (AOAs) and Ca-Al-rich inclusions (CAIs), are among the first solids to have formed in the solar system. The isotopic composition of CAIs is distinct from bulk meteorites, which either results from extreme processing of presolar carriers in the CAI-forming region, or reflects an inherited heterogeneity from the Sun's parental molecular cloud. Amoeboid olivine aggregates are less refractory than CAIs and provide a record of how the isotopic composition of solid material in the disk may have changed in time and space. However, the isotopic composition of AOAs and how this composition relates to that of CAIs and later-formed solids is unknown. Here, using new O, Ti, and Cr isotopic data for eight AOAs from the Allende CV3 chondrite, we show that CAIs and AOAs share a common isotopic composition, indicating a close genetic link and formation from the same isotopic reservoir. Because AOAs are less refractory than CAIs, this observation is difficult to reconcile with a thermal processing origin of the isotope anomalies. Instead, the common isotopic composition of CAIs and AOAs is readily accounted for in a model in which the isotopic composition of infalling material from the Sun's parental molecular cloud changed over time. In this model, CAIs and AOAs record the isotopic composition of the early infall, while later-formed solids contain a larger fraction of the later, isotopically distinct infall. This model implies that CAIs and AOAs record the isotopic composition of the Sun and suggests that the nucleosynthetic isotope heterogeneity of the solar system is predominantly produced by mixing of solar nebula condensates, which acquired their distinct isotopic compositions as a result of time-varied infall from the protosolar cloud.Comment: Published gold open access in Earth and Planetary Science Letter

Constraints on Neon and Argon Isotopic Fractionation in Solar Wind

To evaluate the isotopic composition of the solar nebula from which the planets formed, the relation between isotopes measured in the solar wind and on the Sun's surface needs to be known. The Genesis Discovery mission returned independent samples of three types of solar wind produced by different solar processes that provide a check on possible isotopic variations, or fractionation, between the solar-wind and solar-surface material. At a high level of precision, we observed no significant inter-regime differences in ^(20)Ne/^(22)Ne or ^(36)Ar/^(38)Ar values. For ^(20)Ne/^(22)Ne, the difference between low- and high-speed wind components is 0.24 ± 0.37%; for ^(36)Ar/^(38)Ar, it is 0.11 ± 0.26%. Our measured ^(36)Ar/^(38)Ar ratio in the solar wind of 5.501 ± 0.005 is 3.42 ± 0.09% higher than that of the terrestrial atmosphere, which may reflect atmospheric losses early in Earth's history

Incorporation des gaz rares dans la matière organique primitive du système solaire

The origin of the meteoritic organic matter and associated noble gases is poorly constrained. Experiments have been performed during this thesis in order to better constrain the possible environments of formation. Low pressure adsorption reproduces the concentration and elemental pattern of noble gases in the temperature range 80-100 K, but cannot explain the significant retention of noble gases within the organic structure. In addition, Rayleigh-type distillation experiments induced by adsorption do not show measurable isotopic fractionation. A solvation experiment carried out on insoluble organic matter of Orgueil (CI) reveals the volume trapping of P1 noble gases. This result suggests a mechanical trapping of P1 noble gases in the organic structure. Two syntheses have been carried out in order to reproduce these characteristics. Sublimation- condensation experiments under an ionizing xenon atmosphere reproduces the isotopic fractionation observed for P1 noble gases compared to the solar composition. This result shows that the P1 composition can be generated from a nebula of solar composition. However, this condensation process does not allow the diradicaloids observed by electron paramagnetic resonance to be reproduced. This result strongly suggests an interstellar origin of insoluble organic matter and associated P1 noble gases. Then, the second mechanism was tested : the transformation of nanodiamonds to carbon onions. Nanodiamonds represent an important part of the interstellar carbon and could undergo change to carbon onions under heating or irradiation conditions. Nanodiamonds heating experiments have been carried out. They reveal significant retention of trapped xenon with the maximum temperature release occurring at 800°C. This characteristic, coupled with the detection of carbon onions in primitive meteorites and their genetic link with nanodiamonds, strongly suggests that this structure could be advocated as the P1 noble gas carrier in meteorites.L'origine de la matière organique insoluble des météorites et des gaz rares associés est très mal comprise. Des expériences ont été effectuées lors de cette thèse afin de mieux cerner les environnements plausibles de formation. L'adsorption physique basse pression permet de reproduire les abondances et le fractionnement élémentaire des gaz rares pour un intervalle de température de 80-100 K mais ne permet pas de rendre compte de la forte rétention des gaz rares dans la matière organique. De plus, les phénomènes d'adsoprtion n'induisent pas un fractionnement isotopique mesurable. Une expérience de solvatation sur la matière organique insoluble d'Orgueil (CI) révèle le piégeage dans le volume des gaz rares P1. Ces résultats suggèrent un piégeage d'origine mécanique de ces gaz dans la structure organique. Deux mécanismes ont ainsi été testés pour reproduire ces caractéristiques. La sublimation-condensation de matière organique sous atmosphère de xénon ionisé permet de rendre compte du fractionnement isotopique de 1 %/uma observé pour les gaz rares P1 par rapport à la composante solaire. Ces résultats démontrent la possibilité de produire les caratéristiques du pôle P1 à partir d'une nébuleuse de compositon solaire. Cependant, ce mécanisme ne permet pas de reproduire les di-radicaux observés dans la matière organique insoluble des météorites par résonance paramagnétique électronique. Ce résultat tend à favoriser une origine interstellaire de la matière organique des météorites. A ce titre, un autre mécanisme a été étudié : le changement de phase nanodiamants oignons de carbone. Les nanodiamants représentent une importante quantité du carbone interstellaire et peuvent subir une transformation en oignons de carbone sous des conditions thermiques ou d'irradiations intenses. Des expériences de chauffage de nanodiamants sous une atmosphère de xénon ont été réalisées. Elles révélent la très grande rétention thermique du xénon piégé dans la nouvelle structure avec une température maximum de relâche située à 800°C. Outre leur très grande stabilité thermique, les oignons de carbone ont été observés dans les météorites et leur lien génétique avec les nanodiamants en font un des candidats les plus sérieux au titre de porteur des gaz rares P1

Chondrule heritage and thermal histories from trace element and oxygen isotope analyses of chondrules and amoeboid olivine aggregates

International audienceWe report combined oxygen isotope and mineral-scale trace element analyses of amoeboid olivine aggregates (AOA) and chondrules in ungrouped carbonaceous chondrite, Northwest Africa 5958. The trace element geochemistry of olivine in AOA, for the first time measured by LA-ICP-MS, is consistent with a condensation origin, although the shallow slope of its rare earth element (REE) pattern is yet to be physically explained. Ferromagnesian silicates in type I chondrules resemble those in other carbonaceous chondrites both geochemically and isotopically, and we find a correlation between 16O enrichment and many incompatible elements in olivine. The variation in incompatible element concentrations may relate to varying amounts of olivine crystallization during a subisothermal stage of chondrule-forming events, the duration of which may be anticorrelated with the local solid/gas ratio if this was the determinant of oxygen isotopic ratios as proposed recently. While aqueous alteration has depleted many chondrule mesostases in REE, some chondrules show recognizable subdued group II-like patterns supporting the idea that the immediate precursors of chondrules were nebular condensates