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

Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites

The nature and distribution of sulfides within 17 porphyritic chondrules of the Sahara 97096 EH3 enstatite chondrite have been studied by backscattered electron microscopy and electron microprobe in order to investigate the role of gas–melt interactions in the chondrule sulfide formation. Troilite (FeS) is systematically present and is the most abundant sulfide within the EH3 chondrite chondrules. It is found either poikilitically enclosed in low-Ca pyroxenes or scattered within the glassy mesostasis. Oldhamite (CaS) and niningerite [(Mg,Fe,Mn)S] are present in ≈60% of the chondrules studied. While oldhamite is preferentially present in the mesostasis, niningerite associated with silica is generally observed in contact with troilite and low-Ca pyroxene. The Sahara 97096 chondrule mesostases contain high abundances of alkali and volatile elements (average Na2O = 8.7 wt.%, K2O = 0.8 wt.%, Cl = 7100 ppm and S = 3700 ppm) as well as silica (average SiO2 = 62.8 wt.%). Our data suggest that most of the sulfides found in EH3 chondrite chondrules are magmatic minerals that formed after the dissolution of S from a volatile-rich gaseous environment into the molten chondrules. Troilite formation occurred via sulfur solubility within Fe-poor chondrule melts followed by sulfide saturation, which causes an immiscible iron sulfide liquid to separate from the silicate melt. The FeS saturation started at the same time as or prior to the crystallization of low-Ca pyroxene during the high temperature chondrule forming event(s). Protracted gas–melt interactions under high partial pressures of S and SiO led to the formation of niningerite-silica associations via destabilization of the previously formed FeS and low-Ca pyroxene. We also propose that formation of the oldhamite occurred via the sulfide saturation of Fe-poor chondrule melts at moderate S concentration due to the high degree of polymerization and the high Na-content of the chondrule melts, which allowed the activity of CaO in the melt to be enhanced. Gas–melt interactions thus appear to be a key process that may control the mineralogy of chondrules in the different classes of chondrite

Etude expérimentale des interactions gaz-liquide dans les systèmes silicatés (implications sur la formation des objets primitifs du système solaire)

NANCY/VANDOEUVRE-INPL (545472102) / SudocSudocFranceF

Olivine dissolution in molten silicates: An experimental study with application to chondrule formation

International audienceMg-rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low-Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high-temperature experiments (1450-1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X-ray microtomography. Results indicate that forsterite dissolution in chondrule-like melts is a very fast process with rates that range from 5 μm min-1 to 22 μm min-1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas-melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg-rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica-poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low-Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule

Solubility of uranium oxide in ternary aluminosilicate glass melts

International audienceUranium solubility was measured in melts belonging to the CaO-Al2O3-SiO2 (CAS) and MgO-Al2O3-SiO2 (MAS) systems using the Pt wire loop technique, enabling independent control of the temperature (1400 °C), glass composition, and oxygen fugacity (-16.12)2)[5]Al in each glass system

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

Incorporation of Zn in the destabilization products of muscovite at 1175 degrees C under disequilibrium conditions, and implications for heavy metal sequestration

International audienceThis work reports on the thermal decomposition of muscovite within a granite powder doped with 8.5 wt% ZnO and heated during 10 min to 68 h at 1175°C, and the implications for the sequestration of Zn, and other heavy metalts in such decomposition products. Samples were characterized using analytical scanning and transmission electron microscopy. After 10 min, muscovite is completely pseudomorphosed by Si-rich glass, spinel structure phases, and minor mullite. Spinel phases incorpo-rate Zn, but their compositions depend on their position within the muscovite pseudomorphs. Al-rich oxides crystallize at the core of the pseudomorphs while Zn-Al spinels are located at the rims. The most Al-rich spinels have compositions close to γ-Al2O3, a metastable transition alumina, with up to 5 wt% MgO, 2 wt% Fe2O3, 4 wt% ZnO, and 9 wt% SiO2. The most Zn-rich spinels show compositions intermediate between Al2O3 and gahnite (ZnAl2O4), with up to 31 wt% ZnO and significant contents of MgO (3 wt%), Fe2O3 (5 wt%), and SiO2 (10 wt%). After 68 h, stable spinels are gahnite close to the end-member composition with MgO and Fe2O3 contents below ca. 5 wt%, and SiO2 contents ca. 1 wt%. These results support the existence of a metastable solid solution between γ-Al2O3and gahnite. This experimental work shows that Zn can be incorporated in spinel structures after heating at 1175°C during short durations and Zn is preferentially incorporated in the muscovite pseudomorphs as opposed to the Qtz-Fds glass. Consequently, the thermal breakdown of phyllosilicates can be a viable process to immobilize heavy metals such as Zn

The diffusion coefficients of noble gases (He Ar) in a synthetic basaltic liquid: One-dimensional diffusion experiments

International audienceDiffusion coefficients of noble gases in silicate liquids are poorly known, and, as a result, it is difficult to quantify the importance of kinetic fractionation of noble gas abundances and isotopic compositions during magmatic processes. Nevertheless, diffusive fractionation has been invoked to explain noble gas signatures in MORBs and OIBs, with important implications for magma degassing and noble gas mantle geochemistry. In order to investigate the diffusion of noble gases in magmas, we developed an experimental protocol based on uniaxial diffusion of He and Ar through a column of synthetic basaltic liquid in a Pt tube. At the end of the experiment, the column of silicate liquid was rapidly quenched to a glass recording the diffusion profile. The glass cylinder was cut into a series of slices, which were analyzed for noble gases by traditional noble gas mass spectrometry following total gas extraction by fusion.Using this protocol, we measured He and Ar diffusivities in CMAS glass G1 (50 mol% SiO2, 9 mol% Al2O3, 16 mol% MgO, 25 mol% CaO) at temperatures of 1673 K and 1823 K: DHe = 2.75 ± 0.25 × 10− 6 cm2·s− 1 (T = 1673 K); DHe = 4.77 ± 0.42 × 10− 6 cm2·s− 1 (T = 1823 K); and DAr = 9.3 ± 1.3 × 10− 7 cm2·s− 1 (T = 1673 K). Combining these new high temperature data with diffusion coefficients measured on the same composition just above the glass transition temperature, we determined the activation energy Ea and the pre-exponential factor D0 for He and Ar diffusion in silicate liquids: D0 = 1.72 ± 0.9 × 10− 2 cm2·s− 1 and Ea = 136.5 ± 3.2 kJ/mol for Ar; D0 = 1.8 ± 0.5 × 10− 4 cm2·s− 1 and Ea = 57.6 ± 1.5 kJ/mol for He. Because He and Ar have very different activation energies for diffusion in the liquid state, the ratio DHe/DAr is strongly sensitive to temperature, decreasing from 145 at the glass transition temperature (1005 K) to 2 at 1823 K. The implication is that the kinetic fractionation of He relative to Ar in magmas is likely to be more important during the cooling stages than during the earlier, high temperature stages of magmatic history