2019 |
Aerts, J. W., R. J. M. van Spanning, J. Flahaut, D. Molenaar, P. H. Bland, M. J. Genge, P. Ehrenfreund, and Z. Martins. "Microbial communities in sediments from four mildly acidic ephemeral salt lakes in the Yilgarn Craton (Australia) – Terrestrial analogs to ancient Mars." Frontiers in Microbiology 10 (2019).
Résumé : The Yilgarn Craton in Australia has a large number of naturally occurring shallow ephemeral lakes underlain by a dendritic system of paleodrainage channels. Processes like evaporation, flooding, erosion, as well as inflow of saline, often acidic and ion-rich groundwater contribute to the (dynamic) nature of the lakes and the composition of the sediments. The region has previously been described as an analog environment for early Mars due to its geological and geophysical similarities. Here, we investigated sediment samples of four lake environments aimed at getting a fundamental understanding of the native microbial communities and the mineralogical and (bio)chemical composition of the sediments they are associated with. The dominant mineral phases in the sediments were quartz, feldspars and amphiboles, while halite and gypsum were the only evaporites detected. Element analysis revealed a rich and complex image, in which silicon, iron, and aluminum were the dominant ions, but relative high concentrations of trace elements such as strontium, chromium, zirconium, and barium were also found. The concentrations of organic carbon, nitrogen, and phosphorus were generally low. 16S amplicon sequencing on the Illumina platform showed the presence of diverse microbial communities in all four lake environments. We found that most of the communities were dominated by extremely halophilic Archaea of the Halobacteriaceae family. The dynamic nature of these lakes appears to influence the biological, biochemical, and geological components of the ecosystem to a large effect. Inter- and intra-lake variations in the distributions of microbial communities were significant, and could only to a minor degree be explained by underlying environmental conditions. The communities are likely significantly influenced by small scale local effects caused by variations in geological settings and dynamic interactions caused by aeolian transport and flooding and evaporation events.
Mots-clés : cosmo
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Bekaert, D., M. S. Gudipati, B. Henderson, and B. Marty. "Coulomb explosion of multiply ionized xenon in water ice." Geochemical Journal 53, no. 81 (2019): 69.
Résumé : Laboratory investigations of noble gas trapping in amorphous water ice have been used to predict the noble gas composition of comets and infer on the origin of volatile elements within planetary bodies. However, the recent measurement
of the noble gas composition of ice sublimating from comet 67P/Churyumov-Gerasimenko by the Rosetta mission calls for novel experiments regarding the mechanisms of noble gas trapping and evolution in cometary ice analogues. Here, we
investigated the ionization dynamics of Xe atoms interacting with water ice using the recently developed Resonant Two-Step Laser Ablation Mass Spectrometry (2S-LAI-MS). Xenon-water mixed ice was ablated with an infrared beam set on the maximum absorption wavelength for water (l = 2948 nm) at which xenon atoms are kept neutral. Subsequent multiple ionization of xenon and oxygen resulted in periodic Coulomb explosions of Xen+ components (n OE [1;6]) and ionized water degradation products (OH+, H+, O+, O2+, O3+). Such explosions could only be detected when Xe and water were mixed together in ice, and not when separated in two overlaid layers. This paper discusses the potential mechanisms accounting for the generation of Coulomb explosions in these experiments and its relevance to cometary ice at closer distances to perihelion. We conclude that multiple ionization of xenon and oxygen in our experiment may be due to electron impact processes resembling cometary electron and ion bombardment, whereby energetic particles of hundreds of eV to a few keV are accelerated towards the comet’s nucleus. Electron and ion bombardment could induce significant chemical modifications to, and potentially outgassing from, the cometary surface.
Mots-clés : cosmo
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Cartier, C., and B. J. Wood. "The role of reducing conditions in building Mercury." Elements 15 (2019): 39–45.
Résumé : Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
Mots-clés : cosmo
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Cartier, C., and B. J. Wood. "The role of reducing conditions in building Mercury." Elements 15 (2019): 39–45.
Résumé : Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
Mots-clés : cosmo
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Dalou, C., E. Füri, C. Deligny, L. Piani, G. Caumon, B. Laumonier, J. Boulliung, and M. Edén. "Redox control on nitrogen isotope fractionation during planetary core formation." Proceedings of the National Academy of Sciences of the United States of America (2019).
Résumé : The present-day nitrogen isotopic compositions of Earth’s surficial (15N-enriched) and deep reservoirs (15N-depleted) differ significantly. This distribution can neither be explained by modern mantle degassing nor recycling via subduction zones. As the effect of planetary differentiation on the behavior of N isotopes is poorly
understood, we experimentally determined N-isotopic fractionations during metal–silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW −3.1 < logfO2 < ΔIW −0.5, where ΔIW is the logarithmic
difference between experimental oxygen fugacity [fO2] conditions and that imposed by the coexistence of iron and wüstite) at 1 GPa and 1,400 °C. We developed an in situ analytical method to measure the N-elemental and -isotopic compositions of experimental run products composed of Fe–C–N metal alloys and basaltic melts.
Our results show substantial N-isotopic fractionations between metal alloys and silicate glasses, i.e., from −257 ± 22‰ to −49 ± 1‰ over 3 log units of fO2. These large fractionations under reduced conditions can be explained by the large difference between N bonding in metal alloys (Fe–N) and in silicate glasses (as
molecular N2 and NH complexes). We show that the δ15N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core.
Mots-clés : cosmo magma
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Guitreau, M., and J. Flahaut. "Record of low-temperature aqueous alteration of Martian zircon during the late Amazonian." Nature Communications 10, no. 1 (2019): 2457.
Résumé : Several lines of evidence support the presence of liquid water on Mars at different times. Among those, hydrated minerals testify to past aqueous weathering processes that can be precisely studied in Martian meteorites such as NWA 7533/7034. Bringing constraints on the timing of weathering of the Martian crust would help understand its evolution, the availability of liquid water, and the habitability of Mars. Here we present a new method based on U–Th–Pb isotope systems to assess if zircon crystals underwent low-temperature aqueous alteration, such as exemplified by Hadean-aged detrital crystals from Western Australia. Data for NWA 7533 zircons show evidence for aqueous alteration and modeling of U–Th–Pb isotope system evolution indicates that the latest alteration event occurred during the late Amazonian (227–56 Ma). This finding largely expands the time duration over which liquid water was available near the Martian surface, thereby suggesting that Mars might still be habitable.
Mots-clés : cosmo
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Isono, Y., S. Tachibana, H. Naraoka, F. R. Orthous-Daunay, L. Piani, and Y. Kebukawa. "Bulk chemical characteristics of soluble polar organic molecules formed through condensation of formaldehyde: Comparison with soluble organic molecules in Murchison meteorite." Geochemical Journal 53 (2019): 41–51.
Résumé : Carbonaceous chondrites contain up to 2 wt% organic carbon, which is present as acid and solvent insoluble solid organic matter (IOM) and solvent soluble organic matter (SOM). The extraterrestrial organic matter should record chemical processes occurred in different environments in the early history of the Solar System, and the role of parent body aqueous alteration in the synthesis or subsequent modification of IOM and SOM still requires accurate constraints. We conducted hydrothermal experiments to simulate the synthesis of organic molecules during aqueous alteration on small bodies. Bulk chemical characteristics of soluble organic matter synthesized from formaldehyde in aqueous solutions were studied to compare them with that of chondritic SOM. We found that the redox state of synthesized organic molecules depends on temperature; the molecules become richer in hydrogen at higher temperatures. This can be explained by a cross-disproportionation reaction between organic molecules and formic acid, which occurs as a side reaction of the aldol condensation and works more effectively at higher temperatures. Comparison of the bulk chemical characteristics between the synthesized molecules and SOM extracted from the Murchison meteorite with methanol shows that the soluble
organic molecules in Murchison are more reduced than the synthesized molecules. Considering the temperature condition for aqueous alteration on the CM parent body that is lower than or equivalent to the experimental temperatures, the
reduced nature of Murchison organic molecules requires a reducing environment for them to be formed during hydrothermal alteration or imply that processes other than hydrothermal alteration were responsible for their synthesis. In case of
hydrothermal synthesis, reducing conditions might be established by the interaction between water and iron-bearing silicates or metals on the parent body.
Mots-clés : cosmo
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Marrocchi, Y., R. Euverte, J. Villeneuve, V. Batanova, B. Welsch, L. Ferrière, and E. Jacquet. "Formation of CV chondrules by recycling of amoeboid olivine aggregate-like precursors." Geochimica et Cosmochimica Acta 247, no. 2 (2019): 121–141.
Résumé : We have studied porphyritic olivine-rich chondrules of the carbonaceous chondrite Kaba (CV3) by combined high-resolution X-ray mapping, quantitative electron microprobe analyses, and oxygen isotopic analyses via secondary ion mass spectrometry. These chondrules contain smaller inner-chondrule olivine grains characterized by low refractory element (Ca, Al, Ti) contents, and larger outer-chondrule olivine crystals that are enriched in refractory elements and show complex Ti and Al oscillatory zonings. Our O isotopic survey revealed that many of the inner-chondrule olivines are 16O-richer than the relatively isotopically uniform outer-chondrule olivines. Inner-chondrule olivine crystals—only a minority of which may be derived from earlier generations of chondrules—are likely mostly inherited from nebular condensates similar to AOAs, as they share similar isotopic and chemical features and are thus interpreted as relict grains. Still, being 16O-poorer than most AOAs, they may have experienced significant exchange with a 16O-poor reservoir prior to chondrule formation (even if to a lesser degree than relicts in CM2 and ungrouped C2 chondrites). Subsequent incomplete melting of the relict grains produced Ca-Al-Ti-rich melts that engulfed the remaining relict olivine grains. The complex Ti and Al zoning patterns in outer chondrule (host) olivines, in particular the systematic dilution near the margin, seem to reflect gas-melt interactions (with e.g. SiO (g), Mg (g)) which also buffered the O isotopic composition of chondrule hosts. Together, these results demonstrate that important episodes of recycling of nebular condensates occurred in the solar protoplanetary disk.https://doi.org/10.1016/j.gca.2018.12.038
Mots-clés : cosmo
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Mosenfelder, J. L., A. Von Der Handt, E. Füri, C. Dalou, R. L. Hervig, G. R. Rossman, and M. M. Hirschmann. "Nitrogen incorporation in silicates and metals: Results from SIMS, EPMA, FTIR, and laser-extraction mass spectrometry." American Mineralogist 104, no. 1 (2019): 31–46.
Résumé : A quantitative understanding of nitrogen incorporation in Earth materials is important for constraining volatile evolution in planetary bodies. We used a combination of chemical (SIMS, EPMA, and laser-extraction mass spectrometry) and spectroscopic (FTIR) observations to study nitrogen contents and speciation mechanisms in silicate glasses, metal alloys, and an N-bearing silicate mineral (hyalophane). One suite of Fe-free basaltic glasses was studied by all four methods. Concentrations of N in these glasses determined by EPMA are systematically higher than those measured by laser extraction but agree within mutual 2s uncertainties, demonstrating the general veracity of the EPMA method. SIMS working curves based on measurement of 14N+ and 14N16O- as a function of N content determined by EPMA (or laser extraction) are best fit with exponential functions rather than the linear regressions that are most commonly applied to SIMS data. On the other hand, the relationship based on 12C14N- for C-poor, Fe-free glasses is exceptionally well fit to a linear regression (r2 = 1, p < 0.001), in contrast to expectations from previous work on glasses with lower N contents. Matrix effects on the SIMS signals associated with Fe or H2O content are not justified by the data, but volatile data (both N and H) for hyalophane, which contains 20 wt% BaO, reveal matrix effects possibly induced by its high average molar mass. A combination of FTIR and chemical data, together with a thorough review of the literature, was used to determine incorporation mechanisms for N in the Fe-free glasses. We infer that under reducing conditions at high pressure and temperature N is dissolved in basaltic melts chiefly as NH−2 and NH2–, with N2 and/or nitride (X-N3–) complexes becoming increasingly important at low fO2, increasing N content, and decreasing H content. Our results have implications for future studies seeking to accurately measure N by SIMS and for studies of N partitioning at high pressure relevant to planetary accretion and differentiation.
Mots-clés : cosmo
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Orthous-Daunay, F. R., L. Piani, L. Flandinet, R. Thissen, C. Wolters, V. Vuitton, O. Poch, F. Moynier, I. Sugawara, H. Naraoka et al. "Ultraviolet-photon fingerprints on chondritic large organic molecules." Geochemical Journal 53, no. 32 (2019): 21–32.
Résumé : The organic matter in carbonaceous chondrites is of two kinds : one is called Insoluble Organic Matter, made of extremely large molecules that cannot be named with the usual nomenclature ; one can be extracted by laboratory solvents and analyzed as a molecular mixture. Both are of debated origin. Retracing their natural histories requires putting strong constraints on their possible place of birth and their life time in space environments. It cannot be excluded they were formed in an interstellar medium before accretion on the chondritic parent bodies. As ultraviolet rays are the most common in the star forming regions and during the accretion phase of solar system, we propose to test the resilience of the natural organic matter of the Murchison meteorite against photolysis. The meteoritical soluble molecules were extracted by maceration and artificially exposed to a La photon dose commensurate to the one expected in molecular clouds and disks. The gaseous photolysis products were analyzed on the fly whereas the solid state mixture was solubilized again after irradiation for Orbitrap High Resolution Mass Spectrometry monitoring. We found that ultraviolet photons do modify the molecular mixture, removing H2 and small carbon bearing species, shifting the mass distribution toward lower masses and increasing the number of cycles and double bonds in the molecules structure. A noteworthy effect of the irradiation is its selective preservation of species with a double bond equivalent consistent with aromatic rings in their structure. This is explained by the higher stability of such compounds. As the pristine Murchison extract lacks those features, we estimate it has not undergone significant irradiation after its synthesis. The extract we used for experiment being water insoluble, we assume its reactivity in hydrothermal condition would have been limited and have had no effect on the irradiation fingerprints. As a result the soluble fraction of Murchison was whether formed where the UV photon flux was negligible or it has been accreted quickly and shielded from photolysis in a parent body.
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Rudraswami, N. G., Y. Marrocchi, M. Shyam Prasad, D. Fernandes, J. Villeneuve, and S. Taylor. "Oxygen isotopic and chemical composition of chromites in micrometeorites: Evidence of ordinary chondrite precursors." Meteoritics & Planetary Science 54, no. 6 (2019): 1347–1361.
Résumé : We identified 66 chromite grains from 42 of ~5000 micrometeorites collected from Indian Ocean deep‐sea sediments and the South Pole water well. To determine the chromite grains precursors and their contribution to the micrometeorite flux, we combined quantitative electron microprobe analyses and oxygen isotopic analyses by high‐resolution secondary ion mass spectrometry. Micrometeorite chromite grains show variable O isotopic compositions with δ18O values ranging from −0.8 to 6.0‰, δ17O values from 0.3 to 3.6‰, and Δ17O values from −0.9 to 1.6‰, most of them being similar to those of chromites from ordinary chondrites. The oxygen isotopic compositions of olivine, considered as a proxy of chromite in chromite‐bearing micrometeorites where chromite is too small to be measured in ion microprobe have Δ17O values suggesting a principal relationship to ordinary chondrites with some having carbonaceous chondrite precursors. Furthermore, the chemical compositions of chromites in micrometeorites are close to those reported for ordinary chondrite chromites, but some contribution from carbonaceous chondrites cannot be ruled out. Consequently, carbonaceous chondrites cannot be a major contributor of chromite‐bearing micrometeorites. Based on their oxygen isotopic and elemental compositions, we thus conclude with no ambiguity that chromite‐bearing micrometeorites are largely related to fragments of ordinary chondrites with a small fraction from carbonaceous chondrites, unlike other micrometeorites deriving largely from carbonaceous chondrites.
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Vézinet, A., E. Thomassot, D. G. Pearson, R. A. Stern, Y. Luo, and C. Sarkar. "Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga." Geology (2019).
Résumé : The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with δ18O values up to +9‰ at ca. 3.5 Ga, which are the highest values so far measured in any pre–3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon δ18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics ± clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition.
Mots-clés : cosmo
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2018 |
Avice, G., B. Marty, R. Burgess, A. Hofmann, P. Philippot, K. Zahnle, and D. Zakharov. "Evolution of atmospheric xenon and other noble gases inferred from Archean to Paleoproterozoic rocks." Geochimica et Cosmochimica Acta 232 (2018): 82–100.
Résumé : We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30–40‰ u-1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3 Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9 ± 1.2 (1r) ‰ u-1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u-1 at 3.5 Ga to 12.9‰ u-1 at 3.3 Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7 Ga. However, further studies will be needed to confirm this observation. After 2.7 Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1 Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3 Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth’s surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3 Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time
Mots-clés : cosmo
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Bekaert, D. V., S. Derenne, L. Tissandier, Y. Marrocchi, S. Charnoz, C. Anquetil, and B. Marty. "High-temperature ionization-induced synthesis of biologically relevant molecules in the protosolar nebula." Astrophysical Journal 859, no. 142 (2018).
Résumé : Biologically relevant molecules (hereafter biomolecules) have been commonly observed in extraterrestrial samples, but the mechanisms accounting for their synthesis in space are not well understood. While electron-driven production of organic solids from gas mixtures reminiscent of the photosphere of the protosolar nebula (PSN; i.e., dominated by CO–N2–H2) successfully reproduced key specific features of the chondritic insoluble organic matter (e.g., elementary and isotopic signatures of chondritic noble gases), the molecular diversity of organic materials has never been investigated. Here, we report that a large range of biomolecules detected in meteorites and comets can be synthesized under conditions typical of the irradiated gas phase of the PSN at temperatures=800 K. Our results suggest that organic materials—including biomolecules—produced within the photosphere would have been widely dispersed in the protoplanetary disk through turbulent diffusion, providing a mechanism for the distribution of organic meteoritic precursors prior to any thermal/photoprocessing and subsequent modification by
secondary parent body processes. Using a numerical model of dust transport in a turbulent disk, we propose that organic materials produced in the photosphere of the disk would likely be associated with small dust particles, which are coupled to the motion of gas within the disk and therefore preferentially lofted into the upper layers of the disk where organosynthesis occurs.
Mots-clés : cosmo
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Bekaert, D. V., Y. Marrocchi, A. Meshik, L. Remusat, and B. Marty. "Primordial heavy noble gases in the pristine Paris carbonaceous chondrite." Meteoritics & Planetary Science (2018): 1–20.
Résumé : The Paris carbonaceous chondrite represents the most pristine carbonaceous
chondrite, providing a unique opportunity to investigate the composition of early solar system materials prior to the onset of significant aqueous alteration. A dual origin (namely from the inner and outer solar system) has been demonstrated for water in the Paris meteorite parent body (Piani et al. 2018). Here, we aim to evaluate the contribution of outer solar system (cometary-like) water ice to the inner solar system water ice using Xe isotopes. We report Ar, Kr, and high-precision Xe isotopic measurements within bulk CM 2.9 and CM 2.7 fragments, as well as Ne, Ar, Kr, and Xe isotope compositions of the insoluble organic matter (IOM). Noble gas signatures are similar to chondritic phase Q with no evidence for a cometary-like Xe component. Small excesses in the heavy Xe isotopes relative
to phase Q within bulk samples are attributed to contributions from presolar materials. CM 2.7 fragments have lower Ar/Xe relative to more pristine CM 2.9 fragments, with no systematic difference in Xe contents. We conclude that Kr and Xe were little affected by aqueous alteration, in agreement with (1) minor degrees of alteration and (2) no significant differences in the chemical signature of organic matter in CM 2.7 and CM 2.9 areas (Vinogradoff et al. 2017). Xenon contents in the IOM are larger than previously published data of Xe in chondritic IOM, in line with the Xe component in Paris being pristine and preserved from Xe loss during aqueous alteration/thermal metamorphism
Mots-clés : cosmo
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Boucher, C., T. Lan, J. Mabry, D. V. Bekaert, P. G. Burnard, and B. Marty. "Spatial analysis of the atmospheric helium isotopic composition: Geochemical and environmental implications." Geochimica et Cosmochimica Acta 237 (2018): 120–130.
Résumé : Spatial variations in the atmospheric helium isotopic composition (RA = 3He/4Heair = 1.39 × 10−6) might be induced by localized and/or regional inputs of 3He and/or 4He into the air. It has been suggested that inputs of 4He from hydrocarbon exploitation may generate latitudinal variations in atmospheric 3He/4He. In order to test the possibility of such global variations, we performed high precision analyses of the helium isotopic composition (3He/4He) of sixteen air samples collected in 500 cc metal bottles around the world (79°N–75°S) between 2013 and 2015. In most cases, the 3He/4He of these air samples are indistinguishable from that of air sampled around Nancy (France) within ≤2‰ (95% confidence interval). Only two air samples, collected at Dome C (Antarctica) and at Tokyo (Japan), exhibit statistically higher 3He/4He ratios interpreted as potential 3He excesses of (2.0 ± 1.4)‰ and (1.7 ± 1.5)‰ (95% confidence interval), respectively. Excesses such as these suggest that potential helium isotopic variations in air are likely lower than 4‰ and might be generated temporarily by regional and/or local phenomenon (e.g. auroral precipitation, stratospheric to tropospheric exchanges). Thus, this study supports the use of the atmospheric helium isotopic ratio as an inter-laboratory He standard.
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Broadley, M. W., H. Hagi, R. Burgess, D. Zedgenizov, S. Mikhail, M. Almayrac, A. Ragozin, B. Pomazansky, and H. Sumino. "Plume-lithosphere interaction, and the formation of fibrous diamonds." Geochemical Perspectives Letters 2018, no. 8 (2018): 26–30.
Résumé : Fluid inclusions in diamond provide otherwise inaccessible information on the
origin and nature of carbonaceous fluid(s) in the mantle. Here we evaluate the
role of subducted volatiles in diamond formation within the Siberian cratonic
lithosphere. Specifically, we focus on the halogen (Cl, Br and I) and noble gas
(He, Ne and Ar) geochemistry of fluids trapped within cubic, coated and cloudy
fibrous diamonds from the Nyurbinskaya kimberlite, Siberia. Our data show Br/
Cl and I/Cl ratios consistent with involvement of altered oceanic crust, suggesting
subduction-derived fluids have infiltrated the Siberian lithosphere. 3He/4He
ranging from 2 to 11 RA, indicates the addition of a primordial mantle component
to the SCLM. Mantle plumes may therefore act as a trigger to re-mobilise
subducted carbon-rich fluids from the sub-continental lithospheric mantle, and
we argue this may be an essential process in the formation of fluid-rich diamonds,
and kimberlitic magmatism.
Mots-clés : cosmo magma
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Delon, R., S. Demouchy, Y. Marrocchi, M. A. Bouhifd, F. Barou, P. Cordier, A. Addad, and P. G. Burnard. "Helium incorporation and diffusion in polycrystalline olivine." Chemical Geology 488 (2018): 105–124.
Résumé : Helium is a key tracer of mantle geochemical and isotopic heterogeneities and can constrain our understanding of mantle geodynamics. Nevertheless, the mechanisms of helium storage and transport in mantle minerals remain poorly understood. Polycrystalline olivine was doped with helium at high temperature (1050 ± 25 °C) and high pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments to investigate helium storage and diffusion in Earth’s upper mantle. We also tested the effect of heterogeneous initial concentrations on the extracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. Our results show that two diffusion processes are acting in polycrystalline olivine : (i) a high temperature process with high activation energy ( Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature process with lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two diffusion processes are separated by a transition temperature that depends on the depletion of helium hosted in grain boundaries, i.e., the amount of helium stored at grain boundaries and the temperature and duration of the step heating sequence. Our results confirm that grain boundaries can represent a significant storage site for He. Moreover, we report two different populations of diffusion parameters in the lattice diffusion field, which are interpreted as diffusion in interstitials (Ea= 95 ± 15 kJ·mol−1 and log(D0)=−8.26 ± 2.13) and Mg vacancies (Ea= 168 ± 19 kJ·mol−1 and log(D0)=− 3.59 ± 2.12). Similar diffusion parameters populations are observed in literature data after reprocessing the diffusivities. Furthermore, we determine grain boundary diffusion parameters : Ea = 57 ± 14 kJ·mol−1 and log(D0)=−9.20 ± 0.99. Applying these results to the upper mantle reveals that an important amount of He can be stored at grain boundaries for typical mantle grain size (22% for a grain size of 1 mm) and that most helium can be stored at grain boundaries for relatively small grain sizes (≤290μm and ≤10μm for segregation factors of 1/10−5 and 1/0.0025, respectively). As a consequence, bulk diffusivities can be significantly higher than lattice diffusivities. Although our study cannot be applied directly to the lower mantle, the similar storage sites and diffusion mechanisms are expected in lower mantle silicates if high pressure does not inhibit diffusion
Mots-clés : cosmo
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Füri, E., E. Deloule, and C. Dalou. "Nitrogen abundance and isotope analysis of silicate glasses by secondary ionization mass spectrometry." Chemical Geology 493 (2018): 327–337.
Résumé : Chondritic meteorites preserve extreme intra-sample 15N/14N variations, which exceed, in some cases, the range of nitrogen isotope ratios observed at the Solar System scale. These observations are based on in situ analyses of CN− molecular ions by secondary ionization mass spectrometry (SIMS) in carbon-rich phases. The distribution of nitrogen and its isotopes in silicate minerals and glasses has not been investigated to this date due to the lack of an appropriate analytical protocol, as well as of suitable N-bearing standards. In order to improve our knowledge of the nitrogen signature of both extraterrestrial and terrestrial silicate samples, we have developed a protocol for determining precise and accurate nitrogen abundances (and isotope ratios) in basaltic glasses using high mass
resolution SIMS. Twelve (C-)N-bearing synthetic basaltic glasses, containing between<1 and 18,443 ± 966 ppm N, form the suite of reference materials for this study. By targeting the CN−, NO−, AlN−, and SiN− secondary molecular ions, nitrogen abundances can be detected down to the ppm level in both carbonbearing and carbon-free glasses. The analytical precision and reproducibility of isotope ratios in the form of 15N16O−/14N16O− is on the order of 11‰ and 10 to 17‰ (2σ), respectively, for reference glasses containing ≥100 ppm N. Thus, nitrogen isotope ratios can be determined with an uncertainty that is small enough to resolve nitrogen isotope variations in extraterrestrial silicates. The study of four chondrules of the ordinary chondrite Semarkona (LL3.0) reveals that the nitrogen distribution in the mesostasis is highly heterogeneous, with concentrations ranging from 0 to 1099 ± 168 ppm. The δ15N values in mesostasis, olivine, and pyroxene
vary between −36 ± 50‰ and +55 ± 72‰, indicating that silicate phases in chondrules do not host particularly 15N-poor nitrogen
Mots-clés : cosmo
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Füri, E., L. Zimmermann, and A. E. Saal. "Apollo 15 green glass He-Ne-Ar signatures – In search for indigenous lunar noble gases." Geochemical Perspectives Letters 8 (2018): 1–5.
Résumé : Identifying indigenous lunar noble gases in samples returned by the Apollo and
Luna missions is highly challenging because contributions from the solar wind
(SW) and/or cosmogenic nuclides have modified the noble gas signature of the
regolith and rocks exposed to space at the lunar surface. Here we re-investigate
the possible presence of indigenous noble gases in pyroclastic Apollo 15426 green
glasses based on precise measurements of He-Ne-Ar isotopic compositions and
abundances. The noble gas content of single glass beads varies by two orders of
magnitude, indicating that they experienced highly variable irradiation histories
as a result of intense regolith stirring by impact gardening. Four out of the twelve spherules stand out by having the highest He-Ne-Ar abundances and by releasing an isotopically ‘solar-like’ noble gas component at high temperatures. While a contribution from indigenous noble gases cannot be ruled out, the data are best accounted for by inward diffusion of, and equilibration with, SW-derived volatiles during prolonged space exposure.
Mots-clés : cosmo
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Cosmochemistry
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