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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Dalou, C., M. M. Hirschmann, S. D. Jacobsen, and C. Le Losq. "Raman spectroscopy study of C-O-H-N speciation in reduced basaltic glasses : Implications for reduced planetary mantles." Geochimica et Cosmochimica Acta 265 (2019): 32–47.
Résumé : To better understand the solution of volatile species in a reduced magma ocean, we identify via Raman spectroscopy the nature of C-O-H-N volatile species dissolved in a series of reduced basaltic glasses. The oxygen fugacity (f O2) during synthesis varied from highly reduced at two log units below the iron-wustite buffer (IW-2.1) to moderately reduced (IW-0.4), spanning much of the magmatic f O2 conditions during late stages of terrestrial accretion. Raman vibrational modes for H2, NH2 – , NH3, CH4, CO, CN–, N2, and OH– species are inferred from band assignments in all reduced glasses. The integrated area of Raman bands assigned to N2, CH4, NH3 and H2 vibrations in glasses increases with increasing molar volume of the melt, whereas that of CO decreases. Additionally, with increasing f O2, CO band areas increase while those of N2 decrease, suggesting that the solubility of these neutral molecules is not solely determined by the melt molar volume under reduced conditions. Coexisting with these neutral molecules, other species as CN–, NH2 – and OH– are chemically bonded within the silicate network. The observations indicate that, under reduced conditions, (1) H2, NH2 – , NH3, CH4, CO, CN–, N2, and OH– species coexist in silicate glasses representative of silicate liquids in a magma ocean (2) their relative abundances dissolved in a magma ocean depend on melt composition, f O2 and the availability of H and, (3) metal-silicate partitioning or degassing reactions of those magmatic volatile species must involve changes in melt and vapor speciation, which in turn may influence isotopic fractionation.
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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Mangold, N., J. Flahaut, and V. Ansan. "The surface composition of terrestrial planets." Oxford Research Encyclopedias Planetary Science (2019).
Résumé : Planetary surface compositions are fundamental to an understanding of both the interior activity through differentiation processes and volcanic activity and the external evolution through alteration processes and accumulations of volatiles. While the Moon has been studied since early on using ground-based instruments and returned samples, observing the surface composition of the terrestrial planets did not become practical until after the development of orbital and in situ missions with instruments tracking mineralogical and elemental variations. The poorly evolved, atmosphere-free bodies like the Moon and Mercury enable the study of the formation of the most primitive crusts, through processes such as the crystallization of a magma ocean, and their volcanic evolution. Nevertheless, recent studies have shown more diversity than initially expected, including the presence of ice in high latitude regions. Because of its heavy atmosphere, Venus remains the most difficult planetary body to study and the most poorly known in regards to its composition, triggering some interest for future missions. In contrast, Mars exploration has generated a huge amount of data in the last two decades, revealing a planet with a mineralogical diversity close to that of the Earth. While Mars crust is dominated by basaltic material, recent studies concluded for significant contributions of more felsic and alkali-rich igneous material, especially in the ancient highlands. These ancient terrains also display widespread outcrops of hydrous minerals, especially phyllosilicates, which are key in the understanding of past climate conditions and suggest a volatile-rich early evolution with implications for exobiology. Recent terrains exhibit a cryosphere with ice-rich landforms at, or close to the surface, of mid- and high latitudes, generating a strong interest for recent climatic variability and resources for future manned missions. While Mars is certainly the planetary body the most similar to Earth, the observation of specific processes such as those linked to interactions with solar wind on atmosphere-free bodies, or with a thick acidic atmosphere on Venus, improve our understanding of the differences in evolution of terrestrial bodies. Future exploration is still necessary to increase humankind’s knowledge and further build a global picture of the formation and evolution of planetary surfaces.
Mots-clés : cosmo
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Marrocchi, Y., and L. Piani. "The tumultuous childhood of the Solar System." Nature Astronomy 3, no. 10 (2019): 889–890.
Résumé : The peculiar carbon isotopic compositions of carbonates in the Tagish Lake meteorite suggest that D-type asteroids accreted in the outer part of the protoplanetary disk — beyond 10 au — before being dispersed sunwards to the main asteroid belt.
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.
Mots-clés : cosmo
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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.
Mots-clés : cosmo
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Vacher, L. G., M. Piralla, M. Gounelle, M. Bizzarro, and Y. Marrocchi. "Thermal evolution of hydrated asteroids inferred from oxygen isotopes." The Astrophysical Journal Letters 882 (2019): L20.
Résumé : Chondrites are fragments of unmelted asteroids that formed due to gravitational instabilities in turbulent regions of the Solar protoplanetary disk. Hydrated chondrites are common among meteorites, indicating that a substantial fraction of the rocky bodies that formed early in the solar system accreted water ice grains that subsequently melted due to heat released by the radioactive decay of 26Al. However, the thermal histories of asteroids are still largely unknown ; increased knowledge would provide fundamental information on their timing of accretion and their physical characteristics. Here we show that hydrated meteorites (CM chondrites) contain previously uncharacterized calcium carbonates with peculiar oxygen isotopic compositions (Δ17O≈−2.5‰), which artificially produce the massindependent trend previously reported for carbonates. Based on these isotopic data, we propose a new model to quantitatively estimate the precipitation temperatures of secondary phases (carbonates and serpentine). It reveals that chondritic secondary phases recorded a gradual increase in temperature during the extent of aqueous alteration, from −10°C to a maximum of 250°C. We also show that the thermal path of C-type asteroids is independent of the initial oxygen isotopic composition of the primordial water ice grains that they accreted. Our estimated temperatures for hydrated asteroids remain lower than those experienced by other carbonaceous chondrites, providing strong constraints for modeling the formation conditions and size distribution of water-rich asteroids, especially in anticipation of the return of samples of water-rich asteroids to Earth by the OSIRIS-REx and Hayabusa2 missions.
Mots-clés : cosmo
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Vacher, L. G., L. Truche, F. Faure, L. Tissandier, R. Mosser-Ruck, and Y. Marrocchi. "Deciphering the conditions of tochilinite and cronstedtite formation in CM chondrites from low temperature hydrothermal experiments." Meteoritics & Planetary Science 54 (2019): 1870–1889.
Résumé : Tochilinite/cronstedtite intergrowths are commonly observed as alteration products in CM chondrite matrices, but the conditions under which they formed are still largely underconstrained due to their scarcity in terrestrial environments. Here, we report low temperature (80 °C) anoxic hydrothermal experiments using starting assemblages similar to the constituents of the matrices of the most pristine CM chondrite and S-rich and S-free fluids. Cronstedtite crystals formed only in S-free experiments under circumneutral conditions with the highest Fe/Si ratios. Fe-rich tochilinite with chemical and structural characteristics similar to chondritic tochilinite was observed in S-bearing experiments. We observed a positive correlation between the Mg content in the hydroxide layer of synthetic tochilinite and temperature, suggesting that the composition of tochilinite is a proxy for the alteration temperature in CM chondrites. Using this relation, we estimate the mean precipitation temperatures of tochilinite to be 120–160 °C for CM chondrites. Given the different temperature ranges of tochilinite and cronstedtite in our experiments, we propose that Fe-rich tochilinite crystals resulted from the alteration of metal beads under S-bearing alkaline conditions at T = 120–160 °C followed by cronstedtite crystals formed by the reaction of matrix amorphous silicates, metal beads, and water at a low temperature (50–120 °C).
Mots-clés : cosmo
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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 47, no. 7 (2019): 605–608.
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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Verdier-Paoletti, M. J., Y. Marrocchi, L. G. Vacher, J. Gattacceca, A. Gurenko, C. Sonzogni, and M. Gounelle. "Testing the genetic relationship between fluid alteration and brecciation in CM chondrites." Meteoritics & Planetary Science 54 (2019): 1692–1709.
Résumé : Boriskino is a poorly studied CM chondrite with numerous millimeter‐ to centimeter‐scale clasts exhibiting sharp boundaries. Clast textures and mineralogies attest to diverse geological histories with various degrees of aqueous alteration. We conducted a petrographic, chemical, and isotopic study on each clast type of the breccia to investigate if there exists a genetic link between brecciation and aqueous alteration, and to determine the controlling parameter of the extent of alteration. Boriskino is dominated by CM2 clasts for which no specific petrographic type could be assigned based on the chemical compositions and modal abundances of constituents. One clast stands out and is identified as a CM1 lithology, owing to its lack of anhydrous silicates and its overall abundance of dolomite‐like carbonates and acicular iron sulfides. We observe that alteration phases near clast boundaries exhibit foliation features, suggesting that brecciation postdated aqueous alteration. We measured the O‐isotopic composition of Ca‐carbonates and dolomite‐like carbonates to determine their precipitation temperatures following the methodology of Verdier‐Paoletti et al. (2017). Both types of carbonates yield similar ranges of precipitation temperatures independent of clast lithology, ranging from −13.9 ± 22.4 (2σ) to 166.5 ± 47.3 °C, precluding that temperature alone accounts for the differences between the CM1 and CM2 lithologies. Instead, we suggest that initial water/rock ratios of 0.75 and 0.61 for the CM1 and CM2 clasts, respectively, might control the extent of aqueous alteration. Based on these estimates, we suggest that Boriskino clasts originated from a single parent body with heterogeneous distribution of water either due to local differences in the material permeability or in the initial content of ice available. These conditions would have produced microenvironments with differing geochemical conditions thus leading to a range of degrees of aqueous alteration.
Mots-clés : cosmo
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Villeneuve, J., M. Chaussidon, Y. Marrocchi, Z. Deng, and E. B. Watson. "High‐precision in situ silicon isotopic analyses by multicollector secondary ion mass spectrometry in olivine and lowcalcium pyroxene." Rapid Communications in Mass Spectrometry 33 (2019): 1589–1597.
Résumé : High‐precision determination of silicon isotopes can be achieved by in situ multi‐collector secondary ion mass spectrometry (MS‐SIMS). The accuracy of the analyses is, however, sensitive to ion yields and instrumental mass fractionations (IMFs) induced by the analytical procedure. These effects vary from one instrument to another, with the analytical settings, and with the composition and nature of the sample. Because ion yields and IMF effects are not predictable and rely on empirical calibrations, high‐accuracy analyses require suitable sets of standards. Methods : Here, we document calibrations of ion yields and matrix effects in a set of 23 olivine standards and 3 low‐Ca pyroxene for silicon isotopic measurements in both polarities using Cameca IMS 1270 E7 and IMS 1280 HR2 ion probes set with the cesium (Cs) or radiofrequency (RF) source. Results : Silicon ion yields show (i) strong variations with the chemical composition, and (ii) an opposite behavior between the secondary positive and negative polarities. The magnitude of IMF along the fayalite‐forsterite (olivine) series shows a complex behavior, increasing overall by ≈7‰ (secondary positive) and ≈15‰ (secondary negative) with increasing olivine Mg#. A drastic change in olivine IMF occurs at Mg#≈70 in both polarities. The magnitude of IMF for low‐Ca pyroxene from Mg#= 70–100 is almost constant in both polarities, i.e. ≈0.1‰ in secondary positive and ≈0.15‰ in secondary negative. The analytical uncertainties on individual analyses were ± 0.05–0.15‰ (2 S.E.) with both sources, and the external errors for each standard material were ≈ ±0.05–0.5‰ (2 S.E.) with the Cs source and ≈ ±0.03–0.15‰ (2 S.E.) with the RF source. Conclusions : The IMF effect of Si isotopes in silicates shows complex behaviors that vary with the chemistry and the settings of the instrument. We developed a suitable set of standards in order to perform high‐accuracy in situ measurements of Si isotopes in olivine and low‐Ca pyroxene characterized by varying chemical compositions by MC‐SIMS.
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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Cosmochemistry
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