2021 |
Bernadou, F., F. Gaillard, E. Füri, Y. Marrocchi, and A. Slodczyk. "Nitrogen solubility in basaltic silicate melt – Implications for degassing processes." Chemical Geology 573 (2021): 120192.
Résumé : The distribution of nitrogen between the different terrestrial reservoirs (core-mantle-atmosphere) and how this may have changed since the earliest planetary stages is uncertain. In particular, the primordial degassing processes of the magma ocean and its role in the formation of the atmosphere remains to be quantified. Since no geological samples can capture this early degassing process, we need to go through the thermodynamic modeling of the nitrogen solubility in silicate melt. We hence performed experiments on basaltic samples at fluid saturation in the C-H-O-N system, using an Internally Heated Pressure Vessel (IHPV) and Piston Cylinder (PC) in the pressure range 0.8 kbar to 10 kbar, temperature between 1200 and 1300 ◦C, and a wide range of fO2 conditions from IW + 4.9 to IW-4.7 (IW standing for the Iron-Wustite redox buffer). The nitrogen concentration in the quenched silicate melts at fluid saturation was analysed by secondary ion mass spectrometry (SIMS), and the speciation of the dissolved C-O-H species was determined by Fourier transform infrared spectroscopy (FTIR). We identified two nitrogen species in the silicate melt : N2 dominating at fO2 > IW and N3- at lower fO2. Using these data and a database constraining nitrogen concentration at fluid saturation from 1 bar to 10 kbar pressure, we calibrated a solubility law for nitrogen in basalts defining its P-T-fO2 dependences. This model expands the model of Libourel et al. (2003) to high pressure and higher C-O-H activities. It can be used to investigate the nitrogen degassing processes for different pressure, temperature and fO2 conditions relevant to planetary accretion and modern volcanism.
Mots-clés : cosmo magma
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Bouden, N., J. Villeneuve, Y. Marrocchi, E. Deloule, E. Füri, A. Gurenko, L. Piani, E. Thomassot, P. Peres, and F. Fernandes. "Triple oxygen isotope measurements by multi-collector secondary ion mass spectrometry." Frontiers in Earth Science (2021): 8:601169.
Résumé : Secondary ion mass spectrometry (SIMS) is a powerful technique for in situ triple oxygen isotope measurements that has been used for more than 30 years. Since pioneering works performed on small-radius ion microprobes in the mid-80s, tremendous progress has been made in terms of analytical precision, spatial resolution and analysis duration. In this respect, the emergence in the mid-90s of the large-radius ion microprobe equipped with a multi-collector system (MC-SIMS) was a game changer. Further developments achieved on CAMECA MC-SIMS since then (e.g., stability of the electronics, enhanced transmission of secondary ions, automatic centering of the secondary ion beam, enhanced control of the magnetic field, 1012Ω resistor for the Faraday cup amplifiers) allow nowadays to routinely measure oxygen isotopic ratios (18O/16O and 17O/16O) in various matrices with a precision (internal error and reproducibility) better than 0.5‰ (2σ), a spatial resolution smaller than 10 μm and in a few minutes per analysis. This paper focuses on the application of the MC-SIMS technique to the in situ monitoring of mass-independent triple oxygen isotope variations.
Mots-clés : cosmo
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Boulliung, J., C. Dalou, L. Tissandier, E. Füri, and Y. Marrocchi. "Nitrogen diffusion in silicate melts under reducing conditions." American Mineralogist 106 (2021): 662–666.
Résumé : The behavior of nitrogen during magmatic degassing and the potential kinetic fractionation between N and other volatile species (H, C, O, noble gases) are poorly known due to the paucity of N diffusion data in silicate melts. To better constrain N mobility during magmatic processes, we investigated N diffusion in silicate melts under reducing conditions. We developed uniaxial diffusion experiments at 1 atm, 1425 °C, and under nominally anhydrous reducing conditions (fO2 ≤ IW-5.1, where IW is oxygen fugacity, fO2, reported in log units relative to the iron-wüstite buffer), in which N was chemically dissolved in silicate melts as nitride (N3–). Although several experimental designs were tested (platinum, amorphous graphite, and compacted graphite crucibles), only N diffusion experiments at IW-8 in compacted graphite crucibles for simplified basaltic andesite melts were successful. Measured N diffusivity (DN) is on the order of 5.3 ± 1.5 × 10–12 m2 s–1, two orders of magnitude lower than N chemical diffusion in soda-lime silicate melts (Frischat et al. 1978). This difference suggests that nitride diffusivity increases with an increasing degree of melt depolymerization. The dependence of N3– diffusion on melt composition is greater than that of Ar. Furthermore, N3– diffusion in basaltic-andesitic melts is significantly slower than that of Ar in similarly polymerized andesitic-tholeiitic melts at magmatic temperatures (1400–1450 °C ; Nowak et al. 2004). This implies that N/Ar ratios can be fractionated during reducing magmatic processes, such as during early Earth’s magma ocean stages.
Mots-clés : cosmo magma
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Deng, Z., M. Chaussidon, D. S. Ebel, J. Villeneuve, J. Moureau, and F. Moynier. "Simultaneous determination of mass-dependent Mg isotopic variations and radiogenic 26Mg by laser ablation-MC-ICP-MS and implications for the formation of chondrules." Geochimica et Cosmochimica Acta 299 (2021): 163–183.
Résumé : Improvements in our understanding of the formation of chondrules requires a better knowledge of the thermal histories and the nature of their solid precursors. We present an in situ nanosecond laser ablation multi-collector inductively-coupled-plasma mass-spectrometry (LA-MC-ICP-MS) technique to measure simultaneously mass-dependent Mg isotopic fractionations and radiogenic 26Mg in chondritic components, thus allowing us to investigate within a chronological framework the thermal processes redistributing Mg in chondrules and their precursors. The internal 26Al-26Mg isochrons provide initial 26Al/27Al ratios from 5.46 (± 0.38) × 10−5 to 6.14 (± 0.92) × 10−5 for amoeboid olivine aggregates (AOAs) and Ca-, Al-rich inclusions (CAIs), and from 0.16 (± 0.08) × 10−5 to 1.87 (± 0.92) × 10−5 for chondrules from Allende and Leoville chondrites, which are consistent with the previously reported values. The combination of these values with up to 2.5‰ variation of the 25Mg/24Mg ratio within the studied chondrules shows that: (i) AOAs and the precursors of chondrules were likely formed via condensation of rapid-cooling gas reservoirs, and (ii) Mg stable isotopes are probably at disequilibrium between olivines and mesostases in some chondrules, likely due to Mg loss by vaporization during chondrule formation. We use these new observations to propose that Mg isotopes can likely serve as a tracer for the thermal histories of chondrules. We present here a scenario taking into account Mg loss by vaporization from chondrule melt and Mg gain into the melt by olivine dissolution. The existing Mg isotopic observations in chondrule melts and olivines can be explained in a scenario with a homogeneous distribution of Mg isotopes and initial 26Al in the accretion disk, provided that chondrule precursors have been heated up to sufficiently high peak temperatures (up to 2123 K) and stayed above 1800 K for several tens of minutes to allow for significant Mg evaporation. These conditions are most consistent with a shock wave model for the origin of chondrules.
Mots-clés : cosmo
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Florin, G., B. Luais, O. Alard, and T. Rushmer. "Condensation and evaporation processes during CB chondrite formation : Insights from Ge isotopes and highly siderophile element abundances." Meteoritics & Planetary Science (2021): 1–21.
Résumé : We analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CBa), Gujba (CBa), and HaH 237 (CBb). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ74/70Ge mass-dependent isotopic values of 0.99 +- 0.30‰ (CBa) and −0.65 +- 0.10‰ (CBb). Based on our observations of these three samples, the isotopic compositions of metal in CBa chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CBb chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ74/70Ge and [Ge] from core to rim. We suggest that these δ74Ge zonations result from nearequilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium ; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge ; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes ; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims.
Mots-clés : cosmo
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Gaillard, F., M. A. Bouhifd, E. Füri, V. Malavergne, Y. Marrocchi, L. Noack, G. Ortenzi, M. Roskosz, and S. Vulpius. "The diverse planetary ingassing/outgassing paths produced over billions of years of magmatic activity." Space Science Reviews 217, no. 22 (2021).
Résumé : The C-H-O-N-S elements that constitute the outgassed atmosphere and exosphere
have likely been delivered by chondritic materials to the Earth during planetary accretion and subsequently processed over billions of years of planetary differentiation. Although these elements are generally considered to be volatile, a large part of the accreted C-HO-N-S on Earth must have been sequestered in the core and mantle, with the remaining part concentrated at the Earth’s surface (exosphere: atmosphere+ocean+crust). The likely reason for this is that, depending on the prevailing pressure (P), temperature (T) and oxidation state (oxygen fugacity, fO2) in the planet’s interior, the C-H-O-N-S elements can behave as siderophile, lithophile, refractory, magmatophile, or atmophile. It is not clear if these elements might be sequestered in the interiors of planets elsewhere, since the governing parameters of P-T-fO2 during the diverse magmatic processes controlling magmatic differentiation vary greatly over time and from planet to planet. The magma ocean outgassed the first atmosphere, which was probably also the largest in terms of mass, but its nature and composition remain poorly known. Meanwhile, a significant, but unknown, part
Mots-clés : cosmo magma
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Grewal, D. S., R. Dasgupta, and B. Marty. "A very early origin of isotopically distinct nitrogen in inner Solar System protoplanets." Nature Astronomy (2021).
Résumé : Understanding the origin of life-essential volatiles such as nitrogen (N) in the Solar System and beyond is critical to evaluate the potential habitability of rocky planets1–5. Whether the inner Solar System planets accreted these volatiles from their inception or had an exogenous delivery from the outer Solar System is, however, not well understood. Using previously published data of nucleosynthetic anomalies of nickel, molybdenum, tungsten and ruthenium in iron meteorites along with their 15N/14N ratios, here we show that the earliest formed protoplanets
in the inner and outer protoplanetary disk accreted isotopically distinct N. While the Sun and Jupiter captured N from nebular gas6, concomitantly growing protoplanets in the inner and outer disk possibly sourced their N from organics
and/or dust—with each reservoir having a different N isotopic composition. A distinct N isotopic signature of the inner Solar System protoplanets coupled with their rapid accretion7,8 suggests that non-nebular, isotopically processed N
was ubiquitous in their growth zone between 0 and ~0.3 Myr after Solar System formation. Because the 15N/14N ratio of the bulk silicate Earth falls between that of the inner and outer Solar System reservoirs, we infer that N in the present-day
rocky planets represents a mixture of both inner and outer Solar System material.
Mots-clés : cosmo
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Jacquet, E., M. Piralla, P. Kersaho, and Y. Marrocchi. "Origin of isolated olivine grains in carbonaceous chondrites." Meteoritics & Planetary Science 56, no. 1 (2021): 13–33.
Résumé : We report microscopic, cathodoluminescence, chemical, and O isotopic measurements of FeO-poor isolated olivine grains (IOG) in the carbonaceous chondrites Allende (CV3), Northwest Africa 5958 (C2-ung), Northwest Africa 11086 (CM2-an), and Allan Hills 77307 (CO3.0). The general petrographic, chemical, and isotopic similarity with bona fide type I chondrules confirms that the IOG derived from them. The concentric CL zoning, reflecting a decrease in refractory elements toward the margins, and frequent rimming by enstatite are taken as evidence of interaction of the IOG with the gas as standalone objects. This indicates that they were splashed out of chondrules when these were still partially molten. CaO-rich refractory forsterites, which are restricted to Δ17O <-4°/°° likely escaped equilibration at lower temperatures because of their large size and possibly quicker quenching. The IOG thus bear witness to frequent collisions in the chondrule-forming regions.
Mots-clés : cosmo
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Marrocchi, Y., G. Avice, and J. A. Barrat. "The Tarda meteorite: A window into the formation of D-type asteroids." Tha Astrophysical Journal Letters 913, no. L9 (2021).
Résumé : Dynamic models of solar system evolution suggest that D-type asteroids formed beyond Saturnʼs orbit and represent invaluable witnesses of the prevailing conditions in the outer solar system. Here, we report a comprehensive petrographic and isotopic characterization of the carbonaceous chondrite Tarda, a recent fall recovered in the Moroccan Sahara. We show that Tarda shares strong similarities with the D-type-derived chondrite Tagish Lake, implying that Tarda represents a rare sample of D-type asteroids. Both Tarda and Tagish Lake are characterized by the presence of rare 16O-rich chondrules and chondrule fragments, high C/H ratios, and enrichments in deuterium, 15N, and 13C. By combining our results with literature data on carbonaceous chondrites related to C-type asteroids, we show that the outer solar system 4.56 Gy ago was characterized by largescale oxygen isotopic homogeneities in (i) the water–ice grains accreted by asteroids and (ii) the gas controlling the formation of FeO-poor chondrules. Conversely, the zone in which D-type asteroids accreted was significantly enriched in deuterium relative to the formation regions of C-type asteroids, features likely inherited from unprocessed, D-rich, molecular-cloud materials.
Mots-clés : cosmo
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Piani, L., Y. Marrocchi, L. G. Vacher, and H. Hurimoto. "Origin of hydrogen isotopic variations in chondritic water and organics." Earth and Planetary Science Letters (2021): 117008.
Résumé : Chondrites are rocky fragments of asteroids that formed at different times and heliocentric distances in the early solar system. Most chondrite groups contain water-bearing minerals, attesting that both water-ice and dust were accreted on their parent asteroids. Nonetheless, the hydrogen isotopic composition (D/H) of water in the different chondrite groups remains poorly constrained, due to the intimate mixture of hydrated minerals and organic compounds, the other main H-bearing phase in chondrites. Building on our recent works using in situsecondary ion mass spectrometry analyses, we determined the H isotopic composition of water in a large set of chondritic samples (CI, CM, CO, CR, and C-ungrouped carbonaceous chondrites) and report that water in each group shows a distinct and unique D/H signature. Based on a comparison with literature data on bulk chondrites and their water and organics, our data do not support a preponderant role of parent-body processes in controlling the D/H variations among chondrites. Instead, we propose that the water and organic D/H signatures were mostly shaped by interactions between the protoplanetary disk and the molecular cloud that episodically fed the disk over several million years. Because the preservation of D-rich interstellar water and/or organics in chondritic materials is only possible below their respective sublimation temperatures (160 and 350–450 K), the H isotopic signatures of chondritic materials depend on both the timing and location at which their parent body formed.
Mots-clés : cosmo
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Piralla, M., R. Tartèse, Y. Marrocchi, and K. H. Joy. "Apatite halogen and hydrogen isotope constraints on the conditions of hydrothermal alteration in carbonaceous chondrites." Meteoritics & Planetary Science 56, no. 4 (2021): 809–828.
Résumé : Apatite has been widely used for assessing the volatile inventory and hydrothermal fluid compositions of asteroidal and planetary bodies. We report the OH, F, and Cl abundances, as well as the hydrogen isotope composition, of apatite in the CM1-2 chondrite Boriskino and in the C1-ungrouped Bench Crater meteorite. Apatite in both meteorites is halogen-poor, close to the hydroxylapatite endmember composition, and characterized by average δDSMOW values of −226 +-59% and 233 +-92%, respectively. Compared to apatite, the matrix in Bench Crater is depleted in D with a δDSMOW value of −16 +-119‰. Comparing apatite and water H isotope compositions yields similar apatite-water D/H fractionation ΔDApatite-Water of approximately 120–150‰ for both chondrites, suggesting that apatite formed at similar temperatures. Combining a lattice strain partitioning model with apatite F and Cl abundances in Boriskino and Bench Crater yields low F and Cl abundances <300 μg g−1 in apatite-forming fluids, and fluid F/Cl ratios that are roughly consistent with the bulk F/Cl ratios of other CI and CM chondrites. This suggests that hydrothermal alteration on these meteorite parent bodies took place under closed-system conditions. Based on the OH abundance estimates for the apatite-forming fluids, we estimated the pH values of alteration fluids to be of approximately 10–13. Such alkaline fluid compositions are consistent with previous modeling and suggest that apatite formed late, toward the end of completion of hydrothermal alteration processes on the Boriskino and Bench Crater parent bodies
Mots-clés : cosmo
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2020 |
Aerts, J. W., A. Riedo, D. J. Melton, S. Martini, J. Flahaut, U. J. Meierhenrich, C. Meinert, L. Myrgorodska, R. Lindner, and P. Ehrenfreud. "Biosignature analysis of Mars soil analogs from the Atacama Desert : Challenges and implications for future missions to Mars." Astrobiology 20, no. 6 (2020).
Résumé : The detection of biosignatures on Mars is of outstanding interest in the current field of astrobiology and drives various fields of research, ranging from new sample collection strategies to the development of more sensitive detection techniques. Detailed analysis of the organic content in Mars analog materials collected from extreme environments on Earth improves the current understanding of biosignature preservation and detection under conditions similar to those of Mars. In this article, we examined the biological fingerprint of several locations in the Atacama Desert (Chile), which include different wet and dry, and intermediate to high elevation salt flats (also named salars). Liquid chromatography and multidimensional gas chromatography mass spectrometry measurement techniques were used for the detection and analysis of amino acids extracted from the salt crusts and sediments by using sophisticated extraction procedures. Illumina 16S amplicon sequencing was used for the identification of microbial communities associated with the different sample locations. Although amino acid load and organic carbon and nitrogen quantities were generally low, it was found that most of the samples harbored complex and versatile microbial communities, which were dominated by (extremely) halophilic microorganisms (most notably by species of the Archaeal family Halobacteriaceae). The dominance of salts (i.e., halites and sulfates) in the investigated samples leaves its mark on the composition of the microbial communities but does not appear to hinder the potential of life to flourish since it can clearly adapt to the higher concentrations. Although the Atacama Desert is one of the driest and harshest environments on Earth, it is shown that there are still sub-locations where life is able to maintain a foothold, and, as such, salt flats could be considered as interesting targets for future life exploration missions on Mars.
Mots-clés : cosmo
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Anzures, B. A., S. W. Parman, R. E. Milliken, O. Namur, C. Cartier, and S. Wang. "Effect of sulfur speciation on chemical and physical properties of very reduced mercurian melts." Geochimica et Cosmochimica Acta 286 (2020): 1–18.
Résumé : The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5–4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated fO2 between IW-3 and IW-7). Due to decreasing O availability at these low fO2 conditions, and an abundance of S2-, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250–1475 °C and fO2 between IW-2.5 to IW-7.5. SK-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As fO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ˜IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ˜IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low fO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low fO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.
Mots-clés : cosmo
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Avice, G., and B. Marty. "Perspectives on atmospheric evolution from noble gas and nitrogen isotopes on Earth, Mars & Venus." Space Science Reviews 126 (2020): 36.
Résumé : The composition of an atmosphere has integrated the geological history of the entire planetary body. However, the long-term evolutions of the atmospheres of the terrestrial planets are not well documented. For Earth, there were until recently only few direct records of atmosphere’s composition in the distant past, and insights came mainly from geochemical or physical proxies and/or from atmospheric models pushed back in time. Here we review innovative approaches on new terrestrial samples that led to the determination of the elemental and isotopic compositions of key geochemical tracers, namely noble gases and nitrogen. Such approaches allowed one to investigate the atmosphere’s evolution through geological period of time, and to set stringent constraints on the past atmospheric pressure and on the salinity of the Archean oceans. For Mars, we review the current state of knowledge obtained from analyses of Martian meteorites, and from the direct measurements of the composition of the present-day atmosphere by rovers and spacecrafts. Based on these measurements, we explore divergent models of the Martian and Terrestrial atmospheric evolutions. For Venus, only little is known, evidencing the critical need for dedicated missions.
Mots-clés : cosmo
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Barosch, J., D. C. Hezel, Y. Marrocchi, A. Gurenko, and C. Lenting. "An unusual compound object in Yamato 793408 (H3.2-an) : The missing link between compound chondrules and macrochondrules ?" Meteoritics & Planetary Science 55 (2020): 1458–1470.
Résumé : We found a large ( 2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no 16O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s−1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.
Mots-clés : cosmo
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Barosch, J., D. C. Hezel, L. Sawatzki, L. Halbauer, and Y. Marrocchi. "Sectioning effects of porphyritic chondrules : Implications for the PP/POP/PO classification and correcting modal abundances of mineralogically zoned chondrules." Meteoritics & Planetary Science 55, no. 5 (2020): 993–999.
Résumé : Mineralogically zoned chondrules are a common chondrule type in chondrites. They consist of olivine cores, surrounded by low-Ca pyroxene rims. By serial sectioning porphyritic chondrules from carbonaceous, ordinary, and enstatite chondrites, we demonstrate that the 2-D textural appearances of these chondrules largely depend on where they are cut. The same chondrule may appear as a porphyritic pyroxene (PP) chondrule when sectioned through the low-Ca pyroxene rim, and as a porphyritic olivine-pyroxene (POP) or porphyritic olivine (PO) chondrule when sectioned close or through its equator. Chondrules previously classified into PP/POP/PO chondrules might therefore not represent different types, but various sections through mineralogically zoned chondrules. Classifying chondrule textures into PP, POP, and PO has therefore no unequivocal genetic meaning, it is merely descriptive. Sectioning effects further introduce a systematic bias when determining mineralogically zoned chondrule fractions from 2-D sections. We determined correction factors to estimate 3-D mineralogically zoned chondrule fractions when these have been determined in 2-D sections : 1.24 for carbonaceous chondrites, 1.29 for ordinary chondrites, and 1.62 for enstatite chondrites. Using these factors then shows that mineralogically zoned chondrules are the dominant chondrule type in chondrites with estimated 3-D fractions of 92% in CC, 52% in OC, and 46% in EC.
Mots-clés : cosmo
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Bekaert, D. V., M. W. Broadley, and B. Marty. "The origin and fate of volatile elements on Earth revisited in light of noble gas data obtained from comet 67P/Churyumov-Gerasimenko." Scientific Reports 10 (2020): 5796.
Résumé : The origin of terrestrial volatiles remains one of the most puzzling questions in planetary sciences. The timing and composition of chondritic and cometary deliveries to Earth has remained enigmatic due to the paucity of reliable measurements of cometary material. This work uses recently measured volatile elemental ratios and noble gas isotope data from comet 67P/Churyumov-Gerasimenko (67P/C-G), in combination with chondritic data from the literature, to reconstruct the composition of Earth’s ancient atmosphere. Comets are found to have contributed ~20% of atmospheric heavy noble gases (i.e., Kr and Xe) but limited amounts of other volatile elements (water, halogens and likely organic materials) to Earth. These cometary noble gases were likely mixed with chondritic – and not solar – sources to form the atmosphere. We show that an ancient atmosphere composed of chondritic and cometary volatiles is more enriched in Xe relative to the modern atmosphere, requiring that 8–12 times the present-day inventory of Xe was lost to space. This potentially resolves the long-standing mystery of Earth’s “missing xenon”, with regards to both Xe elemental depletion and isotopic fractionation in the atmosphere. The inferred Kr/H2O and Xe/H2O of the initial atmosphere suggest that Earth’s surface volatiles might not have been fully delivered by the late accretion of volatile-rich carbonaceous chondrites. Instead, “dry” materials akin to enstatite chondrites potentially constituted a significant source of chondritic volatiles now residing on the Earth’s surface. We outline the working hypotheses, implications and limitations of this model in the last section of this contribution
Mots-clés : cosmo
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Bermingham, K. R., E. Füri, K. Lodders, and B. Marty. "The NC-CC isotope dichotomy : Implications for the chemical and isotopic evolution of the early solar system." Space Science Reviews 216, no. 8 (2020): 1–29.
Résumé : Understanding the formation of our planetary system requires identification of the materials from which it originated and the accretion processes that produced the planets. The compositional evolution of the solar system can be constrained by synthesizing astronomical datasets and numerical models with elemental and isotopic compositions from objects that directly sampled the disk : meteorites and their constituents (chondrules, refractory inclusions, and matrix). This contribution reviews constraints on early solar system evolution provided by the so-called non-carbonaceous (NC) and carbonaceous chondrite (CC) groups and their relationship to the volatile element characteristics of chondritic meteorites. In previous work, the NC or CC character of a parent body was used to infer its accretion location in the protoplanetary disk. The NC groups purportedly originated in the inner disk, and the CC groups were derived from the outer disk, where the NC and CC regions of the disk may have been separated early on by proto-Jupiter, a pressure maximum, or a dust trap in the disk. The tenet that all CC parent bodies accreted in the outer disk is, in part, based on evidence that a handful of CC meteorites are enriched in volatile species compared to NC meteorites. Here, it is reviewed if and how the volatile element and nucleosynthetic isotope compositions of meteorites can be linked to accretion locations within the disk. The nucleosynthetic isotope compositions of whole rock meteorite samples contrast the trends found for their major volatile element compositions (i.e., C, N, and O). Although there may be an increase in volatile abundances when comparing some stony NC and CC meteorites and their inferred accretion locations within the disk, this is not necessarily a general rule. The difficulties with inferring parent body accretion locations are discussed. It is found that it cannot always be assumed that parent bodies which formed in the CC reservoir are “volatile-rich” relative to those that formed in the NC reservoir which are “volatile-poor”. Consequently, tracing the origin of terrestrial volatiles using the NC-CC isotope dichotomy remains challenging
Mots-clés : cosmo
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Boulliung, J., E. Füri, C. Dalou, L. Tissandier, L. Zimmermann, and Y. Marrocchi. "Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts." Geochimica et Cosmochimica Acta 284 (2020): 120–133.
Résumé : Knowledge of N solubility in silicate melts is key for understanding the origin of terrestrial N and the distribution andexchanges of N between the atmosphere, the silicate magma ocean, and the core forming metal. To place constraints onthe incorporation mechanism(s) of N in silicate melts, we investigated the effect of the oxygen fugacity (fO2) and melt com-position on the N solubility through N equilibration experiments at atmospheric pressure and high temperature (1425°C).Oxygen fugacity (expressed in log units relative to the iron-wu ̈stite buffer, IW) was varied from IW –8 to IW +4.1, and meltcompositions covered a wide range of polymerization degrees, defined by the NBO/T ratio (the number of non-bridging oxy-gen atoms per tetrahedrally coordinated cations). The N contents of the quenched run products (silicate glasses) were ana-lyzed byin-situsecondary ion mass spectrometry and bulk CO2laser extraction static mass spectrometry, yielding resultsthat are in excellent agreement even for N concentrations at the (sub-)ppm level. The data obtained here highlight the fun-damental control offO2and the degree of polymerization of the silicate melt on N solubility. Under highly reduced conditions(fO2= IW –8), the N solubility increased with increasing NBO/T from 17.4 ± 0.4 ppm.atm-1/2in highly polymerized melts(NBO/T = 0) to 6710 ± 102 ppm.atm-1/2in depolymerized melts (NBO/T˜2.0). In contrast, under less reducing conditions(fO2> IW –3.4), N solubility is very low (≤2 ppm.atm-1/2), irrespective of the NBO/T value. Our results provide constraintson N solubility in enstatite chondrite melts and in the shallow part of a planetary magma ocean. The nitrogen storage capacityof an enstatite chondrite melt, which may approximate that of planetesimals that accreted and melted early in the inner SolarSystem, varies between ˜60 and ˜6000 ppm at IW –5.1 and IW –8, respectively. In contrast, a mafic to ultra-mafic magmaocean could have incorporated ˜0.3 ppm to ˜35 ppm N under thefO2conditions inferred for the young Earth (i.e., IW –5 toIW). The N storage capacity of a reduced magma ocean (i.e., IW –3.4 to IW) in equilibrium with a N-rich atmosphere is ≤1 ppm, comparable to the N content of the present-day mantle. However under more reducing conditions (i.e., IW –5 toIW –4), the N storage capacity is significantly higher (˜35 ppm) ; in this case, Earth would have lost N to the atmosphereand/or N would have been transported into and stored within its deep interior (i.e., deep mantle, core).
Mots-clés : cosmo
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Broadley, M. W., D. V. Bekaert, B. Marty, A. Yamaguchi, and J. A. Barrat. "Noble gas variations in ureilites and their implications for ureilite parent body formation." Geochimica et Cosmochimica Acta 270 (2020): 325–337.
Résumé : Ureilites are equilibrated carbon-rich olivine-pyroxene rocks from the partially melted mantle of a large (>500 km diameter) heterogeneous parent body. Recently the ureilite parent body was interpreted as an incomplete mixture of material from two carbon-rich chondritic reservoirs, one (Mg-rich) with reduced iron, low Δ17O and low δ13C, and the other with oxidised iron, high Δ17O and high δ13C. Here we analyse noble gases (Ar, Kr and Xe) in six equilibrated (unbrecciated) ureilites from Northwest Africa (NWA 2236, NWA 7686, NWA 8049, NWA 8172, NWA 11032 and NWA 11368). We observe weak positive and negative correlations of Δ17O and Mg# with the elemental ratios of Ar/Xe and Kr/Xe, respectively, as well as a weak positive correlation of Mg# with the heavy isotopes of Xe. These correlations broadly support the idea of the two-component mixing hypothesis. Our analyses further suggest that the Mg-rich endmember was rich in Xe from presolar grains (HL-Xe) while the Mg-poorer component may have contained solar-derived noble gases. The observed correlations are less straightforward to reconcile with a recent model for the origin of the ureilite parent body, involving oxidation of metal by H2O from accreted ice with ‘heavy’ oxygen isotopes.
Mots-clés : cosmo
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Cosmochemistry
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