2022 |
Dalou, C., C. Deligny, and E. Füri. "Nitrogen isotope fractionation during magma ocean degassing : tracing the composition of early Earth’s atmosphere." Geochemical Perspectives Letters 20 (2022): 27–31.
Résumé : The evolution of the nitrogen concentration and isotopic composition during the degassing of Earth’s magma ocean, and thus in the primitive atmosphere, is key to understanding how habitable conditions developed on Earth. To constrain nitrogen degassing from the magma ocean, we determined the variations of theNcontent at N2 gas saturation, N speciation, and N isotopic composition of a magma ocean analogue (basaltic komatiite) at oxygen fugacities (fO2) from IW−4.2 to IW (where IW is the logarithmic difference between experimental fO2 and that at Fe-FeO equilibrium). We performed a series of N degassing experiments in a piston cylinder at 1.5 GPa and 1550 °C in pure forsterite capsules. N concentrations in the mafic silicate melts decreased from 13,481 ± 735 ppm under the most reducing conditions to 798 ± 4 ppm at IW, controlled by N speciation (as determined by Raman spectroscopy), which changed from nitride (±N-H complexes) to molecular N2 with increasing fO2. Nitrogen occurs solely as N2 in the degassed gas, regardless of fO2. Nitrogen isotopic compositions (as determined by secondary ion mass spectroscopy) became significantly lighter in the degassed melt (quenched glass), down to −41 ± 13‰relative to the initial composition (measured in an undegassed sample), following open system degassing trends (variable with fO2 conditions), indicative of Rayleigh fractionation. These findings imply that an atmosphere in equilibrium with a reduced magma ocean would be N-depleted, whereas with increasing magma ocean fO2 conditions, the primitive atmosphere would have become more enriched in N2 gas.
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 (2022).
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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Grocolas, T., P. Bouilhol, G. Caro, and S. Mojzsis. "Eoarchean subduction-like magmatism recorded in 3750 Ma mafic–ultramafic rocks of the Ukaliq supracrustal belt (Québec)." Contributions to Mineralogy and Petrology 177, no. 39 (2022).
Résumé : Our understanding of the nature of crustal formation in the Eoarchean is limited by the scarcity and poor preservation of the oldest rocks and variable and imperfect preservation of protolith magmatic signatures. These limitations hamper our ability to place quantitative constraints on thermomechanical models for early crustal genesis and hence on the operative geodynamic regimes at that time. The recently discovered ca. 3.75 Ga Ukaliq supracrustal enclave (northern Québec) is mainly composed of variably deformed and compositionally diverse serpentinized ultramafic rocks and amphibolitized mafic schists whose metamorphic peak, inferred from phase equilibria modeling, was below 720 °C. Inferred protoliths to the Ukaliq ultramafic rocks include cumulative dunites, pyroxenites, and gabbros, whereas the mafic rocks were probably picrites, basalts, and basaltic andesites. The bulk-rock and mineral chemistry documents the partial preservation of cumulative pyroxenes and probably amphiboles and demonstrates the occurrence of a clinopyroxene-dominated, tholeiitic suite and an orthopyroxene-dominated, boninite-like suite. Together with the presence of negative μ142Nd anomalies in the boninitic basalts, two liquid lines of descent are inferred : (i) a damp tholeiitic sequence resulting from the fractionation of a basaltic liquid produced by mantle decompression ; and (ii) a boninitic suite documenting the evolution of an initially primitive basaltic andesite liquid produced by flux melting. Petrographic observations, thermodynamic modeling, bulk-rock and mineral chemistry, and 142Nd isotopic compositions identify the Ukaliq supracrustal belt as the remnant of an Eoarchean arc crust produced by the recycling of Hadean crust in a similar way as modern-style subduction.
Mots-clés : cosmo magma
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Hammouda, T., M. Boyet, P. Frossard, and C. Cartier. "The message of oldhamites from enstatite chondrites." Progress in Earth and Planetary Science 9, no. 13 (2022).
Résumé : We have determined rare-earth element (REE) abundances in oldhamites (CaS) from 13 unequilibrated and equilibrated enstatite chondrites (5 EH and 8 EL) and in a few enstatites by in situ, laser ablation ICP-MS. In EH chondrites, oldhamite REE patterns vary from the most primitive petrographic types (EH3) to the most metamorphosed types (EH5). In EH3, CI-normalized REE patterns are convex downward with strong positive Eu and Yb anomalies, whereas EH5 display flat patterns with enrichments reaching about 80 times CI abundances. The positive anomalies of Eu and Yb found in oldhamites of primitive EH chondrites indicate that they represent the condensation of a residual gas fraction, in a manner similar to fine-grained CAIs of carbonaceous chondrites. The early condensate may have been preserved in the matrix of unequilibrated EH. Equilibrated EH oldhamite patterns may result from metamorphic evolution and REE redistribution on the EH parent body. On the contrary, all the oldhamites from EL chondrites (EL3 to EL6) display a single kind of patterns, which is convex upward and is about 100 times enriched relative to CI, with a negative Eu anomaly. In addition, the EL pattern is similar to that of oldhamites from aubrites (enstatite achondrites). The latter observation suggests that oldhamites of all EL metamorphic types (including primitive ones) bear the signature of a magmatic event accompanied by FeS loss as vapor, prior to the assembly of the EL parent body. Given the difficulty of obtaining precise ages on enstatite chondrites, it is not possible to discuss the chronology of the events recorded by the oldhamite REE patterns.
Mots-clés : cosmo
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Ito, G., J. Flahaut, O. Gonzalez-Maurel, B. Godoy, V. Payet, and M. Barthez. "Remote sensing survey of Altiplano-Puna volcanic complex rocks and minerals for planetary analog use." Remote Sensing 14, no. 9 (2022): 2081.
Résumé : The Altiplano-Puna Volcanic Complex (APVC) of the Central Andes is an arid region with extensive volcanism, possessing various geological features comparable to those of other solar system objects. The unique features of the APVC, e.g., hydrothermal fields and evaporite salars, have been used as planetary analogs before, but the complexity of the APVC presents a wealth of opportunities for more analog studies that have not been exploited previously. Motivated by the potential of using the APVC as an analog of the volcanic terrains of solar system objects, we mapped the mineralogy and silica content of the APVC up to 100,000 km2 in northern Chile based on a combination of remote sensing data resembling those of the Moon and Mars. The band ratio indices of Landsat 8 Operational Land Imager multispectral images and mineral classifications based on spectral hourglass approach using Earth Observing-1 Hyperion hyperspectral images (both in the visible to shortwave infrared wavelengths) were used to map iron-bearing and alteration minerals. We also used Hyperion imagery to detect feldspar spectral signatures and demonstrated that feldspar minerals can be detected on non-anorthosites, which may influence interpretations of feldspar spectral signatures on Mars. From the Terra Advanced Spaceborne Thermal Emission and Reflection Radiometer Global Emissivity Dataset, we derived the silica percentage of non-evaporite rocks within errors of approximately 2–3 wt.% SiO2 for those in the 60–70 wt.% range (about 8 wt.% errors for the 50–60 wt.% range). Based on an integrated assessment of the three datasets, we highlighted three regions of particular interest worthy of further field investigation. We also evaluated the benefits and limitations of all three remote sensing methods for mapping key minerals and capturing rock diversity, based on available samples and existing geological maps.
Mots-clés : cosmo
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Maltese, A., G. Caro, O. P. Pandey, D. Upadhyay, and K. Mezger. "Direct evidence for crust-mantle differentiation in the late Hadean." Communications Earth & Environment 3, no. 12 (2022).
Résumé : Constraints on the evolution of the silicate Earth between 4.5 and 3.8 billion years ago are limited by the scarcity of pristine geological material from that period. The geodynamic evolution of the early Earth, prior to the preserved rock record, is thus mainly inferred from numerical modelling. To evaluate the geological significance of these simulations, geochronological constraints pertaining to the evolution of the Hadean crust are required. Here we show using Neodymium isotope variations generated by decay of now-extinct 146-Samarium that Paleoarchean rocks from the Singhbhum Craton, India derived from a Hadean depleted mantle reservoir that differentiated 4.19+0.06−0.12 billion years ago. The event postdates Neodymium model ages of mantle depletion inferred from other Archean rocks by 200 million years. This geochronological record is mirrored in the Hafnium isotope composition of the oldest zircon grains, suggesting that Hadean mantle differentiation proceeded via distinct pulses of large-scale magmatic activity and crustal rejuvenation.
Mots-clés : cosmo
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Pinto, G., Y. Marrocchi, E. Jacquet, and F. Olivares. "Formation of chondrule fine-grained rims from local nebular reservoirs." Meteoritics & Planetary Science 57, no. 5 (2022): 1004–1017.
Résumé : Chondrules are commonly surrounded by fine-grained rims (FGRs) whose origin remains highly debated ; both nebular and parent body settings are generally proposed. Deciphering their origin, however, is of fundamental importance as they could clarify the matrix–chondrule relationship and thus constrain the formation and transport conditions of chondrules in the circumsolar disk. Here, we report a systematic survey of FGRs in CO, CM, CV, and CR chondrites ; we compare (i) the thickness of FGRs to the size of their host chondrules and (ii) the frequency of FGRs to the modal abundance of matrix in the respective host chondrites. Although FGRs show textural variations depending on the petrologic type of the considered chondrites, our data show a positive correlation between apparent rim thickness and the radius of the host chondrule in all chondrite groups. We also found a positive correlation between the evaluated percentages of rimmed chondrules and the modal abundance of matrix material in the chondrites. We show that this relationship could not result from parent body processes, whether matrix compaction or FGR fragmentation. Therefore, we propose that FGRs were accreted under warm conditions at the end of chondrule-forming events. Our results thus support (i) a nebular origin for FGR, whose abundances are directly related to the abundance of available dust in regions of chondrite accretion ; and (ii) the accretion of chondrites from locally formed chondrules and matrix, suggesting limited radial transport in the protoplanetary disk.
Mots-clés : cosmo
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Regnault, M., Y. Marrocchi, M. Piralla, J. Villeneuve, V. Batanova, N. Schnuriger, and E. Jacquet. "Oxygen isotope systematics of chondrules in Rumuruti chondrites : Formation conditions and genetic link with ordinary chondrites." Meteoritics & Planetary Science 57, no. 1 (2022): 122–135.
Résumé : Rumurutiites (R chondrites) are rare, highly oxidized chondrites belonging to the noncarbonaceous superclan and characterized by low chondrule abundances. Although textural and chemical features of Rumurutiite chondrules resemble those of ordinary chondrites (OCs), their formation conditions and potential genetic link remain debated. Here, we report high-resolution elemental X-ray mapping analyses and in situ O isotopic measurements of olivine grains from five chondrules and eight isolated olivine grains (IOGs) in the NWA 12482 R3 chondrite. The chondrules show chemical zonings similar to their counterparts in ordinary and carbonaceous chondrites (CCs), implying that gas–melt interaction processes between chondrule precursors and SiO- and Mg-rich gas were operative throughout the circumsolar disk. Our isotopic data show that R chondrules are isotopically similar to ordinary chondrules, although differences in their abundances of relict olivine grains and chondrule textural characteristics suggest different formation environments, with R chondrules being formed from 16O-poorer precursors. As with chondrules in OCs, the O isotopic characteristics of R chondrules and IOGs suggest limited transport between CC and noncarbonaceous reservoirs.
Mots-clés : cosmo
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Schnuriger, N., C. Cartier, J. Villeneuve, V. Batanova, M. Regnault, and Y. Marrocchi. "Spinel in CV chondrules : Investigating precursor legacy and chondrule thermal histories." Meteoritics & Planetary Science 57, no. 5 (2022): 1018–1037.
Résumé : In carbonaceous chondrites, Mg-spinel (MgAl2O4) grains are ubiquitous in refractory inclusions but rarely reported in chondrules, where they may correspond to minerals either (i) inherited from chondrule precursors or (ii) crystallized from chondrule melts. Here, we report high-current quantitative electron microprobe measurements and secondary ion mass spectrometry oxygen isotopic analyses of Mg-spinel-bearing chondrules in the CV3 carbonaceous chondrites Northwest Africa 10235 and Allende. Compared to spinels in refractory inclusions, chondrule spinels are characterized by higher Cr contents and 16O-poorer oxygen isotopic signatures (Δ17O ≡ δ17O−0.52 × δ18O, from −2 to −6‰). Because the similar Δ17O values of chondrule olivine and spinel crystals imply their comagmatic origin, we applied a geothermometer based on the Al-Cr distribution between these minerals to determine their crystallization temperatures. The calculated temperatures range from 1200 to 1640 °C (mean = 1470 °C), most being lower than the estimated liquidus temperature of porphyritic chondrules ( 1600 °C). Our results suggest that chondrules experienced relatively slow cooling rates (slower than a few hundreds of °C h−1), which is in good agreement with models of chondrule formation invoking nonlinear or twostage cooling rates.
Mots-clés : cosmo
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Tachibana, S., H. Sawada, R. Okazaki, Y. Takano, K. Sakamoto, Y. N. Miura, C. Okamoto, H. Yano, S. Yamanouchi, P. Michel et al. "Pebbles and sand on asteroid (162173) Ryugu : In situ observation and particles returned to Earth." Science (2022).
Résumé : The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and sub-surface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 2020 December 6. We describe the morphology of >5 g of returned pebbles and sand. Their diverse color, shape and structure are consistent with the observed materials of Ryugu ; we conclude they are a representative sample of the asteroid
Mots-clés : cosmo
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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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Deligny, C., E. Füri, and E. Deloule. "Origin and timing of volatile delivery (N, H) to the angrite parent body : Constraints from in situ analyses of melt inclusions." Geochimica et Cosmochimica Acta 313 (2021): 243–256.
Résumé : Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites : D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D’Orbigny is characterized by d15N values ranging from 0 ± 25 to +56 ± 29‰and dD values between -348 ± 53 and -118 ± 31‰. This shows that the angrite mantle source sampled by D’Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first 4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).
Mots-clés : cosmo
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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., F. Bernadou, M. Roskosz, A. Bouhif, Y. Marrocchi, G. Iacono-Marziano, M. Moreira, B. Scaillet, and G. Rogerie. "Redox controls during magma ocean degassing." Earth and Planetary Science Letters 577 (2021): 117255.
Résumé : Nitrogen, carbon, hydrogen and sulfur are essential elements for life and comprise about 1% of terrestrial planet masses. These elements dominate planetary surfaces due to their volatile nature, but the Earth’s interior also constitutes a major C-H-N-S reservoir. Resolving the origin of the surficial versus deep volatile reservoirs requires the past 4.5 Giga-years of mantle outgassing and ingassing processes to be reconstructed, involving many unknowns. As an alternative, we propose to define the primordial distribution of volatiles resulting from degassing of the Earth’s magma ocean (MO). The equilibrium partitioning of C-H-O-N-S elements between the MO and its atmosphere is calculated by means of solubility laws, extrapolated to high temperatures and over a large range of redox conditions. Depending on the redox conditions, the amount of volatiles, and the size of the MO considered, we show that the last MO episode may have degassed 40-220 bar atmospheres, whereas hundreds to thousands of ppm of C-H-O-N-S can be retained in the magma. Two contrasting scenarios are investigated: reduced vs. oxidized MO. For reduced cases (<IW−2), an H-C ±N-rich atmosphere can be formed, whereas the atmosphere under oxidizing conditions (>IW +2) would be dry and C-N-S-rich. An intermediate redox state produces a C-N atmosphere. In many cases, the present-day surficial abundances (atmosphere +ocean +crust) of C and N, the most volatile elements, are very close to the calculated primordial MO -atmosphere distribution. This probably means that lithospheric recycling and post-magma ocean degassing only moderately alter the surficial abundances of these elements. Sulfur, in contrast, must have been mostly outgassed by post-MO events. Changes in redox conditions during magma ocean degassing played a first order role in the composition of the primordial atmosphere of planets. We suggest that the more oxidized conditions on Venus due to H-loss may have played a role in the growth of a dry MO atmosphere on this planet compared to an H-bearing one on Earth. To verify these first order assertions, constraints on volatile behaviorunder extreme magma ocean conditions and upon magma ocean solidification are urgently needed.
Mots-clés : cosmo
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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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Hewins, R. H., P. M. Zanetta, B. Zanda, C. Le Guillou, J. Gattacceca, C. Sognzoni, S. Pont, L. Piani, T. Rigaudier, H. Leroux et al. "NORTHWEST AFRICA (NWA) 12563 and ungrouped C2 chondrites : Alteration styles and relationships to asteroids." Geochimica et Cosmochimica Acta 311 (2021): 238–273.
Résumé : Many asteroids in the main belt have spectra like those of Mighei-type CM chondrites, but some Near Earth Objects (NEO) resemble less well known types of C2 chondrite. Northwest Africa (NWA) 12563, a new find with affinities to C2 chondrites, could help us understand the differences between observations of CM2 chondrites and bodies that are currently being studied by the Hayabusa2 and OSIRIS-REx space missions. NWA 12563 contains 14% chondrules supported by 86% fine grained matrix consistent with CM2 chondrites, but differs from them in other respects. In both matrix and chondrules, olivine is unaltered and pyroxene shows incipient alteration. Metal in chondrules is pseudomorphed by serpentine, and mesostasis is replaced by serpentine-saponite and chlorite. Many Type I chondrules have highly irregular shapes resulting from fracturing and selective metal replacement. Type II porphyritic chondrules are clusters of phenocrysts set in matrix-like material. Type II chondrules may be kinked and partially disbarred. The matrix of NWA 12563 differs from CM2 chondrites in the absence of tochilinite-cronstedtite intergrowths. It contains hydrated and oxidized amorphous silicate (Fe3+/PFe 75%) richer in magnesium than in other chondrites (with embedded sulfides). Serpentine-saponite is also present, as well as abundant framboidal magnetite. NWA 12563 has similarities to a number of ungrouped magnetite-rich and 18O-rich chondrites (Bells, Essebi, Niger I, WIS 91600, Tagish Lake, and MET 00432) that we call C2-ung1, as opposed to C2-ung2 chondrites (poorer in 18O and magnetite). The oxygen isotopic composition coupled with a magnetic susceptibility of log v = 4.67 places NWA 12563 with these ungrouped chondrites in a cluster distinct from CM2 chondrites. NWA 12563 is closest to WIS 91600 among the C2-ung1 chondrites in alteration style and light element compositions. WIS 91600, however, has suffered light thermal metamorphism, suggesting that NWA 12563 might represent its altered but unheated precursor material within the same parent body if it were zoned. The average Vis-NIR spectrum of NWA 12563 matches the asteroid taxonomic class K and resembles that of CO3 Frontier Mountain (FRO) 95002, but its spectra range from very ‘‘red” in dark matrix areas and very ‘‘blue” in magnetite-rich areas. The average MIR spectrum shows features indicating phyllosilicates, aliphatic CH compounds, hydrated silicates, and olivine. It is significantly different from those of other chondrites including FRO 95002, and closest to Bells (from which it differs in carbon isotopic composition) and WIS91600. The variety of mineralogical, chemical and isotopic properties among C2-ung1 chondrites requires several different parent bodies. However, the high abundance of magnetite common to this cluster of ungrouped chondrites, and to a lesser extent CI chondrites, indicates that they should be considered as possible material from Bennu, which has an 18 mm magnetite signal in its spectrum not seen in the CM2 chondrites (Hamilton et al., 2019).
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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Cosmochemistry
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