2020 |
Cartier, C., O. Namur, L. R. Nittler, S. Z. Weider, E. Crapster-Pregont, A. Vorburger, E. A. Franck, and B. Charlier. "No FeS layer in Mercury? Evidence from Ti/Al measured by MESSENGER." Earth and Planetary Science Letters 534 (2020): 116108.
Résumé : In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury’s surface has a slightly subchondritic Ti/Al ratio of 0.035 ±0.008. Multiphase equilibria experiments show that at the conditions of Mercury’s core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13 km). We also show that Mercury’s metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury’s surface.
Mots-clés : cosmo
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Faure, F. "Early silica crust formation in planetesimals by metastable silica-rich liquid immiscibility or cristobalite crystallisation: the possible origin of silica-rich chondrules." Scientific Reports (2020).
Résumé : The formation and differentiation processes of planetesimals—small bodies in the solar system— remain actively debated. Planetesimal differentiation is known to have occurred early (<1.5 Myr after the formation of Ca-Al-rich inclusions), as attested by the ages of iron meteorites. Metal-silicate segregation implies global-scale melting, induced by heat released from short-lived radiogenic isotopes, and the consequent generation of a silicate magma ocean. Thermodynamic calculations show that silicate magma crystallisation would have induced silicate-silicate differentiation, leading to the formation of a thick olivine-dominated “mantle” and a thin basaltic “crust”. However, thermodynamic modelling of magma ocean crystallisation does not produce any silica phases. Here I experimentally show that crystallisation of a chondritic liquid does not follow the thermodynamically predicted path. Silica phases are generated early (before 55% differentiation) as a function of initial magma ocean temperature. As cristobalite or liquid silica phases are less dense than residual liquids or olivine, silica phases could have formed proto-crusts that would have acted as buoyant lids at the surfaces of planetesimals, allowing the eventual accretion and preservation of debris (chondrites). Moreover, the destruction of such a crust by impacts could provide an explanation for the origin of the silica reservoir that condensed around some chondrules.
Mots-clés : cosmo
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Flahaut, J., J. Carpenter, J. P. Williams, M. Anand, I. A. Crawford, W. van Westrenen, E. Füri, L. Xiao, and S. Zhao. "Regions of interest (ROI) for future exploration missions to the lunar South Pole." Planetary and Space Science 180 (2020): 104750.
Résumé : The last decades have been marked by increasing evidence for the presence of near-surface volatiles at the lunarpoles. Enhancement in hydrogen near both poles, UV and VNIR albedo anomalies, high CPR in remotely sensedradar data have all been tentatively interpreted as evidence for surface and/or subsurface water ice. Lunar waterice and other potential cold-trapped volatiles are targets of interest both as scientific repositories for under-standing the evolution of the Solar System and for exploration purposes. Determining the exact nature, extent andorigin of the volatile species at or near the surface in the lunar polar regions however requiresin situmeasure-ments via lander or rover missions. A number of upcoming missions will address these issues by obtainingin situdata or by returning samples from the lunar surface or shallow subsurface. These all rely on the selection ofoptimal landing sites. The present paper discusses potential regions of interest (ROI) for combined volatile andgeologic investigations in the vicinity of the lunar South Pole. We identified eleven regions of interest (including abroad area of interest (>200 km-200 km) at the South Pole, together with smaller regions located near Cabeus,Amundsen, Ibn Bajja, Wiechert J and Idel’son craters), with enhanced near-surface hydrogen concentration(H>100 ppm by weight) and where water ice is expected to be stable at the surface, considering the present-daysurface thermal regime. Identifying more specific landing sites for individual missions is critically dependent onthe mission’s goals and capabilities. We present detailed case studies of landing site analyses based on the missionscenario and requirements of the upcoming Luna-25 and Luna-27 landers and Lunar Prospecting Rover case study.Suitable sites with promising science outcomes were found for both lander and rover scenarios. However, therough topography and limited illumination conditions near the South Pole reduce the number of possible landingsites, especially for solar-powered missions. It is therefore expected that limited Sun and Earth visibility at lati-tudes>80°will impose very stringent constraints on the design and duration of future polar missions.
Mots-clés : cosmo
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Florin, G., B. Luais, T. Rushmer, and O. Alard. "Influence of redox processes on the germanium isotopic composition of ordinary chondrites." Geochimica et Cosmochimica Acta 269 (2020): 270–291.
Résumé : Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state, which increases from the H to L to LL groups. This is demonstrated by the decrease in metal content (H = 8 vol%, L = 4 vol%, and LL = 2 vol%), and by a positive correlation between D17O and %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largest variation in oxidation state, but there is an ongoing debate on the processes that control this variation. To constrain the causes of the variations in the oxidation state with respect to the associated nebular versus parent bodies processes, we investigated the elemental and isotopic variations of germanium (moderately siderophile and volatile) in the bulk sample, as well as in the metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LL ordinary chondrites. We found that d74/70Gemetal is a proxy for the d74/70Gebulk composition and that each OC group is distinguishable by their d74/70Gemetal, which increases from -0.51 ± 0.09‰ for H chondrites, -0.31 ± 0.06‰ for L chondrites, and, finally, to -0.26 ± 0.09‰ for LL chondrites (2r SD). Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, between d74/70Gemetal and %Fa, as well as oxygen isotopes (d17O, d18O and D17O), that was not a consequence of a ‘‘size sorting effect” on chondrules (i.e., chondrule mixing) or metamorphic processes in the parent bodies but, rather, was the result of nebular processes. We propose that the correlation between the d74/70Ge values and %Fa, D17O, d18O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high d74/70Ge and D17O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge] = 4–7 ppm and d74/70Ge = 3–2.5‰, is needed to change the d74/70Ge from H to LL, which corresponds to a ∆17O ≈ 8–7‰. This value agrees with the ∆17O ≈ 7‰ composition of the accreted nebular component reported by Choi et al. (1998). During thermal metamorphism, phyllosilicates destabilize, liberating germanium that will be incorporated in the metal, then leading to its high d74/70Ge signature. High-temperature metamorphism can explain the lack of d74/70Gemetal variation with the petrologic type in the OC, even for the type 3 chondrites (T ≈ 675 °C), implying a complete reaction even at low petrologic types. In addition, metal-silicate re-equilibration in response to thermal metamorphism results in a decrease in D74/70Gemetal-silicate from 0.33‰ to 0.06‰, within the H chondrite group, which is interpreted as the result of d74/70Gesilicate variation. The mean positive D74/70Gemetal-silicate fractionation factor of +0.22 ± 0.36‰ (error propagation on individual error) also displays a remarkable similarity to the direction of isotopic fractionation with other germanium isotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicate reservoirs. We propose that the D74/70Gemetal-silicate and the negative d74/70Ge values of OCs are inherited from metal-silicate melting and partial exchange before planetesimal accretion in a light isotope-enriched gas. Finally, the d74/70Gemetal-∆17Osilicate correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.
Mots-clés : cosmo
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Joy, K. H., R. Tartèse, S. Messenger, M. E. Zolensky, Y. Marrocchi, D. R. Frank, and D. A. Kring. "The isotopic composition of volatiles in the unique Bench Crater carbonaceous chondrite impactor found in the Apollo 12 regolith." Earth and Planetary Science Letters 540 (2020): 116265.
Résumé : Projectiles striking the Moon have modified its crust and delivered volatile elements to its interior and surface. Direct evidence of impactor origins is recorded by the rare occurrence of sub-cm sized meteorite fragments identified in Apollo samples and lunar meteorites. The Bench Crater meteorite is a millimetre-sized carbonaceous chondrite collected in regolith on the rim of Bench impact crater at the Apollo 12 landing site. Transmission electron microscopy has previously shown that Bench Crater contains abundant hydrated silicates, establishing the survivability of hydrated material impacting the lunar surface. To provide further information on the volatile inventory of the Bench Crater meteorite, we report here the isotope compositions of hydrogen, nitrogen, carbon and oxygen. This is the first direct isotopic analysis of meteoritic material delivered to the lunar surface and provides context for volatile and organic element signatures in lunar regolith samples, and the survivability of volatile material delivered to planetary surfaces during impact bombardment. The Bench Crater meteorite is characterised by δD values ranging between −36 ± 40 and 200 ± 40‰, and bulk average C of −13 ± 30‰, and N of −40 ± 36‰ (all uncertainties at the 2σ confidence level). The oxygen isotope compositions measured in situ in matrix silicates and magnetite in Bench Crater are consistent with those measured in matrix and magnetite in CI and CM chondrite falls. Altogether, these new H, C, N and O isotope data, coupled to mineralogical and geochemical observations, suggest that Bench Crater may have been derived from an asteroidal parent body not represented in the terrestrial meteorite collection. This is a crucial outcome in the current context of sample-return missions to carbonaceous asteroids, and more broadly for investigating the flux of material delivered to the Earth-Moon system through time.
Mots-clés : cosmo
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Martinot, M., J. Flahaut, S. Besse, C. Quantin-Nataf, and W. van Westrenen. "Mineralogical survey of the anorthositic Feldspathic Highlands Terrane crust using Moon Mineralogy Mapper data." Icarus 345 (2020): 113747.
Résumé : Spectroscopic data from the Moon Mineralogy Mapper (M) instrument are used to study the mineralogy of the central peak or peak ring of 75 craters located in the lunar anorthositic Feldspathic Highlands Terrane (FHT-a), as defined by Jolliff et al. (2000). The thickness of South-Pole Aitken (SPA) ejecta at the location of the selected craters is estimated. Crustal thickness models are used with empirical cratering equations to estimate the depth of origin of the material excavated in the studied central peaks, and its distance to the crust-mantle interface. The goal of this survey is to study the composition of the FHT-a crust, and the extent of its potential lateral and vertical heterogeneities. High-Calcium Pyroxene (HCP) and featureless spectra are mostly detected throughout the entire FHT-a, whereas the number of pure plagioclase detections is small. No relationship between the central peak composition and the distance to SPA or the depth within the SPA ejecta is observed. The SPA ejecta material cannot be spectrally distinguished from crustal material. We interpret the paucity of plagioclase spectra in the FHT-a, which contrasts with more frequent plagioclase detections in the central peaks of craters sampling the crust in younger lunar terranes using identical spectroscopic techniques Martinot et al. (2018b), as a possible effect of terrane maturation, or of mixing with mafic components that mask their signature in the visible near-infrared. Overall, the FHT-a appears homogeneous laterally. However, data hint at a pyroxene compositional change with increasing depth, from high-calcium content in the upper crust towards less calcic compositions with increasing depth, which is consistent with prior studies of the architecture of the lunar crust.
Mots-clés : cosmo
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Piralla, M., Y. Marrocchi, M. J. Verdier-Paoletti, L. G. Vacher, J. Villeneuve, L. Piani, D. V. Bekaert, and M. Gounelle. "Primordial water and dust of the Solar System: Insights from in situ oxygen measurements of CI chondrites." Geochimica et Cosmochimica Acta 269 (2020): 451–464.
Résumé : As the chemical compositions of CI chondrites closely resemble that of the Sun’s photosphere, their oxygen isotopic compositions represent a powerful tool to constrain the origin and dynamics of dust and water ice grains in the protoplanetary
disk. However, parent-body alteration processes make straightforward estimation of the primordial isotopic compositions of CI chondritic water and anhydrous minerals difficult. In this contribution, we used in situ SIMS measurements to determine
the oxygen isotope compositions of mechanically isolated olivine and carbonate grains from the CI chondrite Orgueil and carbonates in a polished section of the CI chondrite Ivuna. Most CI olivine grains have Earth-like O isotopic compositions
(D17O ≈ 0‰) plotting at the intersection of the terrestrial fractionation line and the primitive chondrule minerals line. Ca-carbonates from Orgueil and Ivuna define a trend with d17O = (0.50 ± 0.05) x d18O + (0.9 ± 1.4) that differs from massindependent variations observed in secondary phases of other carbonaceous chondrites. These data show that CIs are chemically solar but isotopically terrestrial for oxygen isotopes. This supports models suggesting that primordial Solar System dust was 16O-poor (D17O ≈ 0‰) relative to the 16O-rich nebular gas. Based on results, mass balance calculations reveal that the pristine O isotopic compositions of carbonaceous chondrite matrices differ significantly from the CI composition, except for CR chondrites (calculated D17O values of CM, CO, CV and CR matrices being –3.97 ± 1.19‰, –4.33 ± 1.45‰, –7.95± 1.95‰, and –0.07 ± 1.16‰, respectively). This confirms an open chondrule-matrix system with respect to oxygen isotopes where chondrule compositions reflect complex processes of chondrule precursor recycling and gas-melt interactions. As the Mg-Si-Fe chondrule budget is also partially controlled by gas-melt interactions, the complementary formation of chondrules and matrix from a single solar-like reservoir -if it exists- require that (i) this reservoir must have been in a closed system with the gas or (ii) the gas had a CI composition to satisfy the elemental mass balance.
Mots-clés : cosmo
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Schnuriger, N., J. Flahaut, M. Martinot, and S. D. Chevrel. "Long-lived volcanism expressed through mare infilling, domes and IMPs in the Arago region of the Moon." Planetary and Space Science 185 (2020): 104901.
Résumé : Mare Tranquillitatis corresponds to the deposit of successive Early to Late Imbrian basaltic units filling the Tranquillitatis basin on the Moon. The present study focuses on the western half of the mare, in the vicinity of the Arago crater (6.16°N, 21.42°E). High resolution datasets from recent remote sensing missions were used to reconstruct the geologic history of the area, which includes a variety of geological features such as: 8 extrusive domes, numerous wrinkle ridges, a sinuous rille and 37 Irregular Mare Patches (IMPs). We performed crater counting to date the domes emplacement and estimated the domes lava rheologic properties (plastic viscosities, lava effusion rates, and durations of effusion) using their geometric characteristics. As a result we classify the Arago domes into three groups: E1-type domes (Arago 1 and 8), H1-type domes (Arago 4 to 7), and B-type domes (Arago 2 and 3) respectively emplaced ~ 3.7 Ga, ~3.4 Ga, and ~2.8 Ga ago. IMPs are observed in the younger mare unit and on the top of the Arago 6 dome; they likely correspond to a late stage of waning mare volcanism in the area. Both IMPs and domes have a composition similar to the surrounding mare in the VNIR spectral domain, consistent with mafic materials. The exceptionally long-lived volcanism and its diversity recorded in the Arago region may be related to both a Th-rich anomaly reported nearby and to the large-scale magma center responsible for the Lamont positive Bouguer anomaly. In addition, volcanic features of the Arago region are superimposed on a Ti-rich mare unit visited by the Apollo 11 crew 175 km to the south at Tranquility Base. The geological and historical richness of this region makes it a compelling site for future science and/or In Situ Resources Utilization (ISRU) driven missions to the Moon.
Mots-clés : cosmo
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Villeneuve, J., Y. Marrocchi, and E. Jacquet. "Silicon isotopic compositions of chondrule silicates in carbonaceous chondrites and the formation of primordial solids in the accretion disk." Earth and Planetary Science Letters 542 (2020): 116318.
Résumé : We determined the silicon isotopic compositions of silicates (olivine and low-Ca pyroxene) in type I and type II chondrules of the carbonaceous chondrites Allende, Kaba, NWA (Northwest Africa) 5958, and MIL (Miller Range) 07342. Type I chondrule silicates show large, mass-dependent Si isotopic fractionations, with Si values ranging from −7‰ to +2.6‰, whereas the Si values of type II chondrule silicates are close to zero and vary by less than 2‰. When present, Mg-rich relict olivine grains in type II chondrules show larger Si variations than their FeO-rich counterparts. In type I chondrules, low-Ca pyroxenes yield systematically lighter Si values than Mg-rich olivines. Our results show that type I chondrules are complex objects whose Si isotopic compositions derived from their precursors and SiO-rich gas-melt interactions. This corroborates that type I chondrules are nebular products that formed under open-system conditions. Our data also suggest that at least some type II chondrules derived from their type I counterparts. Overall, this demonstrates that recycling was common during the evolution of the protoplanetary disk
Mots-clés : cosmo
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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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Barnouin, O. S., M. G. Daly, et al & The OSIRIS-REx Team., and B. Marty. "Shape of (101955) Bennu indicative of a rubble pile with internal stiffness." Nature Geoscience 12 (2019): 247–252.
Résumé : The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu’s shape. Here we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu’s top-like shape, considerable macroporosity and prominent surface boulders suggest that it is a rubble pile. High-standing, north–south ridges that extend from pole to pole, many long grooves and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin, which led to its current shape. Today, Bennu might follow a different evolutionary pathway, with an interior stiffness that permits surface cracking and mass wasting.
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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Bekaert, D., Y. Marrocchi, A. Meshik, L. Remusat, and B. Marty. "Primordial heavy noble gases in the pristine Paris carbonaceous chondrite." Meteoritics and Planetary Science 54, no. 2 (2019): 395–414.
Résumé : The Paris carbonaceous chondrite represents the most pristine carbonaceous chondrite, providing a unique opportunity to investigate the composition of early solar system materials prior to the onset of significant aqueous alteration. A dual origin (namely from the inner and outer solar system) has been demonstrated for water in the Paris meteorite parent body (Piani et al. 2018). Here, we aim to evaluate the contribution of outer solar system (cometary‐like) water ice to the inner solar system water ice using Xe isotopes. We report Ar, Kr, and high‐precision Xe isotopic measurements within bulk CM 2.9 and CM 2.7 fragments, as well as Ne, Ar, Kr, and Xe isotope compositions of the insoluble organic matter (IOM). Noble gas signatures are similar to chondritic phase Q with no evidence for a cometary‐like Xe component. Small excesses in the heavy Xe isotopes relative to phase Q within bulk samples are attributed to contributions from presolar materials. CM 2.7 fragments have lower Ar/Xe relative to more pristine CM 2.9 fragments, with no systematic difference in Xe contents. We conclude that Kr and Xe were little affected by aqueous alteration, in agreement with (1) minor degrees of alteration and (2) no significant differences in the chemical signature of organic matter in CM 2.7 and CM 2.9 areas (Vinogradoff et al. 2017). Xenon contents in the IOM are larger than previously published data of Xe in chondritic IOM, in line with the Xe component in Paris being pristine and preserved from Xe loss during aqueous alteration/thermal metamorphism
Mots-clés : cosmo
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Boulesteix, T., M. Cathelineau, E. Deloule, M. Brouand, H. Toubon, P. Lach, and N. Fiet. "Ilmenites and their alteration products, sinkholes for uranium and radium in roll-front deposits after the example of South Tortkuduk (Kazakhstan)." Journal of Geochemical Exploration 206 (2019): 106343.
Résumé : The approximate determination of average Ra/U disequilibria in orebodies is one of the most common causes of errors in U reserve estimations. In roll-front deposits, the disequilibria are however frequently distributed following complex geometries, which must be fully understood to prevent major U reserve overestimates and costly unproductive extractive operations. The processes responsible for disruption of the radioactive equilibria and the U and Ra carriers in such complex natural systems remain poorly constrained. In this contribution, we propose an innovative approach, mixing orebody to sub-grain scale studies to unravel the distribution of U and Ra and the processes responsible for their concentration and uncoupling. Using mineral separations, gamma spectrometry and mineral-chemical analyses, we identified the Fe-Ti clusters (altered ilmenite + pyrite/marcasite) as the microsites for coffinite precipitation and Ra concentration. To understand the influence of such clusters on the distribution of U and Ra at the deposit scale, wholerock Ra/U disequilibria were measured and mapped at a series of ten drill holes along a profile crosscutting the studied roll-front. The main Ra/U disequilibria are encountered around the mineralization in low U content zones. They are controlled by two main processes. (1) In the oxidized zones, the immobility of 230Th with respect to the U produces patches of Ra disequilibria (carried by the altered U minerals). (2) In the immediate vicinity of the roll-front, the dissolution of the mineralization produces an Ra flux trapped by the alteration products of ilmenites, as definitely confirmed by direct SIMS measurements. Such a process is responsible for the Ra disequilibria envelope located downstream of the richest ores, also known as Ra halo. The highest Ra/U ratios correspond to oxidized upstream samples, but most other high Ra/U ratios are from reduced downstream samples close to the mineralization. Such a low to medium U content envelope with high Ra/U ratios constitutes the main cause of U reserve overestimations.
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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DellaGiustina, D. N., J. P. Emery, et al &the OSIRIS-REx Team., and B. et al. Marty. "Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis." Nature Astronomy 3 (2019): 341–351.
Résumé : Establishing the abundance and physical properties of regolith and boulders on asteroids is crucial for understanding the formation and degradation mechanisms at work on their surfaces. Using images and thermal data from NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft, we show that asteroid (101955) Bennu’s surface is globally rough, dense with boulders, and low in albedo. The number of boulders is surprising given Bennu’s moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu’s surface particles span from the disruption of the asteroid’s parent body (boulders) to recent in situ production (micrometre-scale particles).
Mots-clés : cosmo
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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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Cosmochemistry
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