2019 |
Bekaert, D., M. S. Gudipati, B. Henderson, and B. Marty. "Coulomb explosion of multiply ionized xenon in water ice." Geochemical Journal 53, no. 81 (2019): 69.
Résumé : Laboratory investigations of noble gas trapping in amorphous water ice have been used to predict the noble gas composition of comets and infer on the origin of volatile elements within planetary bodies. However, the recent measurement
of the noble gas composition of ice sublimating from comet 67P/Churyumov-Gerasimenko by the Rosetta mission calls for novel experiments regarding the mechanisms of noble gas trapping and evolution in cometary ice analogues. Here, we
investigated the ionization dynamics of Xe atoms interacting with water ice using the recently developed Resonant Two-Step Laser Ablation Mass Spectrometry (2S-LAI-MS). Xenon-water mixed ice was ablated with an infrared beam set on the maximum absorption wavelength for water (l = 2948 nm) at which xenon atoms are kept neutral. Subsequent multiple ionization of xenon and oxygen resulted in periodic Coulomb explosions of Xen+ components (n OE [1;6]) and ionized water degradation products (OH+, H+, O+, O2+, O3+). Such explosions could only be detected when Xe and water were mixed together in ice, and not when separated in two overlaid layers. This paper discusses the potential mechanisms accounting for the generation of Coulomb explosions in these experiments and its relevance to cometary ice at closer distances to perihelion. We conclude that multiple ionization of xenon and oxygen in our experiment may be due to electron impact processes resembling cometary electron and ion bombardment, whereby energetic particles of hundreds of eV to a few keV are accelerated towards the comet’s nucleus. Electron and ion bombardment could induce significant chemical modifications to, and potentially outgassing from, the cometary surface.
Mots-clés : cosmo
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Cartier, C., and B. J. Wood. "The role of reducing conditions in building Mercury." Elements 15 (2019): 39–45.
Résumé : Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
Mots-clés : cosmo
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Cartier, C., and B. J. Wood. "The role of reducing conditions in building Mercury." Elements 15 (2019): 39–45.
Résumé : Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
Mots-clés : cosmo
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Marrocchi, Y., R. Euverte, J. Villeneuve, V. Batanova, B. Welsch, L. Ferrière, and E. Jacquet. "Formation of CV chondrules by recycling of amoeboid olivine aggregate-like precursors." Geochimica et Cosmochimica Acta 247, no. 2 (2019): 121–141.
Résumé : We have studied porphyritic olivine-rich chondrules of the carbonaceous chondrite Kaba (CV3) by combined high-resolution X-ray mapping, quantitative electron microprobe analyses, and oxygen isotopic analyses via secondary ion mass spectrometry. These chondrules contain smaller inner-chondrule olivine grains characterized by low refractory element (Ca, Al, Ti) contents, and larger outer-chondrule olivine crystals that are enriched in refractory elements and show complex Ti and Al oscillatory zonings. Our O isotopic survey revealed that many of the inner-chondrule olivines are 16O-richer than the relatively isotopically uniform outer-chondrule olivines. Inner-chondrule olivine crystals—only a minority of which may be derived from earlier generations of chondrules—are likely mostly inherited from nebular condensates similar to AOAs, as they share similar isotopic and chemical features and are thus interpreted as relict grains. Still, being 16O-poorer than most AOAs, they may have experienced significant exchange with a 16O-poor reservoir prior to chondrule formation (even if to a lesser degree than relicts in CM2 and ungrouped C2 chondrites). Subsequent incomplete melting of the relict grains produced Ca-Al-Ti-rich melts that engulfed the remaining relict olivine grains. The complex Ti and Al zoning patterns in outer chondrule (host) olivines, in particular the systematic dilution near the margin, seem to reflect gas-melt interactions (with e.g. SiO (g), Mg (g)) which also buffered the O isotopic composition of chondrule hosts. Together, these results demonstrate that important episodes of recycling of nebular condensates occurred in the solar protoplanetary disk.https://doi.org/10.1016/j.gca.2018.12.038
Mots-clés : cosmo
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Mosenfelder, J. L., A. Von Der Handt, E. Füri, C. Dalou, R. L. Hervig, G. R. Rossman, and M. M. Hirschmann. "Nitrogen incorporation in silicates and metals: Results from SIMS, EPMA, FTIR, and laser-extraction mass spectrometry." American Mineralogist 104, no. 1 (2019): 31–46.
Résumé : A quantitative understanding of nitrogen incorporation in Earth materials is important for constraining volatile evolution in planetary bodies. We used a combination of chemical (SIMS, EPMA, and laser-extraction mass spectrometry) and spectroscopic (FTIR) observations to study nitrogen contents and speciation mechanisms in silicate glasses, metal alloys, and an N-bearing silicate mineral (hyalophane). One suite of Fe-free basaltic glasses was studied by all four methods. Concentrations of N in these glasses determined by EPMA are systematically higher than those measured by laser extraction but agree within mutual 2s uncertainties, demonstrating the general veracity of the EPMA method. SIMS working curves based on measurement of 14N+ and 14N16O- as a function of N content determined by EPMA (or laser extraction) are best fit with exponential functions rather than the linear regressions that are most commonly applied to SIMS data. On the other hand, the relationship based on 12C14N- for C-poor, Fe-free glasses is exceptionally well fit to a linear regression (r2 = 1, p < 0.001), in contrast to expectations from previous work on glasses with lower N contents. Matrix effects on the SIMS signals associated with Fe or H2O content are not justified by the data, but volatile data (both N and H) for hyalophane, which contains 20 wt% BaO, reveal matrix effects possibly induced by its high average molar mass. A combination of FTIR and chemical data, together with a thorough review of the literature, was used to determine incorporation mechanisms for N in the Fe-free glasses. We infer that under reducing conditions at high pressure and temperature N is dissolved in basaltic melts chiefly as NH−2 and NH2–, with N2 and/or nitride (X-N3–) complexes becoming increasingly important at low fO2, increasing N content, and decreasing H content. Our results have implications for future studies seeking to accurately measure N by SIMS and for studies of N partitioning at high pressure relevant to planetary accretion and differentiation.
Mots-clés : cosmo
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2018 |
Avice, G., B. Marty, R. Burgess, A. Hofmann, P. Philippot, K. Zahnle, and D. Zakharov. "Evolution of atmospheric xenon and other noble gases inferred from Archean to Paleoproterozoic rocks." Geochimica et Cosmochimica Acta 232 (2018): 82–100.
Résumé : We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30–40‰ u-1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3 Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9 ± 1.2 (1r) ‰ u-1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u-1 at 3.5 Ga to 12.9‰ u-1 at 3.3 Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7 Ga. However, further studies will be needed to confirm this observation. After 2.7 Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1 Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3 Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere similar to, or lower than, the modern one, thus requiring other processes than a high pN2 to keep the Earth’s surface warm despite a fainter Sun. The nitrogen isotope composition of the atmosphere at 3.3 Ga was already modern-like, attesting to inefficient nitrogen escape to space since that time
Mots-clés : cosmo
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Bekaert, D. V., S. Derenne, L. Tissandier, Y. Marrocchi, S. Charnoz, C. Anquetil, and B. Marty. "High-temperature ionization-induced synthesis of biologically relevant molecules in the protosolar nebula." Astrophysical Journal 859, no. 142 (2018).
Résumé : Biologically relevant molecules (hereafter biomolecules) have been commonly observed in extraterrestrial samples, but the mechanisms accounting for their synthesis in space are not well understood. While electron-driven production of organic solids from gas mixtures reminiscent of the photosphere of the protosolar nebula (PSN; i.e., dominated by CO–N2–H2) successfully reproduced key specific features of the chondritic insoluble organic matter (e.g., elementary and isotopic signatures of chondritic noble gases), the molecular diversity of organic materials has never been investigated. Here, we report that a large range of biomolecules detected in meteorites and comets can be synthesized under conditions typical of the irradiated gas phase of the PSN at temperatures=800 K. Our results suggest that organic materials—including biomolecules—produced within the photosphere would have been widely dispersed in the protoplanetary disk through turbulent diffusion, providing a mechanism for the distribution of organic meteoritic precursors prior to any thermal/photoprocessing and subsequent modification by
secondary parent body processes. Using a numerical model of dust transport in a turbulent disk, we propose that organic materials produced in the photosphere of the disk would likely be associated with small dust particles, which are coupled to the motion of gas within the disk and therefore preferentially lofted into the upper layers of the disk where organosynthesis occurs.
Mots-clés : cosmo
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Bekaert, D. V., Y. Marrocchi, A. Meshik, L. Remusat, and B. Marty. "Primordial heavy noble gases in the pristine Paris carbonaceous chondrite." Meteoritics & Planetary Science (2018): 1–20.
Résumé : The Paris carbonaceous chondrite represents the most pristine carbonaceous
chondrite, providing a unique opportunity to investigate the composition of early solar system materials prior to the onset of significant aqueous alteration. A dual origin (namely from the inner and outer solar system) has been demonstrated for water in the Paris meteorite parent body (Piani et al. 2018). Here, we aim to evaluate the contribution of outer solar system (cometary-like) water ice to the inner solar system water ice using Xe isotopes. We report Ar, Kr, and high-precision Xe isotopic measurements within bulk CM 2.9 and CM 2.7 fragments, as well as Ne, Ar, Kr, and Xe isotope compositions of the insoluble organic matter (IOM). Noble gas signatures are similar to chondritic phase Q with no evidence for a cometary-like Xe component. Small excesses in the heavy Xe isotopes relative
to phase Q within bulk samples are attributed to contributions from presolar materials. CM 2.7 fragments have lower Ar/Xe relative to more pristine CM 2.9 fragments, with no systematic difference in Xe contents. We conclude that Kr and Xe were little affected by aqueous alteration, in agreement with (1) minor degrees of alteration and (2) no significant differences in the chemical signature of organic matter in CM 2.7 and CM 2.9 areas (Vinogradoff et al. 2017). Xenon contents in the IOM are larger than previously published data of Xe in chondritic IOM, in line with the Xe component in Paris being pristine and preserved from Xe loss during aqueous alteration/thermal metamorphism
Mots-clés : cosmo
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Boucher, C., T. Lan, J. Mabry, D. V. Bekaert, P. G. Burnard, and B. Marty. "Spatial analysis of the atmospheric helium isotopic composition: Geochemical and environmental implications." Geochimica et Cosmochimica Acta 237 (2018): 120–130.
Résumé : Spatial variations in the atmospheric helium isotopic composition (RA = 3He/4Heair = 1.39 × 10−6) might be induced by localized and/or regional inputs of 3He and/or 4He into the air. It has been suggested that inputs of 4He from hydrocarbon exploitation may generate latitudinal variations in atmospheric 3He/4He. In order to test the possibility of such global variations, we performed high precision analyses of the helium isotopic composition (3He/4He) of sixteen air samples collected in 500 cc metal bottles around the world (79°N–75°S) between 2013 and 2015. In most cases, the 3He/4He of these air samples are indistinguishable from that of air sampled around Nancy (France) within ≤2‰ (95% confidence interval). Only two air samples, collected at Dome C (Antarctica) and at Tokyo (Japan), exhibit statistically higher 3He/4He ratios interpreted as potential 3He excesses of (2.0 ± 1.4)‰ and (1.7 ± 1.5)‰ (95% confidence interval), respectively. Excesses such as these suggest that potential helium isotopic variations in air are likely lower than 4‰ and might be generated temporarily by regional and/or local phenomenon (e.g. auroral precipitation, stratospheric to tropospheric exchanges). Thus, this study supports the use of the atmospheric helium isotopic ratio as an inter-laboratory He standard.
Mots-clés : cosmo
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Broadley, M. W., H. Hagi, R. Burgess, D. Zedgenizov, S. Mikhail, M. Almayrac, A. Ragozin, B. Pomazansky, and H. Sumino. "Plume-lithosphere interaction, and the formation of fibrous diamonds." Geochemical Perspectives Letters 2018, no. 8 (2018): 26–30.
Résumé : Fluid inclusions in diamond provide otherwise inaccessible information on the
origin and nature of carbonaceous fluid(s) in the mantle. Here we evaluate the
role of subducted volatiles in diamond formation within the Siberian cratonic
lithosphere. Specifically, we focus on the halogen (Cl, Br and I) and noble gas
(He, Ne and Ar) geochemistry of fluids trapped within cubic, coated and cloudy
fibrous diamonds from the Nyurbinskaya kimberlite, Siberia. Our data show Br/
Cl and I/Cl ratios consistent with involvement of altered oceanic crust, suggesting
subduction-derived fluids have infiltrated the Siberian lithosphere. 3He/4He
ranging from 2 to 11 RA, indicates the addition of a primordial mantle component
to the SCLM. Mantle plumes may therefore act as a trigger to re-mobilise
subducted carbon-rich fluids from the sub-continental lithospheric mantle, and
we argue this may be an essential process in the formation of fluid-rich diamonds,
and kimberlitic magmatism.
Mots-clés : cosmo magma
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Delon, R., S. Demouchy, Y. Marrocchi, M. A. Bouhifd, F. Barou, P. Cordier, A. Addad, and P. G. Burnard. "Helium incorporation and diffusion in polycrystalline olivine." Chemical Geology 488 (2018): 105–124.
Résumé : Helium is a key tracer of mantle geochemical and isotopic heterogeneities and can constrain our understanding of mantle geodynamics. Nevertheless, the mechanisms of helium storage and transport in mantle minerals remain poorly understood. Polycrystalline olivine was doped with helium at high temperature (1050 ± 25 °C) and high pressure (0.30 ± 0.01 GPa), followed by step heating extraction experiments to investigate helium storage and diffusion in Earth’s upper mantle. We also tested the effect of heterogeneous initial concentrations on the extracted diffusivities, and demonstrate the robustness of diffusion parameters obtained in this study. Our results show that two diffusion processes are acting in polycrystalline olivine : (i) a high temperature process with high activation energy ( Ea) where diffusion is only controlled by lattice diffusion, and (ii) a lower temperature process with lower Ea where diffusion is controlled by both grain boundary and lattice diffusion. These two diffusion processes are separated by a transition temperature that depends on the depletion of helium hosted in grain boundaries, i.e., the amount of helium stored at grain boundaries and the temperature and duration of the step heating sequence. Our results confirm that grain boundaries can represent a significant storage site for He. Moreover, we report two different populations of diffusion parameters in the lattice diffusion field, which are interpreted as diffusion in interstitials (Ea= 95 ± 15 kJ·mol−1 and log(D0)=−8.26 ± 2.13) and Mg vacancies (Ea= 168 ± 19 kJ·mol−1 and log(D0)=− 3.59 ± 2.12). Similar diffusion parameters populations are observed in literature data after reprocessing the diffusivities. Furthermore, we determine grain boundary diffusion parameters : Ea = 57 ± 14 kJ·mol−1 and log(D0)=−9.20 ± 0.99. Applying these results to the upper mantle reveals that an important amount of He can be stored at grain boundaries for typical mantle grain size (22% for a grain size of 1 mm) and that most helium can be stored at grain boundaries for relatively small grain sizes (≤290μm and ≤10μm for segregation factors of 1/10−5 and 1/0.0025, respectively). As a consequence, bulk diffusivities can be significantly higher than lattice diffusivities. Although our study cannot be applied directly to the lower mantle, the similar storage sites and diffusion mechanisms are expected in lower mantle silicates if high pressure does not inhibit diffusion
Mots-clés : cosmo
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Füri, E., E. Deloule, and C. Dalou. "Nitrogen abundance and isotope analysis of silicate glasses by secondary ionization mass spectrometry." Chemical Geology 493 (2018): 327–337.
Résumé : Chondritic meteorites preserve extreme intra-sample 15N/14N variations, which exceed, in some cases, the range of nitrogen isotope ratios observed at the Solar System scale. These observations are based on in situ analyses of CN− molecular ions by secondary ionization mass spectrometry (SIMS) in carbon-rich phases. The distribution of nitrogen and its isotopes in silicate minerals and glasses has not been investigated to this date due to the lack of an appropriate analytical protocol, as well as of suitable N-bearing standards. In order to improve our knowledge of the nitrogen signature of both extraterrestrial and terrestrial silicate samples, we have developed a protocol for determining precise and accurate nitrogen abundances (and isotope ratios) in basaltic glasses using high mass
resolution SIMS. Twelve (C-)N-bearing synthetic basaltic glasses, containing between<1 and 18,443 ± 966 ppm N, form the suite of reference materials for this study. By targeting the CN−, NO−, AlN−, and SiN− secondary molecular ions, nitrogen abundances can be detected down to the ppm level in both carbonbearing and carbon-free glasses. The analytical precision and reproducibility of isotope ratios in the form of 15N16O−/14N16O− is on the order of 11‰ and 10 to 17‰ (2σ), respectively, for reference glasses containing ≥100 ppm N. Thus, nitrogen isotope ratios can be determined with an uncertainty that is small enough to resolve nitrogen isotope variations in extraterrestrial silicates. The study of four chondrules of the ordinary chondrite Semarkona (LL3.0) reveals that the nitrogen distribution in the mesostasis is highly heterogeneous, with concentrations ranging from 0 to 1099 ± 168 ppm. The δ15N values in mesostasis, olivine, and pyroxene
vary between −36 ± 50‰ and +55 ± 72‰, indicating that silicate phases in chondrules do not host particularly 15N-poor nitrogen
Mots-clés : cosmo
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Füri, E., L. Zimmermann, and A. E. Saal. "Apollo 15 green glass He-Ne-Ar signatures – In search for indigenous lunar noble gases." Geochemical Perspectives Letters 8 (2018): 1–5.
Résumé : Identifying indigenous lunar noble gases in samples returned by the Apollo and
Luna missions is highly challenging because contributions from the solar wind
(SW) and/or cosmogenic nuclides have modified the noble gas signature of the
regolith and rocks exposed to space at the lunar surface. Here we re-investigate
the possible presence of indigenous noble gases in pyroclastic Apollo 15426 green
glasses based on precise measurements of He-Ne-Ar isotopic compositions and
abundances. The noble gas content of single glass beads varies by two orders of
magnitude, indicating that they experienced highly variable irradiation histories
as a result of intense regolith stirring by impact gardening. Four out of the twelve spherules stand out by having the highest He-Ne-Ar abundances and by releasing an isotopically ‘solar-like’ noble gas component at high temperatures. While a contribution from indigenous noble gases cannot be ruled out, the data are best accounted for by inward diffusion of, and equilibration with, SW-derived volatiles during prolonged space exposure.
Mots-clés : cosmo
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Florentin, L., E. Deloule, F. Faure, and D. Mangin. "Chemical 3D-imaging of glass inclusions from allende (CV3) olivine via SIMS: A new insight on chondrule formation conditions." Geochimica et Cosmochimica Acta 230 (2018): 83–93.
Résumé : Natural glass inclusions – hosted in Mg-rich olivines from Allende (CV3) type I chondrules – and synthetic melt inclusions – trapped in forsterite crystallized from CMAS (CaO-MgO-Al2O3-SiO2) melts – were mapped by Secondary Ion Mass Spectrometry(SIMS) for CMAS major oxides. The first ever 3D chemical images of extra-terrestrial glass inclusions were obtained, along with chemical depth profiles for each oxide. Results show similar patterns for both synthetic glass inclusions (trapped in olivine formed by slow crystallization in a magmatic liquid) and natural inclusions from Allende’s olivines. No incompatible-rich boundary layer or diffusion pattern was observed in either case. The absence of an
incompatible-rich boundary layer suggests that the olivine overgrowth surrounding glass inclusions in Allende’s olivines was formed during slow cooling of the host olivine and likely the surrounding chondrule. This provides new constraints on the cooling rates of type I chondrules.
Mots-clés : cosmo
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Holycross, M. E., E. B. Watson, F. M. Richter, and J. Villeneuve. "Diffusive fractionation of Li isotopes in wet, highly silicic melts." Geochemical Perspectives Letters 6 (2018): 39–42.
Résumé : The discovery of large lithium isotopic gradients in geologic media has motivated recent work examining the kinetic fractionation of Li isotopes in silicate materials. Here, piston-cylinder experiments were used to determine Li diffusivities in rhyolitic melts containing 6 wt. % H2O at 1 GPa pressure and 790-875 ºC. Lithium transport in wet rhyolitic melt is almost an order of magnitude faster than diffusion in dry obsidian glass over the investigated temperature range. Li isotope profiles collected by ion microprobe show that the kinetic exponent β = 0.228 for diffusive fractionation of Li isotopes in wet rhyolite. This value is very close to β = 0.215 determined by Richter et al. (2003) for Li isotope diffusion in a dry basalt-rhyolite couple at 1350 °C. The similarity of the two values indicates little or no dependence of βLi in silicate melts on either temperature or melt composition. The new data confirm a very high potential for diffusive fractionation of 6Li from 7Li and can be confidently used to model deviations in δ7Li to determine the time-temperature histories of natural rhyolite samples.
Mots-clés : cosmo
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Huang, J., Z. Xiao, J. Flahaut, M. Martinot, J. Head, X. Xiao, M. Xie, and L. Xiao. "Geological characteristics of Von Kármán Crater, Northwestern South Pole‐Aitken Basin: Chang'E‐4 Landing site region." Journal of Geophysical Research (Planets) (2018).
Résumé : Von Kármán crater (diameter =~186 km), lying in the northwestern South Pole‐Aitken basin, was formed in the pre‐Nectarian. The Von Kármán crater floor was subsequently flooded with one or several generations of mare basalts during the Imbrian period. Numerous subsequent impact craters in the surrounding region delivered ejecta to the floor, together forming a rich sample of the SPA basin and farside geologic history. We studied in detail the targeted landing region (45.0–46.0°S, 176.4–178.8°E) of the 2018 Chinese lunar mission Chang'E‐4, within the Von Kármán crater. The topography of the landing region is generally flat at a baseline of ~60 m. Secondary craters and ejecta materials have covered most of the mare unit, and can be traced back to at least four source craters (Finsen, Von Kármán L, Von Kármán L' and Antoniadi) based on preferential spatial orientations and crosscutting relationships. Extensive sinuous ridges and troughs are identified spatially related to Ba Jie crater (diameter = ~4 km). Reflectance spectral variations due to difference in both composition and physical properties are observed among the ejecta from various‐sized craters on the mare unit. The composition trends were used together with crater scaling relationships and estimates of regolith thickness to reconstruct the subsurface stratigraphy. The results reveal a complex geological history of the landing region, and set the framework for the in‐situ measurements of the CE‐4 mission, which will provide unique insights into the compositions of farside mare basalt, SPA compositional zone, including SPA compositional anomaly and Mg‐pyroxene annulus, regolith evolution and the lunar space environment.
Mots-clés : cosmo
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Marrocchi, Y., D. V. Bekaert, and L. Piani. "Origin and abundance of water in carbonaceous asteroids." Earth and Planetary Science Letters 482 (2018): 23–32.
Résumé : The origin and abundance of water accreted by carbonaceous asteroids remains underconstrained, but would provide important information on the dynamic of the protoplanetary disk. Here we report the in situoxygen isotopic compositions of aqueously formed fayalite grains in the Kaba and Mokoia CV chondrites. CV chondrite bulk, matrix and fayalite O-isotopic compositions define the mass-independent continuous trend (δ17O =0.84 ±0.03 ×δ18O −4.25 ±0.1), which shows that the main process controlling the O-isotopic composition of the CV chondrite parent body is related to isotopic exchange between 16O-rich anhydrous silicates and 17O-and 18O-rich fluid. Similar isotopic behaviors observed in CM, CR and CO chondrites demonstrate the ubiquitous nature of O-isotopic exchange as the main physical process in establishing the O-isotopic features of carbonaceous chondrites, regardless of their alteration degree. Based on these results, we developed a new approach to estimate the abundance of water accreted by carbonaceous chondrites (quantified by the water/rock ratio) with CM (0.3–0.4) ≥CR (0.1–0.4) ≥CV (0.1–0.2) >CO (0.01–0.10). The low water/rock ratios and the O-isotopic characteristics of secondary minerals in carbonaceous chondrites indicate they (i) formed in the main asteroid belt and (ii)accreted a locally derived (inner Solar System) water formed near the snowline by condensation from the gas phase. Such results imply low influx of D-and 17O-and 18O-rich water ice grains from the outer part of the Solar System. The latter is likely due to the presence of a Jupiter-induced gap in the protoplanetary disk that limited the inward drift of outer Solar System material at the exception of particles with size lower than 150μm such as presolar grains. Among carbonaceous chondrites, CV chondrites show O-isotopic features suggesting potential contribution of 17–18O-rich water that may be related to their older accretion relative to other hydrated carbonaceous chondrites.
Mots-clés : cosmo
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Marrocchi, Y., J. Villeneuve, V. Batanova, L. Piani, and E. Jacquet. "Oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules." Earth and Planetary Science Letters 496 (2018): 132–141.
Résumé : FeO-poor (type I) porphyritic chondrules formed by incomplete melting of solid dust precursors viaa yet-elusive mechanism. Two settings are generally considered for their formation: (i) a nebular setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca–Al–Ti-poor relative to host olivine crystals. Their variable Δ17O, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120◦triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions.
We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Δ17O values and Ca–Al–Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca–Al–Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca–Al–Ti-rich host olivine crystalsviaepitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i)the dilution of CAT-melts, as attested by the positive Al–Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Δ17O values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Δ17O values, possibly due to variable solid/gas mixing ratios.
Mots-clés : cosmo
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Marty, B., G. Avice, D. V. Bekaert, and M. W. Broadley. "Salinity of the Archaean oceans from analysis of fluid inclusions in quartz." Comptes Rendus Geoscience 350 (2018): 154–163.
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Morino, P., G. Caro, and L. Reisberg. "Differentiation mechanisms of the early Hadean mantle: Insights from combined 176 Hf- 142,143 Nd signatures of Archean rocks from the Saglek Block." Geochimica et Cosmochimica Acta 240 (2018): 43–63.
Résumé : Positive 142Nd anomalies in Eoarchean rocks provide evidence for early (4.40 ± 0.03 Ga ; Morino et al., 2017) depletion of Earth’s mantle. This model age lies within errors of the Pb-Pb “age of the Earth” (Connelly and Bizzarro, 2016), and is similar to model ages inferred for crystallization of the lunar mantle (McLeod et al., 2014), implying that this large-scale event may reflect crystallization of a magma ocean following the Moon-forming impact. However, differentiation mechanisms responsible for the formation of this early depleted mantle reservoir and the depth at which it formed cannot be constrained from the Sm-Nd isotope system alone, because the magnitude of Sm/Nd fractionation during partial melting or fractional crystallization shows little dependence on pressure-controlled changes in mantle mineralogy. In contrast, the Lu-Hf isotope system is highly dependent on mineralogy, notably the presence or absence of garnet, an upper mantle phase, and thus may be used to constrain the pressure of fractionation. This study provides the first 176Lu-176Hf isotopic results on mafic and ultramafic rocks belonging to the Eoarchean (Nulliak) and Mesoarchean suites of the Saglek Block (northern Labrador, 3.2–3.9 Ga). The 176Lu-176Hf dating confirms the distinction between these two groups of rocks and provides ages consistent with those obtained from 147Sm-143Nd dating. The whole rock 176Lu-176Hf errorchrons yield ages and initial epsilon values of 3766 ± 140 Ma, ε176Hfi = 6.0 ± 2.5 and 3023 ± 390 Ma, ε176Hfi = −0.3 ± 2.5 for the Nulliak suite and the Mesoarchean suite respectively. The time-integrated 176Lu/177Hf for the sources of the Nulliak and the Mesoarchean suites considering a time of differentiation at 4.40 ± 0.03 Ga are estimated to be 0.047 ± 0.005 and 0.033 ± 0.005, respectively. For the Mesoarchean samples, the combined 146,147Sm-142,143Nd and 176Lu-176Hf data are consistent with a near-chondritic mantle source. On the other hand, Nulliak ultramafic rocks were derived from a mantle reservoir with superchondritic Lu/Hf and Sm/Nd. The Nulliak parent reservoir, however, does not plot on the ε176Hf-ε143Nd mantle array defined by modern oceanic basalts. Instead, the Nulliak source more likely belongs to a distinct array defined by Eo- and Meso-Archean komatiites. These results are interpreted in the framework of a simple model of crystallization of a primordial magma ocean. It appears that the fractionation observed in the mantle source of Nulliak was most likely generated by crystallization of a garnet-bearing assemblage in the shallow mantle, above the transition zone rather than by perovskite fractionation in the lower mantle. To preserve this depleted reservoir from the rest of the hot and vigorously convecting mantle, the Nulliak mantle source may have been isolated either at the top of the mantle in a buoyant lithosphere or near the core-mantle boundary, with the latter setting being more consistent with the komatiitic nature of the erupted rocks. Given that the garnet signature argues for differentiation of the Nulliak source at relatively shallow depth (few hundred kilometers), its isolation in the deep mantle would require a cumulate overturn following crystallization of the magma ocean.
Mots-clés : cosmo
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Cosmochemistry
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