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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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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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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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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Nicoli, G., E. Thomassot, M. Schannor, A. Vezinet, and I. Jovovic. "Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland." Lithos 296-299 (2018): 1–16.
Résumé : In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0 Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes (δ18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland. Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv)–recycling/reworking sequence: (I) During the 2.60–2.95 Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91 Ga with progressive increase of the discharge of reworked material (X juv from 75% to 50%;δ 18O: 4 – 8.5‰). (II) After a period of crustal stabilization(2.35– 2.60 Ga), discrete juvenile material inputs (δ18 O: 5 – 6 ‰ ) at T DM 2.35 Ga argue for the formation of an oceanic lithosphere and sea floor spreading over a period of ~0.2 Ga (Xjuv from b 25% to 70%). Lateral shortening is set to have started at ca. 2.05 Ga with the accretion of volcanic/magmatic arcs (i.e Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1: 520 ± 60 °C at 6.6 ± 1.4 kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84 – 1.89 Ga. Crustal thickening of ~25 km was accompanied by regional metamorphism M2 (690 ± 20 °C at 6.25 ± 0.25 kbar) and remobilization of pre-existing supracrustal lithologies (X juv ~40%; δ 18 O: 5 – 10.5 ‰).
Rates and durations obtained for sea floor spreading (175 ± 25 Ma), subduction (125 ± 75 Ma) and continental collision (ca. 60 Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere.
Mots-clés : cosmo
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Piani, L., H. Yurimoto, and L. Remusat. "A dual origin for water in carbonaceous asteroids revealed by CM chondrites." Nature Astronomy (2018).
Résumé : Carbonaceous asteroids represent the principal source of water in the inner Solar System and might correspond to the main contributors for the delivery of water to Earth. Hydrogen isotopes in water-bearing primitive meteorites, for example carbonaceous chondrites, constitute a unique tool for deciphering the sources of water reservoirs at the time of asteroid formation. However, fine-scale isotopic measurements are required to unravel the effects of parent-body processes on the pre-accretion isotopic distributions. Here, we report in situ micrometre-scale analyses of hydrogen isotopes in six CM-type carbonaceous chondrites, revealing a dominant deuterium-poor water component (δ D = − 350 ± 40‰) mixed with deuterium-rich organic matter. We suggest that this deuterium-poor water corresponds to a ubiquitous water reservoir in the inner protoplanetary
disk. A deuterium-rich water signature has been preserved in the least altered part of the Paris chondrite (δ DParis ≥ − 69 ± 163‰) in hydrated phases possibly present in the CM rock before alteration. The presence of the deuterium-enriched water signature in Paris might indicate that transfers of ice from the outer to the inner Solar System were significant within the first million years of the history of the Solar System.
Mots-clés : cosmo
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Pignatelli, I., E. Mugnaioli, and Y. Marrocchi. "Cronstedtite polytypes in the Paris meteorite." European Journal of Mineralogy 30 (2018): 349–354.
Résumé : We present the first detailed crystallo-chemical and crystallographic description of cronstedtite with extraterrestrial origin. The analysed crystals occur as pseudomorphs after anhydrous silicates in the Paris meteorite, which is the least altered carbonaceous chondrite currently known. Three-dimensional electron diffraction data have been collected by electron diffraction tomography and
used to identify the cronstedtite polytypes, because of their small size. All identified polytypes are MDO (Maximum degree of order), and the most abundant one is 1T. Many other crystals also belong to the Bailey’s group (or OD subfamily) C, but their stacking disorder hampers the polytype identification. Only in one crystal could the 3T and 2M1 polytypes be recognised. TEM-EDX data indicate that the chemical composition of the crystals is similar, regardless of their polytypic sequence.
Mots-clés : cosmo
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Qian, Y. Q., L. Xiao, S. Y. Zhao, J. N. Zhao, J. Huang, J. Flahaut, M. Martinot, J. W. Head, H. Hiesinger, and G. X. Wang. "Geology and Scientific significance of the Rümker region in Northern Oceanus Procellarum: China's Chang'E‐5 Landing region." Journal of Geophysical Research (Planets) (2018).
Résumé : The Rümker region (41‐45° N, 49‐69° W) is located in northern Oceanus Procellarum of the Moon. Mons Rümker is the most distinctive geological feature in the area. The region is characterized by prolonged lunar volcanism (Late Imbrian Period to Eratosthenian Period), forming multiple geologic units in the area, including very low‐Ti to low‐Ti mare basalts, high‐Ti mare basalts, and volcanic complexes. Each geologic unit has distinct element composition and mineral assemblages. The Rümker region, overlying the Procellarum KREEP Terrain, was selected as the landing region for China's Chang'E‐5 lunar sample return mission. Pre‐landing analysis of the geologic context and scientific potential are reported in this contribution. We conducted detailed geological mapping using image, spectral and altimetry data. Fourteen geological units were defined, a geologic map was constructed, and the geologic history was outlined. The western mare units (Im1, Im2, Im3) are Imbrian‐aged (~3.4‐3.5 Ga) representing the major stage of lunar mare eruptive volcanism. The eastern young mare units (Em3, Em4; <2 Ga) are among the youngest mare basalts on the Moon. They have never been explored in situ or studied in the laboratory. We suggest that samples returned from the eastern mare unit (Em4) could answer many fundamental questions and that this unit should be listed as the top priority landing site for Chang'E‐5 sample return mission.
Mots-clés : cosmo
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Tartèse, R., M. Chaussidon, A. Gurenko, F. Delarue, and F. Robert. "Insights into the origin of carbonaceous chondrite organics from their triple oxygen isotope composition." PNAS 115, no. 34 (2018): 8535–8540.
Résumé : Dust grains of organic matter were the main reservoir of C and N in the forming Solar System and are thus considered to be an essential ingredient for the emergence of life. However, the physical environment and the chemical mechanisms at the origin of these organic grains are still highly debated. In this study, we report highprecision triple oxygen isotope composition for insoluble organic
matter isolated from three emblematic carbonaceous chondrites, Orgueil, Murchison, and Cold Bokkeveld. These results suggest that the O isotope composition of carbonaceous chondrite insoluble organic matter falls on a slope 1 correlation line in the triple oxygen isotope diagram. The lack of detectable mass-dependent O isotopic fractionation, indicated by the slope 1 line, suggests that the bulk of carbonaceous chondrite organics did not form on asteroidal parent bodies during low-temperature hydrothermal events. On the other hand, these O isotope data, together with the H and N isotope characteristics of insoluble organic matter, may indicate that parent bodies of different carbonaceous chondrite types largely
accreted organics formed locally in the protosolar nebula, possibly by
photochemical dissociation of C-rich precursors.
Mots-clés : cosmo
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Vacher, L., Y. Marrocchi, J. Villeneuve, M. J. Verdier-Paoletti, and M. Gounelle. "Collisional and alteration history of the CM parent body." Geochimica et Cosmochimica Acta 239 (2018): 213–234.
Résumé : Boriskino is a little studied CM2 chondrite composed of millimeter-sized clasts of different lithologies and degrees of alteration. Boriskino thus offers a good opportunity to better understand the preaccretionary alteration history and collisional evolution that took place on the CM parent body. The least altered lithology displays 16O-poor Type 1a calcite and aragonite grains (d18O ~ 30–37‰, d17O ~ 15–18‰ and D17O~-2 to 0‰, SMOW) that precipitated early, before the establishment of the petrofabric, from a fluid whose isotopic composition was established by isotopic exchange between a 16O-poor water and 16O-rich anhydrous silicates. In contrast, the more altered lithologies exhibit 16O-rich Type 2a and veins of calcite (d18O ~ 17– 23‰, d17O ~ 6–9‰ and D17O~-4 to -1‰, SMOW) that precipitated after establishment of the deformation, from transported
16O-rich fluid in preexisting fractures. From our petrographic and X-ray tomographic results, we propose that the more altered lithologies of Boriskino were subjected to high intensity impact(s) (10–30 GPa) that produced a petrofabric, fractures and chondrule flattening. Taking all our results together, we propose a scenario for the deformation and alteration history of Boriskino, in which the petrographic and isotopic differences between the lithologies are explained by their separate locations into a single CM parent body. Based on the d13C-d18O values of the Boriskino Type 2a calcite (d13C ~ 30–71‰,PDB), we propose an alternative d13C-d18O model where the precipitation of Type 2a calcite can occurred in an open system environment with the escape of 13C-depleted CH4 produced from the reduction of C-bearing species by H2 released during serpentinization or kamacite corrosion. Assuming a mean precipitation temperature of 110 °C, the observed d13C variability in T2a calcite can be reproduced by the escape of ~15–50% of dissolved carbon into CH4 by Rayleigh distillation.
Mots-clés : cosmo
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2017 |
Avice, G., B. Marty, and R. Burgess. "The origin and degassing history of the Earth’s atmosphere revealed by Archean xenon." Nature Communications (2017).
Résumé : Xenon (Xe) is an exceptional tracer for investigating the origin and fate of volatile elements on Earth. The initial isotopic composition of atmospheric Xe remains unknown, as do the mechanisms involved in its depletion and isotopic fractionation compared with other reservoirs in the solar system. Here we present high precision analyses of noble gases trapped in fluid inclusions of Archean quartz (Barberton, South Africa) that reveal the isotopic composition of the paleo-atmosphere at E3.3 Ga. The Archean atmospheric Xe is massdependently fractionated by 12.9±2.4%u°/°°1 (±2s, s.d.) relative to the modern atmosphere. The lower than today 129Xe excess requires a degassing rate of radiogenic Xe from the mantle higher than at present. The primordial Xe component delivered to the Earth’s atmosphere is distinct from Solar or Chondritic Xe but similar to a theoretical component called U-Xe. Comets may have brought this component to the Earth’s atmosphere during the last stages of terrestrial accretion.
Mots-clés : cosmo
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Chaussidon, M., Z. Deng, J. Villeneuve, J. Moureau, B. Watson, F. Richter, and F. Moynier. "In situ analysis of non-traditional isotopes by SIMS and LA–MC–ICP–MS: key aspects and the example of Mg isotopes in olivines and silicate glasses." Reviews in Mineralogy and Geochemistry 182 (2017): 127–163.
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Faure, F., L. Tissandier, L. Florentin, and K. Devineau. "A magmatic origin for silica-rich glass inclusions hosted in porphyritic magnesian olivines in chondrules: An experimental study." Geochimica et Cosmochimica Acta 204 (2017): 19–31.
Résumé : Rare silica-rich glass inclusions (69 < SiO2 < 82 wt.%) are described within magnesian olivines of porphyritic Type IA chondrules. These glass inclusion compositions are clearly out of equilibrium with their host Mg-olivines and their presence within the olivines is generally attributed to an unclear secondary process such as a late interaction with nebular gases. We performed dynamic crystallisation experiments that demonstrate that these Si-rich glass inclusions are actually magmatic in origin and were trapped inside olivines that crystallized slowly from a magma with a CI, i.e. solar, composition. Their silicarich
compositions are the consequence of the small volumes of inclusions, which inhibit the nucleation of secondary crystalline phase (Ca-poor pyroxene) but allow olivine to continue to crystallize metastably on the walls of the inclusions. We suggest
that Si-rich glass inclusions could be the only reliable relicts of what were the first magmas of the solar system, exhibiting a CI, i.e. non-fractionated, composition.
Mots-clés : cosmo
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Füri, E., E. Deloule, and R. Trappitsch. "The production rate of cosmogenic deuterium at the Moon’s surface." Earth and Planetary Science Letters 474 (2017): 76–82.
Résumé : The hydrogen (D/H) isotope ratio is a key tracer for the source of planetary water. However, secondary processes such as solar wind implantation and cosmic ray induced spallation reactions have modified the primordial D/H signature of ‘water’ in all rocks and soils recovered on the Moon. Here, we re-evaluate the production rate of cosmogenic deuterium (D) at the Moon’s surface through ion microprobe analyses of hydrogen isotopes in olivines from eight Apollo 12 and 15 mare basalts. These in situ measurements are complemented by CO2laser extraction-static mass spectrometry analyses of cosmogenic noble gas nuclides (3He, 21Ne, 38Ar). Cosmic ray exposure (CRE) ages of the mare basalts, derived from their cosmogenic 21Ne content, range from 60 to 422 Ma. These CRE ages are 35% higher, on average, than the published values for the same samples. The amount of D detected in the olivines increases linearly with increasing CRE ages, consistent with a production rate of (2.17 ±0.11) ×10−12mol(g rock)−1Ma−1. This value is more than twice as high as previous estimates for the production of D by galactic cosmic rays, indicating that for water-poor lunar samples, i.e., samples with water concentrations ≤50 ppm, corrected D/H ratios have been severely overestimated
Mots-clés : cosmo
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Florentin, L., F. Faure, E. Deloule, L. Tissandier, A. Gurenko, and D. Lequin. "Origin of Na in glass inclusions hosted in olivine from Allende CV3 and Jbilet Winselwan CM2: Implications for chondrule formation." Earth and Planetary Science Letters 474 (2017): 160–171.
Résumé : Glass inclusions trapped in Mg-rich olivines within type I chondrules from the Allende (CV3) and Jbilet Winselwan (CM2) chondrites were analyzed by EPMA (Electron Probe Microanalysis) for major elements and by SIMS (Secondary Ion Mass Spectrometry) for Cl and S (analyzed here for the first time in chondrule-hosted glass inclusions). The inclusions from Jbilet Winselwan are poor in Na2O, whereas those from Allende are Na-rich, displaying up to 8wt.% Na2O. The source of Na is a central issue in terms of chondrule origins because of the volatility of Na at high temperature. The wide scatter in Na2O contents of olivine-hosted glass inclusions from chondrules has led the community to propose that Na2O came from late interactions of chondrules with a Si/Na-rich gas. To gain new insights into the origins of the Na2O recorded in glass inclusions, heating experiments (up to 1810◦C) were performed on Allende inclusions in an effort to constrain the initial composition of the trapped melts. Our results demonstrate that sodium (although volatile) does not escape from inclusions during heating, thus confirming that glass inclusions behave as closed systems. Furthermore, heated olivines still bear inclusions containing up to 7.2wt.% of Na2O. Olivines are thought to form at temperatures at which Na is volatile. This implies that (1) Na from glass inclusions cannot come from condensation but rather results from trapping in a Na-rich environment, which implies a high pressure, as in a melting planetasimal (2) there may be two distinct origins for the sodium: an indigenous origin for the sodium trapped inside glass inclusions and a gaseous origin for the sodium recorded in mesostasis from chondrules. Consequently, these results are in favor of a planetesimal origin for olivine from chondrules.
Mots-clés : cosmo
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Cosmochemistry
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