2020 |
Anzures, B. A., S. W. Parman, R. E. Milliken, O. Namur, C. Cartier, and S. Wang. "Effect of sulfur speciation on chemical and physical properties of very reduced mercurian melts." Geochimica et Cosmochimica Acta 286 (2020): 1–18.
Résumé : The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5–4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated fO2 between IW-3 and IW-7). Due to decreasing O availability at these low fO2 conditions, and an abundance of S2-, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250–1475 °C and fO2 between IW-2.5 to IW-7.5. SK-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As fO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ˜IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ˜IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low fO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low fO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.
Mots-clés : cosmo
|
|
Barosch, J., D. C. Hezel, Y. Marrocchi, A. Gurenko, and C. Lenting. "An unusual compound object in Yamato 793408 (H3.2-an) : The missing link between compound chondrules and macrochondrules ?" Meteoritics & Planetary Science 55 (2020): 1458–1470.
Résumé : We found a large ( 2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no 16O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s−1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.
Mots-clés : cosmo
|
|
Barosch, J., D. C. Hezel, L. Sawatzki, L. Halbauer, and Y. Marrocchi. "Sectioning effects of porphyritic chondrules : Implications for the PP/POP/PO classification and correcting modal abundances of mineralogically zoned chondrules." Meteoritics & Planetary Science 55, no. 5 (2020): 993–999.
Résumé : Mineralogically zoned chondrules are a common chondrule type in chondrites. They consist of olivine cores, surrounded by low-Ca pyroxene rims. By serial sectioning porphyritic chondrules from carbonaceous, ordinary, and enstatite chondrites, we demonstrate that the 2-D textural appearances of these chondrules largely depend on where they are cut. The same chondrule may appear as a porphyritic pyroxene (PP) chondrule when sectioned through the low-Ca pyroxene rim, and as a porphyritic olivine-pyroxene (POP) or porphyritic olivine (PO) chondrule when sectioned close or through its equator. Chondrules previously classified into PP/POP/PO chondrules might therefore not represent different types, but various sections through mineralogically zoned chondrules. Classifying chondrule textures into PP, POP, and PO has therefore no unequivocal genetic meaning, it is merely descriptive. Sectioning effects further introduce a systematic bias when determining mineralogically zoned chondrule fractions from 2-D sections. We determined correction factors to estimate 3-D mineralogically zoned chondrule fractions when these have been determined in 2-D sections : 1.24 for carbonaceous chondrites, 1.29 for ordinary chondrites, and 1.62 for enstatite chondrites. Using these factors then shows that mineralogically zoned chondrules are the dominant chondrule type in chondrites with estimated 3-D fractions of 92% in CC, 52% in OC, and 46% in EC.
Mots-clés : cosmo
|
|
Bekaert, D. V., M. W. Broadley, and B. Marty. "The origin and fate of volatile elements on Earth revisited in light of noble gas data obtained from comet 67P/Churyumov-Gerasimenko." Scientific Reports 10 (2020): 5796.
Résumé : The origin of terrestrial volatiles remains one of the most puzzling questions in planetary sciences. The timing and composition of chondritic and cometary deliveries to Earth has remained enigmatic due to the paucity of reliable measurements of cometary material. This work uses recently measured volatile elemental ratios and noble gas isotope data from comet 67P/Churyumov-Gerasimenko (67P/C-G), in combination with chondritic data from the literature, to reconstruct the composition of Earth’s ancient atmosphere. Comets are found to have contributed ~20% of atmospheric heavy noble gases (i.e., Kr and Xe) but limited amounts of other volatile elements (water, halogens and likely organic materials) to Earth. These cometary noble gases were likely mixed with chondritic – and not solar – sources to form the atmosphere. We show that an ancient atmosphere composed of chondritic and cometary volatiles is more enriched in Xe relative to the modern atmosphere, requiring that 8–12 times the present-day inventory of Xe was lost to space. This potentially resolves the long-standing mystery of Earth’s “missing xenon”, with regards to both Xe elemental depletion and isotopic fractionation in the atmosphere. The inferred Kr/H2O and Xe/H2O of the initial atmosphere suggest that Earth’s surface volatiles might not have been fully delivered by the late accretion of volatile-rich carbonaceous chondrites. Instead, “dry” materials akin to enstatite chondrites potentially constituted a significant source of chondritic volatiles now residing on the Earth’s surface. We outline the working hypotheses, implications and limitations of this model in the last section of this contribution
Mots-clés : cosmo
|
|
Boulliung, J., E. Füri, C. Dalou, L. Tissandier, L. Zimmermann, and Y. Marrocchi. "Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts." Geochimica et Cosmochimica Acta 284 (2020): 120–133.
Résumé : Knowledge of N solubility in silicate melts is key for understanding the origin of terrestrial N and the distribution andexchanges of N between the atmosphere, the silicate magma ocean, and the core forming metal. To place constraints onthe incorporation mechanism(s) of N in silicate melts, we investigated the effect of the oxygen fugacity (fO2) and melt com-position on the N solubility through N equilibration experiments at atmospheric pressure and high temperature (1425°C).Oxygen fugacity (expressed in log units relative to the iron-wu ̈stite buffer, IW) was varied from IW –8 to IW +4.1, and meltcompositions covered a wide range of polymerization degrees, defined by the NBO/T ratio (the number of non-bridging oxy-gen atoms per tetrahedrally coordinated cations). The N contents of the quenched run products (silicate glasses) were ana-lyzed byin-situsecondary ion mass spectrometry and bulk CO2laser extraction static mass spectrometry, yielding resultsthat are in excellent agreement even for N concentrations at the (sub-)ppm level. The data obtained here highlight the fun-damental control offO2and the degree of polymerization of the silicate melt on N solubility. Under highly reduced conditions(fO2= IW –8), the N solubility increased with increasing NBO/T from 17.4 ± 0.4 ppm.atm-1/2in highly polymerized melts(NBO/T = 0) to 6710 ± 102 ppm.atm-1/2in depolymerized melts (NBO/T˜2.0). In contrast, under less reducing conditions(fO2> IW –3.4), N solubility is very low (≤2 ppm.atm-1/2), irrespective of the NBO/T value. Our results provide constraintson N solubility in enstatite chondrite melts and in the shallow part of a planetary magma ocean. The nitrogen storage capacityof an enstatite chondrite melt, which may approximate that of planetesimals that accreted and melted early in the inner SolarSystem, varies between ˜60 and ˜6000 ppm at IW –5.1 and IW –8, respectively. In contrast, a mafic to ultra-mafic magmaocean could have incorporated ˜0.3 ppm to ˜35 ppm N under thefO2conditions inferred for the young Earth (i.e., IW –5 toIW). The N storage capacity of a reduced magma ocean (i.e., IW –3.4 to IW) in equilibrium with a N-rich atmosphere is ≤1 ppm, comparable to the N content of the present-day mantle. However under more reducing conditions (i.e., IW –5 toIW –4), the N storage capacity is significantly higher (˜35 ppm) ; in this case, Earth would have lost N to the atmosphereand/or N would have been transported into and stored within its deep interior (i.e., deep mantle, core).
Mots-clés : cosmo
|
|
Broadley, M. W., D. V. Bekaert, B. Marty, A. Yamaguchi, and J. A. Barrat. "Noble gas variations in ureilites and their implications for ureilite parent body formation." Geochimica et Cosmochimica Acta 270 (2020): 325–337.
Résumé : Ureilites are equilibrated carbon-rich olivine-pyroxene rocks from the partially melted mantle of a large (>500 km diameter) heterogeneous parent body. Recently the ureilite parent body was interpreted as an incomplete mixture of material from two carbon-rich chondritic reservoirs, one (Mg-rich) with reduced iron, low Δ17O and low δ13C, and the other with oxidised iron, high Δ17O and high δ13C. Here we analyse noble gases (Ar, Kr and Xe) in six equilibrated (unbrecciated) ureilites from Northwest Africa (NWA 2236, NWA 7686, NWA 8049, NWA 8172, NWA 11032 and NWA 11368). We observe weak positive and negative correlations of Δ17O and Mg# with the elemental ratios of Ar/Xe and Kr/Xe, respectively, as well as a weak positive correlation of Mg# with the heavy isotopes of Xe. These correlations broadly support the idea of the two-component mixing hypothesis. Our analyses further suggest that the Mg-rich endmember was rich in Xe from presolar grains (HL-Xe) while the Mg-poorer component may have contained solar-derived noble gases. The observed correlations are less straightforward to reconcile with a recent model for the origin of the ureilite parent body, involving oxidation of metal by H2O from accreted ice with ‘heavy’ oxygen isotopes.
Mots-clés : cosmo
|
|
|
Cartier, C. "Book review: Planetary Geoscience." In European Journal of Mineralogy, 501–503. Vol. 32., 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
|
|
Deng, Z., F. Moynier, J. Villeneuve, N. K. Jensen, D. Liu, P. Cartigny, T. Mikouchi, J. Siebert, A. Agranier, M. Chaussidon et al. "Early oxidation of the martian crust triggered by impacts." Science Advances 6, no. 44 (2020): eabc4941.
Résumé : Despite the abundant geomorphological evidence for surface liquid water on Mars during the Noachian epoch (>3.7 billion years ago), attaining a warm climate to sustain liquid water on Mars at the period of the faint young Sun is a long-standing question. Here, we show that melts of ancient mafic clasts from a martian regolith meteorite, NWA 7533, experienced substantial Fe-Ti oxide fractionation. This implies early, impact-induced, oxidation events that increased by five to six orders of magnitude the oxygen fugacity of impact melts from remelting of the crust. Oxygen isotopic compositions of sequentially crystallized phases from the clasts show that progressive oxidation was due to interaction with an 17O-rich water reservoir. Such an early oxidation of the crust by impacts in the presence of water may have supplied greenhouse gas H2 that caused an increase in surface temperature in a CO2-thick atmosphere.
Mots-clés : cosmo
|
|
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
|
|
Füri, E., L. Zimmermann, E. Deloule, and R. Trappitsch. "Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018." Earth and Planetary Science Letters 550 (2020): 116550.
Résumé : Exposure of rocks and regolith to solar (SCR) and galactic cosmic rays (GCR) at the Moon’s surface results in the production of ‘cosmogenic’ deuterium and noble gas nuclides at a rate that depends on a complex set of parameters, such as the energy spectrum and intensity of the cosmic ray flux, the chemical composition, size, and shape of the target as well as the shielding depth. As the effects of cosmic rays on the D production in lunar samples remain poorly understood, we determine here the D content and noble gas (He-Ne-Ar) characteristics of nominally anhydrous mineral (olivine and pyroxene) grains and rock fragments, respectively, from different documented depths (0 to ≥4.8cm) within Apollo olivine basalt 12018. Deuterium concentrations, determined by secondary ion mass spectrometry, and cosmogenic 3He, 21Ne, and 38Ar abundances, measured by CO2laser extraction static mass spectrometry, are constant over the depth range investigated. Neon isotope ratios (20Ne/22Ne ≈0.86 and 21Ne/22Ne ≈0.85) of the cosmogenic endmember are comparable to the theoretical signature of GCR-produced neon. These observations indicate that the presence of significant amounts of SCR nuclides in the studied sub-samples can be ruled out. Hence, D within the olivines and pyroxenes must have been predominantly produced in situby GCR-induced spallation reactions during exposure at the lunar surface. Comparison of the amount of D with the 21Ne (184 ±26Ma) or 38Ar (193 ±25Ma) exposure ages yields a D production rate that is in good agreement with the value of (2.17 ±0.11) ×10−12mol(grock)−1Ma−1from Füri et al. (2017). These results confirm that cosmic ray effects can substantially alter the hydrogen isotope (D/H) ratio of indigenous ‘water’ in returned extraterrestrial samples and meteorites with long exposure ages.
Mots-clés : cosmo
|
|
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
|
|
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
|
|
Greer, J., G. Caro, N. L. Cates, P. Tropper, W. Bleeker, N. M. Kelly, and S. J. Mojzsis. "Widespread poly-metamorphosed Archean granitoid gneisses and supracrustal enclaves of the southern Inukjuak Domain, Québec (Canada)." Lithos 364-365 (2020): 105520.
Résumé : The ~12,000 km2 Inukjuak Domain in northern Québec is part of the Archean Minto Block in the northwestern Superior Province of Canada. Eoarchean (ca. >3800–3780 Ma) rocks of the Nuvvuagittuq supracrustal belt (NSB) are the best known occurrence of otherwise abundant <1 m to km-scale supracrustal enclaves dispersed throughout the Innuksuac Complex. The supracrustals are dominantly amphibolites, with subordinate intermediate-, mafic- and ultramafic schists, quartzo-feldspathic (trondhjemitic and granodioritic) sills, dikes and sheets, banded iron-formations and quartz-pyroxene±magnetite rocks, and (detrital) fuchsitic quartzites. Supracrustal assemblages are in turn hosted by variably deformed granite-granitoid gneisses metamorphosed to amphibolite facies. Locally, retrogression is expressed as pervasive chloritization and development of jaspilite box veinings. This metamorphic history precludes preservation of original fragile microfossil shapes. Despite its importance as one of the few terranes to host Eoarchean supracrustal assemblages, limited geochronology was previously available for rocks beyond the ~8 km2 NSB. Here, we report new major-, minor-, and trace-element geochemistry and metamorphic petrology coupled with Usingle bondPb zircon geochronology, from rocks within and surrounding the NSB. These include the little-studied but volumetrically significant Voizel suite gneisses. Results show that intra-NSB fold belt rocks of the Central Tonalitic Gneiss (CTG) preserve mainly ca. 3650 Ma zircons. Beyond the NSB, the Voizel gneisses – previously considered contemporaneous with the CTG – are instead about 100 Myr younger (~3550 Ma). Tonalitic (ortho)gneisses at the margin of the NSB were previously assigned a ca. 3650 Ma age, and the surrounding Boizard suite gneisses may be about 2700 Ma. We find the Boizard rocks contain inherited zircon cores up to ca. 3700 Ma, with younger overgrowths dated at ca. 2700 Ma. A tonalitic gneiss that transects another highly deformed supracrustal enclave north of the NSB – dubbed the Ukaliq Supracrustal Belt – yields maximum concordant zircon ages of 3653 ± 16 Ma (2σ). Detrital zircons from Ukaliq and Nuvvuagittuq quartzites and quartz-biotite schists define a maximum age of ca. 3780 Ma. No indication of Usingle bondPb zircon ages older than about 3800 Ma exist in this terrane from our geochronological surveys. New reconnaissance sampling of gneisses to the west of the NSB fold belt yielded more zircon-bearing rocks with ages of ca. 3760 Ma. This discovery of more pre-3700 Ma rocks beyond the NSB outcrops calls attention to the existence of hitherto unidentified but widespread scattered occurrences of Eoarchean and Paleoarchean rocks throughout the region.
Mots-clés : cosmo
|
|
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
|
|
Marrocchi, Y., L. Bonal, J. Gattacceca, L. Piani, P. Beck, R. Greenwood, J. Eschrig, A. Basque, P. M. Nuccio, and F. F. Martin. "The Piancaldoli meteorite : A forgotten primitive LL3.10 ordinary chondrite." Meteoritics & Planetary Science 55, no. 8 (2020): 1924–1935.
Résumé : The Piancaldoli ordinary chondrite fell in northern Italy on August 10, 1968. Preliminary studies led to its classification as an LL3.4 unequilibrated ordinary chondrite. However, recent developments in classification procedures have prompted us to re-examine its mineralogical, petrographic, spectroscopic, chemical, and isotopic features in a multitechnique study. Raman spectra and magnetic properties indicate that Piancaldoli experienced minimal thermal metamorphism, consistent with its high bulk hydrogen content and the Cr contents of ferroan olivines in its type II chondrules. In combination with findings of previous studies, our data thus confirm the variability of Cr contents in ferroan olivines in type II chondrules as a proxy of thermal metamorphism. Furthermore, our results reveal that Piancaldoli is less altered than previously reported and should be reclassified as an LL3.10 unequilibrated ordinary chondrite. Our results also imply that the bulk deuterium enrichment, as observed in Piancaldoli (LL3.10), Bishunpur (LL3.15), and Semarkona (LL3.00), is a specific signature of the most primitive unequilibrated ordinary chondrites. Based on our results, we propose that, to date, Piancaldoli is the second leastaltered unequilibrated ordinary chondrite fall after Semarkona. This work reiterates the importance of meteorite collections worldwide as fundamental resources for studying the formation conditions and evolution of our solar system.
Mots-clés : cosmo
|
|
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
|
|
Marty, B., M. Almayrac, P. H. Barry, D. Bekaert, M. W. Broadley, D. J. Byrne, C. J. Ballentine, and A. Caracausi. "An evaluation of the C/N ratio of the mantle from natural CO2-rich gas analysis : Geochemical and cosmochemical implications." Earth and Planetary Science Letters 551 (2020): 116574.
Résumé : The terrestrial carbon to nitrogen ratio is a key geochemical parameter that can provide information on the nature of Earth’s precursors, accretion/differentiation processes of our planet, as well as on the volatile budget of Earth. In principle, this ratio can be determined from the analysis of volatile elements trapped in mantle-derived rocks like mid-ocean ridge basalts (MORB), corrected for fractional degassing during eruption. However, this correction is critical and previous attempts have adopted different approaches which led to contrasting C/N estimates for the bulk silicate Earth (BSE) (Marty and Zimmermann, 1999 ; Bergin et al., 2015). Here we consider the analysis of CO2-rich gases worldwide for which a mantle origin has been determined using noble gas isotopes in order to evaluate the C/N ratio of the mantle source regions. These gases experienced little fractionation due to degassing, as indicated by radiogenic ⁎ values (where 4He and 40Ar* are produced by the decay of U+Th, and 40K isotopes, respectively) close to the mantle production/accumulation values. The C/N and ratios of gases investigated here are within the range of values previously observed in oceanic basalts. They point to an elevated mantle C/N ratio (∼350-470, molar) higher than those of potential cosmochemical accretionary endmembers. For example, the BSE C/N and ratios (160-220 and , respectively) are higher than those of CM-CI chondrites but within the range of CV-CO groups. This similarity suggests that the Earth accreted from evolved planetary precursors depleted in volatile and moderately volatile elements. Hence the high composition of the BSE may be an inherited feature rather than the result of terrestrial differentiation. The and ratios of the surface (atmosphere plus crust) and of the mantle cannot be easily linked to any known chondritic composition. However, these compositions are consistent with early sequestration of carbon into the mantle (but not N and noble gases), permitting the establishment of clement temperatures at the surface of our planet.
Mots-clés : cosmo
|
|
Piani, L., Y. Marrocchi, T. Rigaudier, L. G. Vacher, D. Thomassin, and B. Marty. "Earth’s water may have been inherited from material similar to enstatite chondrite meteorites." Science 369, no. 6507 (2020): 1110–1113.
Résumé : The origin of Earth’s water remains unknown. Enstatite chondrite (EC) meteorites have similar isotopic composition to terrestrial rocks and thus may be representative of the material that formed Earth. ECs are presumed to be devoid of water because they formed in the inner Solar System. Earth’s water is therefore generally attributed to the late addition of a small fraction of hydrated materials, such as carbonaceous chondrite meteorites, which originated in the outer Solar System where water was more abundant. We show that EC meteorites contain sufficient hydrogen to have delivered to Earth at least three times the mass of water in its oceans. EC hydrogen and nitrogen isotopic compositions match those of Earth’s mantle, so EC-like asteroids might have contributed these volatile elements to Earth’s crust and mantle.
Mots-clés : cosmo
|
|
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
|
|
Cosmochemistry
RSS 2.0