Shock-metamorphic microstructures in quartz grains from Albian sandstones from the Tin Bider impact structure, Algeria

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¹Kassab, ²Ludovic Ferrière, ¹Djelloul Belhai
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13766]
¹Department of Geology, University of Sciences and Technologies Houari Boumediene, Algiers, Algeria
²Natural History Museum Vienna, Burgring 7, Vienna, A-1010 Austria
Published by arrangement with John Wiley & Sons

Tin Bider is a 6-km-diameter complex impact structure, the largest one recognized in Algeria. The crater was excavated in Cretaceous sedimentary rocks composed of, from the base to the top, Albian sandstones, Cenomanian clays, Cenomanian-Turonian limestones, undifferentiated Coniacian to Maastrichtian clays and limestones. The age of the impact event is poorly constrained to <66 Ma by stratigraphy, the youngest geological unit affected by the event being the ˜66 Myr old Maastrichtian limestones. Albian sandstones outcrop in the central sector of the structure and represent the only occurrence at outcrop of this geological unit in the structure. Here we report on a detailed petrographic analysis of eight Albian sandstone samples that were searched for shock-metamorphic features. We confirm the presence of rare shocked quartz grains with planar deformation features (PDFs) and report on their crystallographic orientations as determined using the universal stage microscope. PDFs oriented parallel to the π{10121} and ω{1013} orientations are the most abundant ones. For the first time in impactites from Tin Bider, PDFs with basal (0001) orientation, corresponding to amorphized mechanical Brazil twins, are reported. Our results indicate that locally the peak shock pressure was of at least 20 GPa, but much lower in average for the investigated samples.

Processes and temperatures of FGR formation in chondrites

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^1,2P-M.Zanetta,¹C.Le Guillou,¹H.Leroux,^3,4B.Zandab,^2,3R.Hewins,¹G.Bellino
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.11.019]
¹Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 – UMET – Unité Matériaux et Transformations, F-59000 Lille, France
²IMPMC, Sorbonne Université, MNHN, UPMC Paris, UMR CNRS 7590, 75005 Paris, France
³EPS, Rutgers Univ., Piscataway, NJ 08854, USA
⁴Observatoire de Paris, IMCCE,75014 Paris, France
Copyright Elsevier

In order to understand the nature of the dust that accreted onto chondrules in the nebula and to unravel the conditions of formation of fine grained rims (FGRs), we studied three of the least altered chondrites from different chondrite groups (LL3.00 Semarkona, CO3.0 DOM 08006, CR2.8 QUE 99177) and compared the results with our previous work on the Paris CM chondrite (Zanetta et al., 2021). For each sample, we selected representative rimmed chondrules showing minimal traces of aqueous alteration. We performed high-resolution SEM X-ray chemical mapping to obtain relevant phase abundances and grain size distributions. Four FIB sections were then extracted from each meteorite, two in the rims and two in their adjacent matrix for quantitative TEM analysis. At the microscale, texture, modal abundances and grain size differ depending on the chondrite but also between FGRs and their adjacent matrix. At the nanoscale (i.e. TEM observations), matrices of the four chondrites consist mostly of domains of amorphous silicate associated with Fe-sulfides, Fe-Ni metal, Mg-rich anhydrous silicates and an abundant porosity. The related FGRs in Semarkona (LL) and DOM 08006 (CO) exhibit more compact textures with a lower porosity while FGRs in QUE99177 (CR) are similar to the matrix in terms of porosity. In the three chondrites, FGRs are made of smooth and chemically homogeneous amorphous (or nanocrystalline) silicate with no porosity that encloses domains of porous amorphous silicate bearing Mg-rich anhydrous silicates, Fe-sulfides, Fe-oxides and sometimes metal and Fe-rich olivines. The average compositions in major elements of the amorphous regions are similar for the FGRs and the matrix within a given chondrite (but differ between chondrites). The texture and the chemical homogeneity of the smooth silicate and the fact that it encloses domains of porous amorphous silicate bearing other mineral phases similar to matrix-like material suggests a formation by condensation. Areas that are enclosed in this smooth silicate exhibit Fe-rich olivine formed through Fe interdiffusion that also suggest a thermal modification of the dust accreted to form FGRs. These characteristics indicate a transformation process for the modification of the FGR material similar to the one proposed in our previous work on Paris. We conclude that matrix and FGRs accreted a similar type of dust but FGR material was affected by thermal modification and compaction contemporary with their accretion. For each chondrite, dust accreted onto chondrules under different conditions (dust density, temperature) which led to diverse degrees of compaction/thermal modification of the sub-domains and explain the textural differences observed in FGRs. They accreted on chondrules in a warm environment related to the chondrule formation episode, whereas matrix accreted later in a cooler environment.

Dolomites In Hydrated Fine-Grained Antarctic Micrometeorites: Effective Tools For Analyzing Secondary Processes

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¹E.Dobrică,¹K.K.Ohtaki,²C.Engrand
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.11.018]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science, and Technology, the University of Hawai’i at Mānoa, Honolulu, Hawai‘i 96822 USA
²Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay Campus, France
Copyright Elsevier

We report detailed transmission electron microscope (TEM) observations of carbonates from one hydrated fine-grained Antarctic micrometeorite (H-FgMM). These carbonates show the occurrence of complex chemical variations and microstructures that provide important evidence regarding the formation and evolution of rarely analyzed H-FgMMs. The chemical variations were identified at both micrometer and nanometer scales, indicating that these carbonates formed under localized fluid conditions that suggest a variable chemical microenvironment. Individual carbonates grew from isolated reservoirs of fluid. Moreover, these carbonates contain manganese amounts almost twice as high as those measured in CM chondrites but similar to those identified in CI chondrites. Their particular compositions indicate reducing and progressively evolving conditions in the fluid from which these carbonates precipitated, probably due to water consumption during phyllosilicates formation. In addition to the compositional variability, microstructural features are pervasive in these carbonates, similar to those described in heavily shocked meteorites indicating that these carbonates were probably modified during shock processes after their formation. Since carbonates are highly susceptible to shock metamorphism, we suggest that it is essential to investigate their structure in detail before interpreting the isotopic measurements related to the time of their formation. Additionally, associated with carbonates, ubiquitous phosphates were identified in the micrometeorite analyzed. Future studies of these mineral associations will provide us further insight into the formation and evolution of asteroids, especially since they were both identified in the surface materials of Ryugu and Bennu.

A 187Re-187Os, 87Rb-86Sr, highly siderophile and incompatible trace element study of some carbonaceous, ordinary and enstatite chondrite meteorites

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¹Nicole Phelan et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.11.020]
¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA
Copyright Elsevier

New ¹⁸⁷Re-¹⁸⁷Os, ⁸⁷Rb-⁸⁷Sr, triple O-isotope isotope, bulk rock highly siderophile- (HSE: Os, Ir, Ru, Pt, Pd, Re), major- and trace-element abundance data are reported for a variety of carbonaceous, ordinary and enstatite chondrite meteorites. In addition, new mineral chemical data are reported for the Chelyabinsk LL5 ordinary chondrite fall for comparison with existing chondrite data and to investigate element sequestration into metal and mineral phases within some chondrites. The focus of the study is to link the variations observed in the HSE abundances and Re-Os isotopes with other isotopic and elemental data to explore the relative roles of sample sizes, terrestrial alteration and parent body processes more fully on chondrite meteorite compositions. Trace element variations in Chelyabinsk silicate, oxide and metal grains highlight the importance of geochemical heterogeneity imparted by mineralogical variations and mode effects, as well as sample size. Using a range of sample powder aliquot sizes, it is possible to show that this becomes significant for the HSE at <0.1 g. Variations in high field strength elements relative abundances (HFSE: Ti, Zr, Nb, Ta, Hf) are also identified within individual aliquots of carbonaceous chondrite Ivuna, emphasizing the importance of complete dissolution of refractory phases. The range of fall and find meteorites examined here demonstrates that terrestrial alteration effects revealed for trace elements (e.g., Ba, U, Sr) do not correlate particularly well with Re/Os variations. Instead, the Re/Os ratios of carbonaceous chondrites are susceptible to disturbance, more so than indicated by incompatible trace element systematics, with the Murchison CM2 carbonaceous chondrite showing significant Re/Os fractionation between sample aliquots. For sample aliquots measured that do not show significant mode or terrestrial alteration effects, parent body processes appear to be largely restricted to thermal metamorphism and dehydration. Including data for this study, the combined published dataset for Re-Os isotope and HSE abundances now extends to 33 ordinary, 39 carbonaceous, 27 enstatite and 6 Rumuruti chondrites. The range in absolute HSE abundances among these meteorite groups is ∼30%, with all chondrites having, within uncertainties, the same average Os, Ir, Ru, Pt and Pd abundances. Notably, carbonaceous chondrites have long-term Re/Os ∼8% lower than for the other chondrite groups. If chondrite groups are representative of early planetary feedstocks, then the measured ¹⁸⁷Os/¹⁸⁸Os of ordinary chondrites make them a close match to the composition of the bulk silicate Earth. Assuming ∼0.5% late accretion of ordinary chondrites to Earth, this would result in a long-term Rb/Sr ratio ∼0.6% higher than from late accretion of carbonaceous chondrites, indicating that ordinary chondrites are a potentially attractive source for moderately volatile enrichment.

Alkali magmatism on mars: An unexpected diversity

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¹Sautter V.,^2,3Payre V.
Comptes Rendus – Geoscience 353, 64 Link to Article [DOI 10.5802/CRGEOS.64]
¹IMPMC-UMR, CNRS 7590, Sorbonnne Université, 61 rue Buffon, Paris, 75231, France
²Rice University, Houston, 77005-1892, TX, United States
³Department of Physics and Astronomy, Northern Arizona University, Flagstaff, 86011, AZ, United States