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  • 551  (31)
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  • English  (39)
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  • 1
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    The history of the Tissint meteorite, from its crystallization on Mars to its exposure in space: New geochemical, isotopic, and cosmogenic nuclide data (2021)
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    Publication Date: 2021-07-26
    Description: The Tissint meteorite fell on July 18, 2011 in Morocco and was quickly recovered, allowing the investigation of a new unaltered sample from Mars. We report new high-field strength and highly siderophile element (HSE) data, Sr-Nd-Hf-W-Os isotope analyses, and data for cosmogenic nuclides in order to examine the history of the Tissint meteorite, from its source composition and crystallization to its irradiation history. We present high-field strength element compositions that are typical for depleted Martian basalts (0.174 ppm Nb, 17.4 ppm Zr, 0.7352 ppm Hf, and 0.0444 ppm W), and, together with an extended literature data set for shergottites, help to reevaluate Mars’ tectonic evolution in comparison to that of the early Earth. HSE contents (0.07 ppb Re, 0.92 ppb Os, 2.55 ppb Ir, and 7.87 ppb Pt) vary significantly in comparison to literature data, reflecting significant sample inhomogeneity. Isotope data for Os and W (187Os/188Os = 0.1289 ± 15 and an ε182W = +1.41 ± 0.46) are both indistinguishable from literature data. An internal Lu-Hf isochron for Tissint defines a crystallization age of 665 ± 74 Ma. Considering only Sm-Nd and Lu-Hf chronometry, we obtain, using our and literature values, a best estimate for the age of Tissint of 582 ± 18 Ma (MSWD = 3.2). Cosmogenic radionuclides analyzed in the Tissint meteorite are typical for a recent fall. Tissint's pre-atmospheric radius was estimated to be 22 ± 2 cm, resulting in an estimated total mass of 130 ± 40 kg. Our cosmic-ray exposure age of 0.9 ± 0.2 Ma is consistent with earlier estimations and exposure ages for other shergottites in general.
    Keywords: 549.112
    Language: English
    Type: article
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  • 2
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    Hf-W chronology of a macrochondrule from the L5/6 chondrite Northwest Africa 8192 (2021)
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    Publication Date: 2021-07-26
    Description: A large, igneous-textured, and 2 cm-sized spherical object from the L5/6 chondrite NWA 8192 was investigated for its chemical composition, petrography, O isotopic composition, and Hf-W chronology. The petrography and chemical data indicate that this object closely resembles commonly found chondrules in ordinary chondrites and is therefore classified as a “macrochondrule.* As a result of metal loss during its formation, the macrochondrule exhibits elevated Hf/W, which makes it possible to date this object using the short-lived 182Hf-182W system. The Hf-W data provide a two-stage model age for metal–silicate fractionation of 1.4 ± 0.6 Ma after Ca-Al-rich inclusion (CAI) formation, indicating that the macrochondrule formed coevally to normal-sized chondrules from ordinary chondrites. By contrast, Hf-W data for metal from the host chondrite yield a younger model age of ~11 Ma after CAIs. This younger age agrees with Hf-W ages of other type 5–6 ordinary chondrites, and corresponds to the time of cooling below the Hf-W closure temperature during thermal metamorphism on the parent body. The Hf-W model age difference between the macrochondrule and the host metal demonstrates that the Hf-W systematics of the bulk macrochondrule were not disturbed during thermal metamorphism, and therefore, that the formation age of such objects can still be determined even in strongly metamorphosed samples. Collectively, this study illustrates that chondrule formation was not limited to mm-size objects, implying that the rarity of macrochondrules reflects either that this process was very inefficient, that subsequent nebular size-sorting decimated large chondrules, or that large precursors were rare.
    Keywords: 549.112
    Language: English
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  • 3
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    The brecciated texture of polymict eucrites: Petrographic investigations of unequilibrated meteorites from the Antarctic Yamato collection (2021)
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    Publication Date: 2021-07-26
    Description: We report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench-textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y-793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post-magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late-stage alteration processes that caused deposition of Fe-rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late-stage metasomatism model for their formation.
    Keywords: 549.112
    Language: English
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  • 4
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    Mid-infrared reflectance spectroscopy of aubrite components (2021)
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    Publication Date: 2021-07-26
    Description: Aubrites Peña Blanca Spring and Norton County were studied in the mid-infrared reflectance as part of a database for the MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission to Mercury. Spectra of bulk powder size fractions from Peña Blanca Spring show enstatite Reststrahlen bands (RB) at 9 µm, 9.3 µm, 9.9 µm, 10.4 µm, and 11.6 µm. The transparency feature (TF) is at 12.7 µm, the Christiansen feature (CF) at 8.1–8.4 µm. Micro-FTIR of spots with enstatite composition in Norton County and Peña Blanca Spring shows four types: Types I and II are similar to the bulk powder spectra but vary in band shape and probably display axis orientation. Type III has characteristic strong RB at 9.2 µm, 10.4 µm, and 10.5 µm, and at 11.3 µm. Type IV is characterized by a strong RB at 10.8−11.1 µm. Types III and IV could show signs of incipient shock metamorphism. Bulk results of this study confirm earlier spectral studies of aubrites that indicate a high degree of homogeneity and probably make the results of this study representative for spectral studies of an aubrite parent body. Spectral types I and II occur in all mineralogical settings (mineral clasts, matrix, melt, fragments in melt vein), while spectral type III was only observed among the clasts, and type IV in the melt. Comparison with surface spectra of Mercury does not obtain a suitable fit, only type IV spectra from quenched impact glass show similarity, in particular the 11 µm feature. Results of this study will be available upon request or via the IRIS database (Münster) and the Berlin Emissivity Database (BED).
    Keywords: 549.112
    Language: English
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  • 5
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    Classification of CM chondrite breccias—Implications for the evaluation of samples from the OSIRIS-REx and Hayabusa 2 missions (2021)
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    Publication Date: 2021-07-25
    Description: CM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM-like clasts in three achondrites by chemical analysis of the tochilinite-cronstedtite-intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2-2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0-CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4-CM2.6) or less altered fragments (CM2.7-CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO2 ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO2 ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0-CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS-REx and Hayabusa 2 missions in the future.
    Keywords: 549.112
    Language: English
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  • 6
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    Sectioning effects of porphyritic chondrules: Implications for the PP/POP/PO classification and correcting modal abundances of mineralogically zoned chondrules (2021)
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    Publication Date: 2021-07-24
    Description: 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.
    Keywords: 549.112
    Language: English
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  • 7
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    An unusual compound object in Yamato 793408 (H3.2-an): The missing link between compound chondrules and macrochondrules? (2021)
    Details
    Publication Date: 2021-07-23
    Description: 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.
    Keywords: 549.112
    Language: English
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  • 8
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    Middle to Late Pleistocene environments based on stable organic carbon and nitrogen isotopes of loess-palaeosol sequences from the Carpathian Basin (2021)
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    Publication Date: 2021-11-03
    Description: Stable organic carbon and nitrogen isotopes can be used to interpret past vegetation patterns and ecosystem qualities. Here we present these proxies for two loess-palaeosol sequences from the southern Carpathian Basin to reconstruct the palaeoenvironment during the past 350 ka and establish regional commonalities and differences. Until now, isotopic studies on loess sequences from this region were only conducted on deposits from the last glacial cycle. We conducted methodological tests concerning the complete decalcification of the samples prior to stable isotope analyses. Two decalcification methods (fumigation method and wet chemical acidification), different treatment times, and the reproducibility of carbon isotope analyses were tested. Obtained results indicate that the choice of the decalcification method is essential for organic carbon stable isotope analyses of loess-palaeosol sequences because ratios vary by more than 10‰ between the wet chemical and fumigation methods, due to incomplete carbonate removal by the latter. Therefore, we suggest avoiding the fumigation method for studies on loess-palaeosol sequences. In addition, our data show that samples with TOC content 〈0.2% bear increased potential for misinterpretation of their carbon isotope ratios. For our sites, C3-vegetation is predominant and no palaeoenvironmental shifts leading to a change of the dominant photosynthesis pathway can be detected during the Middle to Late Pleistocene. Furthermore, the potential for further stable nitrogen isotope studies is highlighted, since this proxy seems to reflect especially past precipitation patterns and reveals favourable conditions in the southern Carpathian Basin, especially during interstadials.
    Keywords: 551 ; southern Carpathian Basin ; loess-palaeosol sequences ; stable isotope analyses ; Pleistocene ecosystem reconstruction
    Language: English
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  • 9
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    Compound-Specific Radiocarbon Analysis of (Sub-)Antarctic Coastal Marine Sediments—Potential and Challenges for Chronologies (2021)
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    Publication Date: 2021-11-03
    Description: In Antarctic and Subantarctic environments, 14C-based age determination is often challenging due to unknown reservoir effects, low organic carbon contents of sediments, and high contributions of petrogenic (14C-free) carbon in ice marginal settings. In this study, we evaluate possible benefits and challenges of compound-specific radiocarbon analysis (CSRA) as a tool for age determination of marine Antarctic and Subantarctic sediment sequences. We present a comprehensive data set of 14C ages obtained on bulk organic carbon, carbonates, and on fatty acids (FA) from three coastal marine sediment cores from Subantarctic South Georgia and East Antarctica. Low molecular weight (LMW) FA represent the least 14C-depleted fraction, indicating that the phytoplankton-derived compounds can be a means of dating sediments. In contrast, vascular plant-derived high molecular weight FA are systematically depleted in 14C relative to the low molecular weight homologues, reflecting processes such as soil formation/erosion in the catchment. Comparative age-depth models show significant differences, depending on the material used for the respective models. While the land plant-derived FA may lead to an overestimation of the actual sediment age, LMW FA reveal complex aquatic reservoir effects. Bulk sedimentary organic carbon 14C ages likely provide appropriate age estimates in settings with low petrogenic carbon input in the Antarctic, whereas CSRA has the potential to produce improved age control in settings with high contributions of petrogenic carbon.
    Keywords: 551 ; Antarctica ; marine sediments ; compound‐specific radiocarbon analysis (CSRA)
    Language: English
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  • 10
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    Radiocarbon Evidence for the Contribution of the Southern Indian Ocean to the Evolution of Atmospheric CO2 Over the Last 32,000 Years (2021)
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    Publication Date: 2021-10-28
    Description: It is widely assumed that the ventilation of the Southern Ocean played a crucial role in driving glacial-interglacial atmospheric CO2 levels. So far, however, ventilation records from the Indian sector of the Southern Ocean are widely missing. Here we present reconstructions of water residence times (depicted as ΔΔ14C and Δδ13C) for the last 32,000 years on sediment records from the Kerguelen Plateau and the Conrad Rise (~570- to 2,500-m water depth), along with simulated changes in ocean stratification from a transient climate model experiment. Our data indicate that Circumpolar Deep Waters in the Indian Ocean were part of the glacial carbon pool. At our sites, close to or bathed by upwelling deep waters, we find two pulses of decreasing ΔΔ14C and δ13C values (~21–17 ka; ~15–12 ka). Both transient pulses precede a similar pattern in downstream intermediate waters in the tropical Indian Ocean as well as rising atmospheric CO2 values. These findings suggest that 14C-depleted, CO2-rich Circumpolar Deep Water from the Indian Ocean contributed to the rise in atmospheric CO2 during Heinrich Stadial 1 and also the Younger Dryas and that the southern Indian Ocean acted as a gateway for sequestered carbon to the atmosphere and tropical intermediate waters.
    Keywords: 551 ; radiocarbon ; ventilation ; Southern Ocean ; Younger Dryas ; carbon cycle ; Indian Ocean
    Language: English
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