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  1. More than 20,000 siderite concretions from the Mazon Creek area of northern Illinois, United States are housed in the paleobotanical collections of the Field Museum. A large proportion contain fossil plants of Middle Pennsylvanian age that often have excellent three-dimensional morphology and sometimes anatomical detail. Approximately eighty plant taxa have been recognized from the Mazon Creek Lagerstätte, but few have been studied in detail, and in some cases the systematic affinities of these fossils need reevaluation. The three-dimensional (3D) preservation of Mazon Creek fossil plants makes them ideal candidates for study using x-ray micro-computed tomography (μCT), and here we apply these techniques to more accurately reconstruct the morphology of specimens of Tetraphyllostrobus Gao et Zodrow and Crossotheca Zeiller. The mineralogical composition of the fossil plant preservation was studied using elemental maps and Raman spectroscopy. In-situ spores were studied with differential interference contrast, Airyscan confocal super-resolution microscopy, and scanning electron microscopy, which reveal different features of the spores with different degrees of clarity. Our analyses show that μCT can provide excellent detail on the three-dimensional structure of Mazon Creek plant fossils, with the nature of associated mineralization sometimes enhancing and sometimes obscuring critical information. Results provide guidance for selecting and prioritizing fossil plant specimens preserved in siderite concretions for future research. 
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  2. ABSTRACT

    Extraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment‐dispersed extraterrestrial chrome‐spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre‐LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment‐dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy‐based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome‐spinel from other time windows.

     
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  3. Abstract

    The Hamburg meteorite fell on January 16, 2018, near Hamburg, Michigan, after a fireball event widely observed in the U.S. Midwest and in Ontario, Canada. Several fragments fell onto frozen surfaces of lakes and, thanks to weather radar data, were recovered days after the fall. The studied rock fragments show no or little signs of terrestrial weathering. Here, we present the initial results from an international consortium study to describe the fall, characterize the meteorite, and probe the collision history of Hamburg. About 1 kg of recovered meteorites was initially reported. Petrology, mineral chemistry, trace element and organic chemistry, and O and Cr isotopic compositions are characteristic of H4 chondrites. Cosmic ray exposure ages based on cosmogenic3He,21Ne, and38Ar are ~12 Ma, and roughly agree with each other. Noble gas data as well as the cosmogenic10Be concentration point to a small 40–60 cm diameter meteoroid. An40Ar‐39Ar age of 4532 ± 24 Ma indicates no major impact event occurring later in its evolutionary history, consistent with data of other H4 chondrites. Microanalyses of phosphates with LA‐ICPMS give an average Pb‐Pb age of 4549 ± 36 Ma. This is in good agreement with the average SIMS Pb‐Pb phosphate age of 4535.3 ± 9.5 Ma and U‐Pb Concordia age of 4535 ± 10 Ma. The weighted average age of 4541.6 ± 9.5 Ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system.

     
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