^1,2Masayuki Ikeda,^2,3,4Ryuji Tada
Earth and Planetary Science Letters 537, 116168 Link to Article [https://doi.org/10.1016/j.epsl.2020.116168]
¹Department of Geosciences, Graduate School of Science, Shizuoka University, Shizuoka, 790-8577, Japan
²Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo, 113-0033, Japan
³The Research Center for Earth System Science, Yunnan University, Chenggong District, Kunming, Yunnan Province 650500, China
⁴Institute for Geo-Cosmology, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
Copyright Elsevier

Astronomical solutions for planetary orbits beyond several tens of million years (Myr) ago have large uncertainties due to the chaotic nature of the Solar System, mainly Myr-scale cycles related with the Earth-Mars secular resonance. Our only accessible archive for unraveling the Earth’s orbital variations in the geologic past are sedimentological records, yet their reliability and uncertainties are still debated. Here, we describe Myr-scale orbital signals of early Mesozoic monsoon records from two different sedimentary settings (lake level records of the equatorial Pangea and biogenic silica burial flux of deep-sea Panthalassa), along with a marine carbon isotope compilation. Although most of the dominant multi-Myr cycles are not exactly of the same frequency, 1.8 Myr cycles during ∼216–210 Ma are detected from the two mutually-independent sedimentary settings, and differ from available astronomical solutions. This finding provides not only convincing evidence for the chaotic nature of the Solar System in the geological past, but also additional constraints on astronomical models. On the other hand, besides the orbital cycles, tectonic forcing and consequent climatic perturbations could also have affected the proxies on multi-Myr timescales during episodes of large igneous province emplacement, such as Siberian trap volcanism (252–245 Ma), Wrangellia (233–225 Ma), Central Atlantic Magmatic Province (202–200 Ma), and Karoo-Ferrar volcanism (184–180 Ma). If we can distinguish orbital signals from other effects, such as tectonic and volcanic processes, the multi-Myr cycles in geologic records have the potential to reconstruct the chaotic evolution of the Solar System.

¹S. Kommescher,^1,2R.O.C. Fonseca,^1,3F. Kurzweil,^1,3M.M. Thiemens,¹C. Münker,^1,4P. Sprung
Geochemical Perspectives 13, (In Press) Link to Article [doi: 10.7185/geochemlet.2007]
¹Institut für Geologie und Mineralogie, Universität zu Köln, Germany
²Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Germany
³G-TIME Laboratory, Université Libre de Bruxelles, Belgium
⁴Hot Laboratory Division (AHL), Paul Scherer Institut, Villigen, Switzerland

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^1,2Peter Jenniskens et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13452]
¹SETI Institute, 189 Bernardo Ave, Mountain View, California, 94043 USA
²NASA Ames Research Center, Moffett Field, California, 94035 USA
Published by arrangement with John Wiley & Sons

The trajectory and orbit of the LL7 ordinary chondrite Dishchii’bikoh are derived from low‐light video observations of a fireball first detected at 10:56:26 UTC on June 2, 2016. Results show a relatively steep ~21° inclined orbit and a short 1.13 AU semimajor axis. Following entry in Earth’s atmosphere, the meteor luminosity oscillated corresponding to a meteoroid spin rate of 2.28 ± 0.02 rotations per second. A large fragment broke off at 44 km altitude. Further down, mass was lost to dust during flares at altitudes of 34, 29, and 25 km. Surviving meteorites were detected by Doppler weather radar and several small 0.9–29 g meteorites were recovered under the radar reflection footprint. Based on cosmogenic radionuclides and ground‐based radiometric observations, the Dishchii’bikoh meteoroid was 80 ± 20 cm in diameter assuming the density was 3.5 g/cm³. The meteoroid’s collisional history confirms that the unusual petrologic class of LL7 does not require a different parent body than three previously observed LL chondrite falls. Dishchii’bikoh was ejected 11 Ma ago from parent body material that has a 4471 ± 6 Ma U‐Pb age, the same as that of Chelyabinsk (4452 ± 21 Ma). The distribution of the four known pre‐impact LL chondrite orbits is best matched by dynamical modeling if the source of LL chondrites is in the inner asteroid belt in a low inclined orbit, with the highly inclined Dishchii’bikoh being the result of interactions with Earth before impacting.

¹Hannah H. Kaplan et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13461]
¹Southwest Research Institute, Boulder, Colorado, 80302 USA
Published by arrangement with John Wiley & Sons

The primary objective of the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS‐REx) mission is to return to Earth a pristine sample of carbonaceous material from the primitive asteroid (101955) Bennu. To support compositional mapping of Bennu as part of sample site selection and characterization, we tested 95 spectral indices on visible to near infrared laboratory reflectance data from minerals and carbonaceous meteorites. Our aim was to determine which indices reliably identify spectral features of interest. Most spectral indices had high positive detection rates when applied to spectra of pure, single‐component materials. The meteorite spectra have fewer and weaker absorption features and, as a result, fewer detections with the spectral indices. Indices targeting absorptions at 0.7 and 2.7–3 μm, which are attributable to hydrated minerals, were most successful for the meteorites. Based on these results, we identified a set of 17 indices that are most likely to be useful at Bennu. These indices detect olivines, pyroxenes, carbonates, water/OH‐bearing minerals, serpentines, ferric minerals, and organics. Particle size and albedo are known to affect band depth but had a negligible impact on interpretive success with spectral indices. Preliminary analysis of the disk‐integrated Bennu spectrum with these indices is consistent with expectations given the observed absorption near 3 μm. Our study prioritizes spectral indices to be used for OSIRIS‐REx spectral analysis and mapping and informs the reliability of all index‐derived data products, including a science value map for sample site selection.

¹Nancy L. Chabot,¹Rachel H. Cueva,²Andrew W. Beck,³Richard D. Ash
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13462]
¹Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, 20723 USA
²Department of Petroleum Engineering & Geology, Marietta College, Marietta, Ohio, 45750 USA
³Department of Geology, University of Maryland, College Park, Maryland, 20742 USA
Published by arrangement with John Wiley & Sons

Some of the defining characteristics of the IIG iron meteorite group are their high bulk P contents and massive, coarse schreibersite, which have been calculated to make up roughly 11–14 wt% of each specimen. In this study, we produced two data sets to investigate the formation of schreibersites in IIG irons: measurements of trace elements in the IIG iron meteorite Twannberg and experimental determinations of trace element partitioning into schreibersite. The schreibersite‐bearing experiments were conducted with schreibersite in equilibrium with a P‐rich melt and with bulk Ni contents ranging from 0 to 40 wt%. The partitioning behavior for the 20 elements measured in this study did not vary with Ni content. Comparison of the Twannberg measurements with the experimental results required a correction factor to account for the fact that the experiments were conducted in a simplified system that did not contain a solid metal phase. Previously determined solid metal/P‐rich melt partition coefficients were applied to infer schreibersite/solid metal partitioning behavior from the experiments, and once this correction was applied, the two data sets showed broad similarities between the schreibersite/solid metal distribution of elements. However, there were also differences noted, in particular between the Ni and P contents of the solid metal relative to the schreibersite inferred from the experiments compared to that measured in the Twannberg sample. These differences support previous interpretations that subsolidus schreibersite evolution has strongly influenced the Ni and P content now present in the solid metal phase of IIG irons. Quantitative attempts to match the IIG solid metal composition to that of late‐stage IIAB irons through subsolidus schreibersite growth were not successful, but qualitatively, this study corroborates the striking similarities between the IIAB and IIG groups, which are highly suggestive of a possible genetic link between the groups as has been previously proposed.

¹Christian Vollmer,¹Stella Rombeck,²Julia Roszjar,³Adam R. Sarafian,¹Stephan Klemme
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13453]
¹Institut für Mineralogie, Westfälische Wilhelms‐Universität Münster, Corrensstrasse 24, D‐48149 Münster, Germany
²Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, 1010 Vienna, Austria
³Science and Technology Division, Corning Incorporated, 21 Lynn Morse Rd., Painted Post, New York, New York, 14870 USA
Published by arrangement with John Wiley & Sons

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.

¹O. Pravdivtseva,²F. L. H. Tissot,³N. Dauphas,¹S. Amari
Nature Astronomy(In Press) Link to Article [DOIhttps://doi.org/10.1038/s41550-019-1000-z]
¹Physics Department and McDonnell Center for the Space Sciences, Washington University, Saint Louis, MO, USA
²The Isotoparium, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
³Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL, USA

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¹Dominika Kalasová,¹Tomáš Zikmund,²Pavel Spurný,³Jakub Haloda,²Jiří Borovička,¹Jozef Kaiser
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13460]
¹CEITEC – Central European Institute of Technology, Brno University of Technology, Purkyňova 123, Brno, Czech Republic
²Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
³Czech Geological Survey, Geologická 6, 152 00 Prague, Czech Republic
Published by arrangement with John Wiley & Sons

A very bright and long bolide was observed over the eastern part of the Czech Republic during late local evening on December 9, 2014. This bolide was recorded by professional instruments in the Czech part of the European Fireball Network. Three meteorites weighing in total 87 g were found in the predicted area and were named Žďár nad Sázavou. The complete material composition of the meteorite was obtained from one cut‐off piece using petrography, mineralogy, and scanning electron microscopy (together with X‐ray energy dispersive spectroscopy and wavelength dispersive spectroscopy). X‐ray computed tomography (CT) was applied on all pieces for the determination of their grain and bulk density, digitization of shape, and examination of the structural homogeneity. CT has proved its important role for nondestructive exploration of brecciated meteorites formed by distinct lithologies or petrological types. In this article, we discuss its limits in terms of structural and material resolution based on the correlation of state‐of‐the‐art CT data and SEM images. Furthermore, we introduce a new way of air cavity quantification, which distinguishes the natural porosity of meteorite and cracks related to erosion processes. This helps to discuss the presence of weathering products based on comparison of the meteorite pieces found at different times after impact.

¹E. Dobrică,¹A. J. Brearley
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13458]
¹Department of Earth and Planetary Sciences, MSC03‐2040, 1 University of New Mexico, Albuquerque, New Mexico, 87131‐0001 USA
Published by Arrangement with John Wiley & Sons

We have investigated the fine‐grained matrix of the least‐altered unequilibrated ordinary chondrite (UOC) Semarkona (LL3.00) using different electron microscope techniques. Unlike previous studies, which found that the matrix of Semarkona was extensively altered to phyllosilicates, we have discovered the widespread occurrence of much more pristine amorphous silicates in the sample that we have studied. Detailed TEM study shows that these materials occur pervasively in the matrix as (1) continuous groundmass; (2) distinct, circular to subrounded features, which contain nanometric‐size sulfides and carbides; or (3) distinct objects containing parallel, linear features composed of sulfides and voids. These amorphous silicates have many textural and compositional similarities to the occurrences of amorphous silicates found in pristine carbonaceous chondrites (CCs); however, minor differences were also identified. Most of the textural and chemical differences suggest that these materials formed at different times and locations in the solar nebula, compared to matrix materials in CCs. Nevertheless, their occurrence suggests that the amorphous silicates in Semarkona formed by similar processes to those proposed for amorphous silicates in CCs, that is, rapid cooling that favored disequilibrium condensation of material evaporated during chondrule‐forming events. In addition, the occurrence of minimally altered amorphous silicates in Semarkona demonstrates that the effects of aqueous alteration, which have been widely described in this meteorite, are not pervasive. Instead, our new observations demonstrate that aqueous alteration has affected Semarkona heterogeneously and that locally, regions of much more pristine matrix that have escaped extensive alteration are still preserved within this meteorite. Such materials provide significant new insights into the pristine characteristics of ordinary chondrite matrix material that has not been previously available.

^1,2Patrick J. A. Hill,^1,3Gordon R. Osinski,^1,3Neil R. Banerjee
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13457]
¹Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7 Canada
²Department of Earth and Atmospheric Sciences, University of Alberta, 1‐26 Earth Sciences Building, Alberta, T6G 2E3 Canada
³Institute for Earth and Space Exploration, University of Western Ontario, 1151 Richmond Street, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

By analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast‐rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least‐squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt‐poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late‐stage melt flows (group 1 glasses, occurring in melt‐bearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater).