¹Alexander N. Krot,²Michail I. Petaev,¹Kazuhide Nagashima,¹Elena Dobrică,^3,4Brandon C. Johnson,³Melissa D. Cashion
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13717]
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 906822 USA
²Department of Earth and Planetary Sciences, Harvard University and Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, 02138 USA
³Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana, 47907 USA
⁴Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana, 47907 USA
Published by arrangement with John Wiley & Sons

We report on the mineralogy, petrology, and oxygen isotopic compositions of ferroan olivine–pyroxene-normative cryptocrystalline chondrules (Fe-CCs) in CH chondrites and discuss their origin and the origin of other components in the genetically related CH and CB chondrites. There are two kinds of Fe-CCs: (1) compositionally uniform (Fe/[Fe+Mg] = 0.17–0.34) chondrules with euhedral Fe, Ni-metal grains and (2) metal-free chemically zoned (Fe/[Fe+Mg] = 0.05–0.4) chondrules surrounded by ferroan olivine (Fa44−62) rims; the Fe/(Fe+Mg) ratio increases toward the rims. Both types contain low CaO and Al2O3 (<0.04 wt%), but relatively high contents of MnO and Cr2O3 (up to 1 wt%). Compositionally uniform euhedral Fe, Ni-metal grains are Ni-rich (9–20 wt%) and have subsolar Co/Ni ratio. There is a positive correlation between iron content in the metal grains and Fe/(Fe+Mg) ratio in silicate portion of their host chondrules. Some Fe-CCs experienced postcrystallization solid-state reduction of ferroan silicates to metallic iron. Ferroan cryptocrystalline chondrules and olivine rims have similar oxygen isotopic compositions (interchondrule Δ17O ranges from ˜ −2‰ to 2‰), which are slightly 16O-depleted relative to those of magnesian olivine–pyroxene-normative cryptocrystalline chondrules (Mg-CCs; Δ17O ˜ −2‰) commonly observed in CBs and CHs. We suggest that the Fe-CCs and Mg-CCs formed in the impact plume under different redox conditions (˜IW−1 and ˜IW−3, respectively), which may have been controlled by heterogeneous distribution of water-bearing phases (water ice, hydrated materials) in the collided bodies and/or in the disk. We propose the following impact plume scenario for the origin of Fe-CCs: (1) condensation of ferromagnesian silicate melt around Fe, Ni-metal melt droplets from a highly oxidized portion of the plume; (2) crystallization of euhedral metal grains from the supercooled ferromagnesian silicate melt followed by its solidification; (3) condensation of ferroan olivine rims around solidified Fe-CCs; (4) high-temperature annealing of Fe-CCs and their rims in the plume accompanied by Fe-Mg interdiffusion between ferroan olivine rims and their host chondrules. Subsequently, some Fe-CC experienced solid-state reduction to various degrees, possibly in the reduced portions of gaseous plume. The impact plume-produced or reprocessed components in CBs and CHs include Ca,Al-poor magnesian and ferroan cryptocrystalline chondrules; Ca,Al-rich skeletal olivine chondrules; isotopically uniform, 26Al-poor 16O-depleted (Δ17O ˜ −15 to −5‰) igneous CAIs surrounded by igneous forsterite rims; chemically zoned and unzoned Fe,Ni-metal grains; and metal-sulfide nodules. These objects are dominant in CBs and abundant in CHs. The CH chondrites also contain other high-temperature chondritic components, which avoided processing in the plume and most likely predate the plume event: 26Al-poor and 26Al-rich, mostly 16O-rich CAIs (Δ17O ˜ −40 to −10‰) surrounded by Wark–Lovering rims, and porphyritic chondrules (magnesian [type I], ferroan [type II], and Al-rich) showing a range of Δ17O (from ˜ −10 to ˜ +5‰). Some of these components appear to have been melted in the plume. We conclude that CH and CB chondrites contain multiple generations of chondrules and refractory inclusions formed by different mechanisms at different times and different regions of the protoplanetary disk, consistent with the hypothesis of Wasson and Kallemeyn (1990).

¹Tamara E. Koch,¹Dominik Spahr,¹Beverley J. Tkalcec,¹Miles Lindner,¹David Merges,²Fabian Wilde,¹Björn Winkler,^1,3Frank E. Brenker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13731]
¹Insitute of Geosciences, Goethe University Frankfurt, 60438 Frankfurt am Main, Germany
²Helmholtz-Zentrum Hereon, Max-Planck Strasse 1, 21502 Geesthacht, Germany
³Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, 1680 East-West Road, Honolulu, Hawai‘i, 96822 USA
Published by arrangement with John Wiley & Sons

Chondrules are thought to play a crucial role in planet formation, but the mechanisms leading to their formation are still a matter of unresolved discussion. So far, experiments designed to understand chondrule formation conditions have been carried out only under the influence of terrestrial gravity. In order to introduce more realistic conditions, we developed a chondrule formation experiment, which was carried out at long-term microgravity aboard the International Space Station. In this experiment, freely levitating forsterite (Mg2SiO4) dust particles were exposed to electric arc discharges, thus simulating chondrule formation via nebular lightning. The arc discharges were able to melt single dust particles completely, which then crystallized with very high cooling rates of >105 K h−1. The crystals in the spherules show a crystallographic preferred orientation of the [010] axes perpendicular to the spherule surface, similar to the preferred orientation observed in some natural chondrules. This microstructure is probably the result of crystallization under microgravity conditions. Furthermore, the spherules interacted with the surrounding gas during crystallization. We show that this type of experiment is able to form spherules, which show some similarities with the morphology of chondrules despite very short heating pulses and high cooling rates.

¹Anders Plan,²Gavin G. Kenny,³Timmons M. Erickson,¹Paula Lindgren,¹Carl Alwmark,^1,4,5Sanna Holm-Alwmark,⁶Philippe Lambert,¹Anders Scherstén,¹Ulf Söderlund
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13723]
¹Department of Geology, Lund University, Sölvegatan 12, Lund, 223 62 Sweden
²Department of Geosciences, Swedish Museum of Natural History, Stockholm, SE-104 05 Sweden
³Jacobs—JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
⁴Niels Bohr Institute, University of Copenhagen, Copenhagen, DK-2100 Denmark
⁵Natural History Museum Denmark, University of Copenhagen, Copenhagen, DK-2100 Denmark
⁶CIRIR—Center for International Research and Restitution on Impacts and on Rochechouart, Sciences et Applications, 218 Boulevard Albert 1er, Bordeaux, 33800 France
Published by arrangement with John Wiley & Sons

Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stage III) from the ˜200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4 microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO4 during shock.