¹Lindsay P. Keller,^2,1Eve L. Berger,^1,3Shouliang Zhang,⁴Roy Christoffersen
Meteoritics & Planteray Science (in Press) Link to Article [https://doi.org/10.1111/maps.13732]
¹NASA Johnson Space Center, Mail Code XI3, Houston, Texas, 77058 USA
²Texas State University − Jacobs JETS − NASA Johnson Space Center, Houston, Texas, 77058 USA
³Samsung Austin Semiconductor, Analysis Engineering, 12100 Samsung Blvd, Austin, Texas, 78754 USA
⁴Jacobs, NASA Johnson Space Center, Mail Code X13, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Transmission electron microscope (TEM) imaging techniques combined with focused ion beam sample preparation were used to calibrate the solar energetic particle track production rate in lunar samples. Track density measurements by TEM as a function of depth were obtained from lunar rock 64455 that has a well-constrained exposure age of 2 Myr giving a track production rate of 4.4 ± 0.4 × 104 tracks cm−2 yr−1 for a 2π exposure at 1 AU. The typical space weathering effects in mature lunar soils (both vapor-deposited rims and solar wind-damaged rims) accumulate in ˜106 yr based on the new calibration applied to track densities in individual grains. Solar wind-damaged rim widths in anorthite and olivine follow a power law relationship with track density and achieve steady-state widths in a few Myr. Vapor-deposited rim widths show no correlation with exposure age suggesting that their formation is episodic with the full width of vapor-deposited rims accumulating in a single or a few rare impact events. Solar wind-damaged rim development was modeled using the stopping range of ions in matter code. Modeling shows that the solar wind-damaged rims develop rapidly and approach steady-state values in 105–106 yr. Anorthite and olivine record similar track densities for similar exposure ages, but their structural response to solar wind irradiation differs significantly. Solar wind-damaged rims on olivine are not amorphous in contrast to modeling and high flux laboratory experiments and a model is proposed to account for their different response to solar wind irradiation.

¹Masashi Yoshida,¹Masaaki Miyahara,^1,2,3Hiroki Suga,^4,5Akira Yamaguchi,⁶Naotaka Tomioka,⁷Takeshi Sakai,^7,8Hiroaki Ohfuji,⁸Fumiya Maeda,^7,8,9Itaru Ohira,⁸Eiji Ohtani,¹⁰Seiji Kamada,¹¹Takuji Ohigashi,¹¹Yuichi Inagaki,¹²Yu Kodama,³Naohisa Hirao
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13735]
¹Graduate School of Advanced Science and Engineering, Hiroshima University, Hiroshima, Higashi, 739-8526 Japan
²Department of Earth and Planetary, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Tokyo, Bunkyo-ku, 113-0033 Japan
³Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto Sayo, Hyogo, 679-5198 Japan
⁴National Institute of Polar Research, Tokyo, 190-8518 Japan
⁵Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tokyo, 190-8518 Japan
⁶Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Nankoku, 783-8502 Japan
⁷Geodynamics Research Center, Ehime University, Matsuyama, 790-8577 Japan
⁸Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
⁹Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Tokyo, Toshima-ku, 171-8588 Japan
¹⁰Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578 Japan
¹¹UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki, Aichi, 444-8585 Japan
¹²Marine Works Japan, Kochi, Nankoku, 783-8502 Japan
Published by arrangement with John Wiley & Sons

The (plagioclase) lherzolitic shergottite Northwest Africa (NWA) 7397 consists of poikilitic and non-poikilitic lithologies. Coarse-grained low-Ca pyroxene oikocrysts enclose olivine and chromite grains in the poikilitic lithology. The major constituents of the non-poikilitic lithology are olivine, Ca-pyroxene, and plagioclase. Minor amounts of chromite, ilmenite, alkali feldspar, Ca-phosphate, and iron-sulfide are included in the non-poikilitic lithology. Most plagioclase grains in the non-poikilitic lithology have become maskelynite. A melt pocket occurs in the non-poikilitic lithology. Plagioclase in contact with the melt pocket has dissociated into zagamiite + stishovite. Apatite and merrillite entrained in the melt pocket have transformed into tuite. Olivine in contact with the melt pocket has dissociated into bridgmanite (almost vitrified) + ferroan-periclase. Alteration products, iron oxides and hydroxides, also occur in the dissociated olivine although it is not clear when the aqueous alteration occurred. The dissociation reactions of olivine and plagioclase into the high-pressure polymorphs (bridgmanite, ferroan-periclase, zagamiite, and stishovite) are found from lherzolitic shergottites for the first time. The estimated peak shock-pressure and -temperature conditions recorded in melt pockets of NWA 7397 are ˜23 GPa and 2,000 °C at least, respectively, based on the high-pressure mineral assemblages.

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.

¹C. R. Fisher,¹M. C. Price,¹M. J. Burchell
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13729]
¹Centre for Astrophysics and Planetary Science, School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH UK
Published ba arrangement with John Wiley & Sons

The plumes naturally erupting from the icy satellite Enceladus were sampled by the Cassini spacecraft in high-speed fly-bys, which gave evidence of salt. This raises the question of how salt behaves under high-speed impact, and how it can best be sampled in future missions to such plumes. We present the results of 35 impacts onto aluminum targets by a variety of salts (NaCl, NaHCO3, MgSO4, and MgSO4·7H2O) at speeds from 0.26 to 7.3 km s−1. Using SEM-EDX, identifiable projectile residue was found in craters at all speeds. It was possible to distinguish NaCl and NaHCO3 from each other, and from the magnesium sulfates, but not to separate the hydrous from anhydrous magnesium sulfates. Raman spectroscopy on the magnesium sulfates and NaHCO3 residues failed to find a signal at low impact speeds (<0.5 km s−1) where there was insufficient projectile material deposited at the impact sites. At intermediate speeds (0.5 to 2–3 km s−1), identifiable Raman spectra were found in the impact craters, but not at higher impact speeds, indicating a loss of structure during the high speed impacts. Thus, intact capture of identifiable salt residues on solid metal surfaces requires impact speeds between 0.75 and 2 km s−1.

¹Jinia Sikdar,^1,2Vinai K. Rai
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13722]
¹Physical Research Laboratory, Ahmedabad, 380009 India
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, 85287 USA
Published by arrangement with John Wiley & Sons

Magnesium, a major mineral-forming element of the inner solar system, is partitioned between the silicate and the unique sulfidic phases (niningerite, MgS) of enstatite chondrites (ECs) owing to the formation of EC under exceptionally reducing conditions. In this study, we have carried out mineralogical characterization of the distinct Mg- and Si-bearing phases of unequilibrated ECs (EH3). To evaluate the Mg isotope variations in such reduced planetary bodies, we have analyzed the Mg isotope composition of several enstatitic silicate phases, matrices (composed of mixed proportions of silicate and sulfidic phases), and bulk meteorite fractions micro milled from three EH3 chondrites. We found that the stable Mg isotope composition (expressed as δ25Mg) of the microphase separates of EH3 chondrites ranged from −0.216 ± 0.014‰ to −0.094 ± 0.014‰. Despite the dispersion in Mg isotope values, the average δ25Mg of the silicate fractions of the studied EH3 chondrites was similar to its matrix and bulk meteorite fractions. Mass-dependent Mg isotope fractionation was evinced among the phase separates of EH3 chondrites with the slope of the fractionation line on a δ25Mg versus δ26Mg plot being closer to kinetic fractionation trend. Experimental and theoretical considerations hint that Mg isotope exchange between the silicate and sulfide phases might have generated the observed Mg isotope variations among the microphase separates of EH3 chondrites. Based on our Mg isotope data, in combination with the Si isotope composition obtained in the same aliquot of silicate–matrix fractions of EH3 chondrites and its subsolar Al/Si ratio, we suggest that the high abundance of Si in EC (due to the partitioning of Si among diverse silicates, silica, and metallic phases) and the loss of Mg and refractory components from EC-forming regions could explain the lower Mg/Si ratio of EC compared to that of ordinary and carbonaceous chondrites.

¹S.T.M. Peters,^1,2M.B. Fischer,¹A. Pack,³K. Szilas,⁴P.W.U. Appel,⁵C. Münker,⁶L. Dallai,⁵C.S. Marien
Geochemical Perspectives Letters (in Press) Link to Article [doi: 10.7185/geochemlet.2120]
¹Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstraße 1, 37077 Göttingen, Germany
²Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
³University of Copenhagen, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, 1350 København K, Denmark
⁴Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København K, Denmark
⁵Universität zu Köln, Institut für Geologie und Mineralogie, Zülpicher Str. 49b, 50674 Köln, Germany
⁶CNR-Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 56124 Pisa, Italy

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Francis M.McCubbin,¹Jonathan A.Lewis,²Jessica J.Barnes,³Stephen M.Elardo,¹Jeremy W.Boyce
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.021]
¹NASA Johnson Space Center, Mailcode XI2, 2101 NASA Parkway, Houston, Texas 77058, USA
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, USA
³Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
Copyright Elsevier

We conducted a petrologic study of apatite within eight unbrecciated, non-cumulate eucrites and two monomict, non-cumulate eucrites. These data were combined with previously published data to quantify the abundances of F, Cl, and H2O in the bulk silicate portion of asteroid 4 Vesta (BSV). Using a combination of apatite-based melt hygrometry/chlorometry and appropriately paired volatile/refractory element ratios, we determined that BSV has 3.0–7.2 ppm F, 0.39–1.8 ppm Cl, and 3.6–22 ppm H2O. The abundances of F and H2O are depleted in BSV relative to CI chondrites to a similar degree as F and H2O in the bulk silicate portion of the Moon. This degree of volatile depletion in BSV is similar to what has been determined previously for many moderately volatile elements in 4 Vesta (e.g., Na, K, Zn, Rb, Cs, and Pb). In contrast, Cl is depleted in 4 Vesta by a greater degree than what is recorded in samples from Earth or the Moon. Based on the Cl-isotopic compositions of eucrites and the bulk rock Cl/F ratios determined in this study, the eucrites likely formed through serial magmatism of a mantle with heterogeneous δ37Cl and Cl/F, not as extracts from a partially crystallized global magma ocean. Furthermore, the volatile depletion and Cl-isotopic heterogeneity recorded in eucrites is likely inherited, at least in part, from the precursor materials that accreted to form 4 Vesta and is unlikely to have resulted solely from degassing of a global magma ocean, magmatic degassing of eucrite melts, and/or volatile loss during thermal metamorphism. Although our results can be reconciled with the past presence of wide-scale melting on 4 Vesta (i.e., a partial magma ocean), any future models for eucrite petrogenesis involving a global magma ocean would need to account for the preservation of a heterogeneous eucrite source with respect to Cl/F ratios and Cl isotopes.

¹Maxime Piralla,¹Johan Villeneuve,²Valentina Batanova,³Emmanuel Jacquet,¹Yves Marrocchi
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.08.007]
¹Université de Lorraine, CNRS, CRPG, UMR 7358, Vandœuvre-lès-Nancy 54500, France
²Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble 38000, France
³Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d’Histoire naturelle, Sorbonne Université, CNRS, UMR 7590, CP52, 57 rue Cuvier, Paris 75005, France
Copyright Elsevier

Chondrules are sub-millimetric spheroids that are ubiquitous in chondrites and whose formation mechanism remains elusive. Textural and oxygen isotopic characteristics of chondrules in carbonaceous chondrites (CCs) suggest that they result from the recycling of isotopically heterogeneous early-condensed precursors via gas-melt interactions. Here, we report high-resolution X-ray elemental maps and in situ O isotopic analyses of FeO-poor, olivine-rich chondrules from ordinary chondrites (OCs) to compare the conditions of chondrule formation in these two main classes of chondrites. OC chondrules show minor element (e.g., Ti, Al) zonings at both the chondrule and individual olivine grain scales. Considering the entire isotopic data set, our data define a mass-independent correlation, with olivine grains showing O isotopic variations spanning more than 40 ‰. Though 16O-rich relict olivine grains were identified in OC chondrules, they are much less abundant than in CC chondrules. They appear as two types: (i) those with low minor element abundances and Δ17O < −15 ‰ and (ii) those with varying minor element abundances and less negative Δ17O values averaging −5.5 ‰. The host olivine grains exhibit mass-dependent O isotopic variations within individual chondrules. Our results reveal that similar processes (precursor recycling and interactions between chondrule melts and a SiO- and Mg-rich gas) established the observed features of OC and CC chondrules. The mass-dependent isotopic variations recorded by host olivine grains result from kinetic effects induced by complex evaporation/recondensation processes during the gas-melt interactions. This suggests that OC chondrules formed through enhanced recycling processes, in good agreement with the lower abundances of relict olivine grains in OC chondrules compared to CC chondrules. We use the Δ18O = δ18O − δ17O parameter to demonstrate that there is no genetic relationship between CC and OC chondrules, suggesting limited radial transport in the protoplanetary disk. Finally, to the first order, the Δ18O−Δ17O diagram may allow the non-carbonaceous vs. carbonaceous origin of a given chondrule to be deciphered.