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.

¹C.Deligny,¹E.Füri,¹E.Deloule
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.07.038]
¹Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France
Copyright Elsevier

Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites: D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D’Orbigny is characterized by δ15N values ranging from 0 ± 25 to +56 ± 29‰ and δD values between −348 ± 53 and −118 ± 31‰. This shows that the angrite mantle source sampled by D’Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first ~4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).

Cosmochmistry Papers will be on Summer Break for the next two weeks. We will return on August, 30th.
Enjoy your holiday!

¹Alex I. Sheen,¹Christopher D. K. Herd,¹Jarret Hamilton,^1,2Erin L. Walton
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13726]
¹Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
²Department of Physical Sciences, MacEwan University, City Centre Campus, 10700 104 Ave, Edmonton, Alberta, T5J 4S2 Canada
Published by arrangement with John Wiley & Sons

Baddeleyite (ZrO2) is a common late-stage accessory mineral in basaltic shergottites and is a robust geochronometer for obtaining igneous crystallization ages via high-precision in situ SIMS U-Pb analysis. Amenability to SIMS U-Pb dating depends in large part on the size and abundance of baddeleyite grains, which are generally surveyed using microbeam methods. We examine the petrography, mineralogy, geochemistry, and baddeleyite distribution in eight basaltic shergottites to identify factors that may be used to predict baddeleyite distribution in unknown samples of Mars. Results suggest that fractional crystallization controls baddeleyite occurrence in shergottites to the first order; samples with pyroxene major element compositions extending beyond the 1-bar stability boundary generally have higher baddeleyite abundance compared with samples with pyroxene compositions terminating at or before the stability boundary. In samples which display two pyroxene composition trends (high-Ca and low-Ca), the largest baddeleyite grains tend to be associated with Fe-Ti oxides; in samples where pyroxene composition forms a continuous trend extending beyond the 1-bar stability boundary, the largest baddeleyite grains are typically associated with polymineralic late-stage pockets. Bulk HFSE content and fO2 do not appear to directly influence baddeleyite distribution. Based on our findings, we propose that pyroxene composition is a useful proxy for assessing baddeleyite abundance and distribution in shergottites and may aid in determining a sample’s feasibility for U-Pb geochronology prior to conducting detailed surveys for baddeleyite characterization.