^1,2Thomas Stephan,^1,2,6Reto Trappitsch,³Peter Hoppe,^1,2,4Andrew M. Davis,^1,2,4,5Michael J. Pellin,^1,2Olivia S. Pardo
The Astrophysical Journal 877,101 Link to Article [https://doi.org/10.3847/1538-4357/ab1c60]
¹Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL, USA
³Max Planck Institute for Chemistry, 55128 Mainz, Germany
⁴The Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
⁵Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁶Present address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

We have analyzed molybdenum isotopes, together with strontium and barium isotopes, in 18 presolar silicon carbide grains using the Chicago Instrument for Laser Ionization (CHILI), a resonance ionization mass spectrometer. All observed isotope ratios can be explained by mixtures of pure s-process matter with isotopically solar material. Grain residues were subsequently analyzed for carbon, nitrogen, silicon, and sulfur isotopes, as well as a subset for ²⁶Al–²⁶Mg systematics using the NanoSIMS. These analyses showed that all but one grain are mainstream grains, most probably coming from low-mass asymptotic giant branch (AGB) stars. One grain is of the AB type, for which the origin is still a matter of debate. The high precision of molybdenum isotope measurements with CHILI provides the best estimate to date for s-process molybdenum made in low-mass AGB stars. The average molybdenum isotopic abundances produced by the s-process found in the analyzed mainstream SiC grains are 0% ⁹²Mo, 0.73% ⁹⁴Mo, 13.30% ⁹⁵Mo, 36.34% ⁹⁶Mo, 9.78% ⁹⁷Mo, 39.42% ⁹⁸Mo, and 0.43% ¹⁰⁰Mo. Solar molybdenum can be explained as a combination of 45.9% s-process, 30.6% r-process, and 23.5% p-process contributions. Furthermore, the observed variability in the individual grain data provides insights into the variability of conditions (neutron density, temperature, and timescale) during s-process nucleosynthesis in the grains’ parent stars, as they have subtle effects on specific molybdenum isotope ratios. Finally, the results suggest that the ratio between p– and r-process molybdenum in presolar SiC from many different types of parent stars is Mo _p /Mo _r  = 0.767, the value inferred for the solar system and consistent with what has been found in bulk samples and leachates of primitive meteorites.

¹Sota Arakawaand,¹Taishi Nakamoto
The Astrophysical Journal 877, 84 Link to Article [https://doi.org/10.3847/1538-4357/ab1b3e]
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan

Shock-wave heating within the solar nebula is one of the leading candidates for the source of chondrule-forming events. Here we examine the possibility of compound chondrule formation via optically thin shock waves. Several features of compound chondrules indicate that they are formed via the collisions of supercooled precursors. We evaluate whether compound chondrules can be formed via the collision of supercooled chondrule precursors in the framework of the shock-wave heating model by using semi-analytical methods and discuss whether most of the crystallized chondrules can avoid destruction upon collision in the post-shock region. We find that chondrule precursors immediately turn into supercooled droplets when the shock waves are optically thin, and they can maintain supercooling until the condensation of evaporated fine dust grains. Owing to the large viscosity of supercooled melts, supercooled chondrule precursors can survive high-speed collisions on the order of 1 km s⁻¹ when the temperature is below ~1400 K. From the perspective of the survivability of crystallized chondrules, shock waves with a spatial scale of ~10⁴ km may be potent candidates for the chondrule formation mechanism. Based on our results from one-dimensional calculations, a fraction of compound chondrules can be reproduced when the chondrule-to-gas mass ratio in the pre-shock region is ~2 × 10⁻³, which is approximately half of the solar metallicity.

¹Takaharu Saito,¹Hiroshi Hidaka,²Seung-Gu Lee
The Astrophysical Journal 877, 73 Link to Article [https://doi.org/10.3847/1538-4357/ab1aa5]
¹Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8601, Japan
²Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Republic of Korea

Howardite–eucrite–diogenite meteorites are believed to originate in the crust of the asteroid 4 Vesta, whose differentiation processes are still controversial. In this study, the first 176Lu–176Hf isotopic data of nine diogenites are presented together with their 87Rb–87Sr isotopic compositions and rare earth element (REE) abundances to investigate the differentiation process of diogenites. The 176Lu–176Hf data sets of nine diogenites revealed the significantly higher initial 176Hf/177Hf ratio of diogenites than that of eucrites, while there are no resolvable differences between their ages. Based on the high initial ratio and the early formation of diogenites, their source material is estimated to be the Vestan mantle. The 87Rb–87Sr systematics of nine diogenites are entirely disturbed probably due to impact events on Vesta. The significant variation observed in the REE abundances of nine diogenites suggests their crystallization from compositionally diverse melts. Based on the mantle origin and compositional diversity of diogenites, we propose the crystallization of diogenites from partial melts of the Vestan mantle. The variation of the trace element abundances of diogenites can be explained by the variation of the degree of the partial melting. The timescale between the crystallization and partial melting of the Vestan mantle is estimated to be ~100–600 Ma from the 176Lu–176Hf isotopic data of nine diogenites, while a heat source for the partial melting is uncertain.

¹Bernd Helber,^1,2Bruno Dias,^1,3,4Federico Bariselli,¹Luiza F. Zavalan,⁵Lidia Pittarello,⁶Steven Goderis,⁶Bastien Soens,^6,7,8Seann J. McKibbin,⁶Philippe Claeys,¹Thierry E. Magin
The Astrophysical Journal 876, 120 Link to Article [https://doi.org/10.3847/1538-4357/ab16f0]
¹Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Rhode-Saint-Genèse, Belgium
²Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium
³Research Group Electrochemical and Surface Engineering, Vrije Universiteit Brussel, Brussels, Belgium
⁴Dipartimento di Scienze e Tecnologie Aerospaziali, Politecnico di Milano, Milano, Italy
⁵Department of Lithospheric Research, University of Vienna, Vienna, Austria
⁶Analytical, Environmental, and Geo-Chemistry, Vrije Universiteit Brussel, Brussels, Belgium
⁷Institute of Earth and Environmental Science, University of Potsdam, Potsdam-Golm, Germany
⁸Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, Göttingen, Germany

Meteoroids largely disintegrate during their entry into the atmosphere, contributing significantly to the input of cosmic material to Earth. Yet, their atmospheric entry is not well understood. Experimental studies on meteoroid material degradation in high-enthalpy facilities are scarce and when the material is recovered after testing, it rarely provides sufficient quantitative data for the validation of simulation tools. In this work, we investigate the thermo-chemical degradation mechanism of a meteorite in a high-enthalpy ground facility able to reproduce atmospheric entry conditions. A testing methodology involving measurement techniques previously used for the characterization of thermal protection systems for spacecraft is adapted for the investigation of ablation of alkali basalt (employed here as meteorite analog) and ordinary chondrite samples. Both materials are exposed to a cold-wall stagnation point heat flux of 1.2 MW m−2. Numerous local pockets that formed on the surface of the samples by the emergence of gas bubbles reveal the frothing phenomenon characteristic of material degradation. Time-resolved optical emission spectroscopy data of ablated species allow us to identify the main radiating atoms and ions of potassium, calcium, magnesium, and iron. Surface temperature measurements provide maximum values of 2280 K for the basalt and 2360 K for the chondrite samples. We also develop a material response model by solving the heat conduction equation and accounting for evaporation and oxidation reaction processes in a 1D Cartesian domain. The simulation results are in good agreement with the data collected during the experiments, highlighting the importance of iron oxidation to the material degradation.

¹Gert Nolze,²Klaus Heide
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.125538]
¹Dept 5.1, Federal Institute for Materials, Research and Testing (BAM), Unter den Eichen 87, 12205 Berlin, Germany
²Institut für Geowissenschaften (IGW), Friedrich-Schiller-Universität Jena (FSU), Germany
Copyright Elsevier

Roaldite – Fe4N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.
Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}R, which is most likely parallel to {120}K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.

^1,2,3E.S.Steenstra,²J.Berndt, ¹S.Klemme,¹A.Rohrbach,¹E.S.Bullock,³W.van Westrenen
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.10.006]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Institute of Mineralogy, University of Münster, Germany
³Faculty of Science, Vrije Universiteit Amsterdam, The Netherlands
Copyright Elsevier

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) – temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.

It was found that the measured SCSS values agree well with the SCSS values predicted from a previous thermodynamic model for high-FeO* melts. To assess the possibility of sulfide liquid saturation of the source regions of non-cumulate eucrites and angrites, their S abundances were compared with the calculated SCSS values for their source regions. Results show that if eucritic and angritic source regions were saturated with FeS liquid, significant degassing (> 50–80%) of S must have occurred during or following their magmatic emplacement. Such loss is inconsistent with the S, Cl, Zn and Rb isotopic compositions of non-cumulate eucrites. Sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of Cu and HSE in non-cumulate eucrites and angrites (Riches et al., 2012).

Additional calculations were performed to further explore the timing and extent of S loss during crystallization of the Vestan magma ocean. The assumption of chondritic bulk S abundances of bulk Vesta would correspond with extremely high S contents of the eucrite source region(s), even after consideration of depletion of S due to core formation. In light of the S, Cl, Zn and Rb stable isotopic compositions of eucrites, the S abundances in eucrites are most consistent with the hypothesis that the Vestan mantle was already strongly depleted in S (>70–80 %) by the time of Vestan magma ocean crystallization, resulting in more realistic S contents of the eucrite source region(s). The depletion of S could have been established during initial accretion of Vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation (Wu et al., 2018). Modeling of the new experimentally determined sulfide-silicate partition coefficients and previously reported Vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early Vestan magma ocean crystallization is also unlikely. The lack of sulfide liquid saturation in the source regions of non-cumulate eucrites and angrites, as well as during early Vestan magma ocean solidification, shows that current geochemical models of core formation and late accretion remain valid for these bodies.

^1,2J. Chen,²B.L. Jolliff,²A. Wang,²R.L. Korotev,²K. Wang,²P.K. Carpenter,²H. Chen,¹Z. Ling,¹X. Fu,¹Y. Ni,¹H. Cao,¹Y. Huang
Journal of Geophysical Research , Planets  (in Press) Link to Article [https://doi.org/10.1029/2019JE006084]
¹Institute of Space Sciences, Shandong Provincial Key Laboratory of OpticaJ Astronomy and
Solar-TerrestriaJ Environment, Shandong University, Weihai 264209, China.
²Departrnent of Earth & Planetary Sciences and the McDonnell Center for the Space
Sciences, Washington University in St. Louis, MO 63130, USA.
²The Australian National Uruversity, Canberra ACT 2600, Australia.
Published by arrangement with John Wiley & Sons

We present comprehensive compositional and mineralogical results on two basaltic lunar meteorites Northwest Africa (NWA) 4734 and NWA 10597 to constrain their igneous mineralogy and metamorphic characteristics and examine the potential pairing relationship among them and other meteorites (e.g., basaltic lunar meteorites collected from LaPaz Icefield, Antarctica (LAPs)). NWA 4734 and NWA 10597 are low‐Ti (3.2–3.5 wt.% TiO₂), low‐Al (10–12 wt.% Al₂O₃), low‐K (880–1300 ppm K) mare basalts derived from evolved parental magma (Mg# (molar Mg/[Mg+Fe]×100) = 33.6–38.3) and are mostly composed of pyroxene (52.7–55.5 vol.%), plagioclase/maskelynite (27.5–29.3 vol.%), olivine (6.7–7.6 vol.%), and late‐formed components (i.e., mesostasis). Pyroxene and olivine in these two meteorites exhibit a multimodal compositional distribution, indicating multiple generations of these mafic minerals, which correspond to different evolution phases (e.g., magma chamber, ascending, and eruption) during the solidification of the basaltic parental magma. Immiscibility played an important role in the evolution of the late‐stage melts, inducing fractionation involving Fe‐rich and Si, K‐rich melts within mesostasis. Considering the extensive partial transformation of plagioclase to maskelynite across the sections, the average shock pressure endured by NWA 4734 and NWA 10597 is determined to be 23–29 GPa, which is consistent with the pressure condition (below 29.8 GPa) recorded by various silica polymorphs. Similar textures, bulk composition, modal mineral proportions, and mineral compositions indicate that NWA 4734, NWA 10597, and LAPs were most likely originated from the same region and both experienced intensive shock events after their crystallization.

¹Harries, D.
ACS Earth and Space Chemistry (in Press) Link to Article [DOI: 10.1021/acsearthspacechem.9b00081]
¹Institute of Geosciences, Friedrich Schiller University Jena, Carl-Zeiss-Promenade 10, Jena, 07745, Germany

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^1,5,6Seann J. MCKIBBIN,^1,7Lidia PITTARELLO,¹Christina MAKARONA,^2,3
Christopher HAMANN,^2,3Lutz HECHT,^4,8Stepan M. CHERNONOZHKIN,¹Steven GODERIS,¹Philippe CLAEYS
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13392]
¹Analytical, Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels 1050, Belgium
²Museum fur Naturkunde, Leibniz-Institut fur Evolutions- und Biodiversitatsforschung, Invalidenstraße 43,
10115 Berlin, Germany
³Institut fur Geologische Wissenschaften, Freie Universitat Berlin, Malteserstraße 74-100, 12249 Berlin, Germany
⁴GeoRessources, Faculte des Sciences et Technologies, Universite de Lorraine, Rue Jacques Callot, BP 70239, 54506,
Vandoeuvre-les-Nancy CEDEX, France
⁵Present address: Institut f€ur Erd- und Umweltwissenschaften, Universitat Potsdam, Haus 27, Karl-Liebknecht-Straße 24-25,
Potsdam-Golm 14476, Germany
⁶Present address: Geowissenschaftliches Zentrum, Abteilung Isotopengeologie, Georg-August-Universitat Göttingen,
Goldschmidtstraße 1, Göttingen 37073, Germany
⁷Present address: Department of Lithospheric Research, Universit€at Wien, UZA 2, Althanstraße 14, Vienna A-1090,Austria
⁸Department of Chemistry, Universiteit Gent, Krijgslaan 281-S12, Ghent 9000, BelgiumPresent address: Department of Chemistry, Universiteit Gent, Krijgslaan 281-S12, Ghent 9000, Belgium
Published by arrangement with John Wiley & Sons

Main group pallasite meteorites are samples of a single early magmatic planetesimal, dominated by metal and olivine but containing accessory chromite, sulfide, phosphide, phosphates, and rare phosphoran olivine. They represent mixtures of core and mantle materials, but the environment of formation is poorly understood, with a quiescent core–mantle boundary, violent core–mantle mixture, or surface mixture all recently suggested. Here, we review main group pallasite data sets and petrologic characteristics, and present new observations on the low‐MnO pallasite Brahin that contains abundant fragmental olivine, but also rounded and angular olivine and potential evidence of sulfide–phosphide liquid immiscibility. A reassessment of the literature shows that low‐MnO and high‐FeO subgroups preferentially host rounded olivine and low‐temperature P₂O₅‐rich phases such as the Mg‐phosphate farringtonite and phosphoran olivine. These phases form after metal and silicate reservoirs back‐react during decreasing temperature after initial separation, resulting in oxidation of phosphorus and chromium. Farringtonite and phosphoran olivine have not been found in the common subgroup PMG, which are mechanical mixtures of olivine, chromite with moderate Al₂O₃ contents, primitive solid metal, and evolved liquid metal. Lower concentrations of Mn in olivine of the low‐MnO PMG subgroup, and high concentrations of Mn in low‐Al₂O₃ chromites, trace the development and escape of sulfide‐rich melt in pallasites and the partially chalcophile behavior for Mn in this environment. Pallasites with rounded olivine indicate that the core–mantle boundary of their planetesimal may not be a simple interface but rather a volume in which interactions between metal, silicate, and other components occur.

¹Hoppe, P.,^2,4,5,6Pignatari, M.,³Amari, S.
Springer Proceedings in Physics 219, 373-376 Link to Article [DOI: 10.1007/978-3-030-13876-9_68]
¹Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, Mainz, 55128, Germany
²E. A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, United Kingdom
³McDonnell Center for the Space Sciences and Physics Department, Washington University, St. Louis, MO 63130, United States
⁴NuGrid Collaboration, East Lansing, United States
⁵JINA-CEE, East Lansing, United States
⁶Konkoly Observatory, Konkoly Thege Miklos ut 15-17, Budapest, 1121, Hungary

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