^1,2Frank Gyngard, ¹Manavi Jadhav, ²Larry R. Nittler, ³Rhonda M. Stroud, ¹Ernst Zinner
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.002]
¹Laboratory for Space Sciences and the Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63130, USA
²Department of Terrestrial Magnetism, Carnegie Institution, Washington DC 20015, USA
³US Naval Research Laboratory, Code 6360, Washington DC 20375, USA
Copyright Elsevier

We report the morphology, microstructure, and isotopic composition of the largest SiC stardust grain known to have condensed from a supernova. The 25-μm diameter grain, termed Bonanza, was found in an acid-resistant residue of the Murchison meteorite. Grains of such large size have neither been observed around supernovae nor predicted to form in stellar environments. The large size of Bonanza has allowed the measurement of the isotopic composition of more elements in it than any other previous presolar grain, including: Li, B, C, N, Mg, Al, Si, S, Ca, Ti, Fe, and Ni. Bonanza exhibits large isotopic anomalies in the elements C, N, Mg, Si, Ca, Ti, Fe, and Ni typical of an astrophysical origin in ejecta of a Type II core-collapse supernova and comparable to those previously observed for other presolar SiC grains of type X. Additionally, we extracted multiple focused ion beam lift-out sections from different regions of the grain. Our transmission electron microscopy demonstrates that the crystalline order varies at the micrometer scale, and includes rare, higher order polytype domains (e.g., 15R). Analyses with STEM-EDS show Bonanza contains a heterogeneous distribution of subgrains with sizes ranging from < 10 nm to >100 nm of Ti(N,C); Fe, Ni-rich grains with variable Fe:Ni; and (Al,Mg)N. Bonanza also has the highest ever inferred initial 26Al/27Al ratio, consistent with its supernova origin. This unique grain affords us the largest expanse of data, both microstructurally and isotopically, to compare with detailed calculations of nucleosynthesis and dust condensation in supernovae.

¹M. Matthes,¹M. Fischer-Gödde, ¹T.S. Kruijer, ¹T. Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.009]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Copyright Elsevier

To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotopic systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial 107Pd/108Pd = (2.57±0.07) × 10-5. The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from 196Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial 107Pd/108Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9±0.4 Ma and 8.9±0.6 Ma after CAI formation, where this age range reflects uncertainties in the initial 107Pd/108Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and, hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500 K/Ma through the Pd-Ag closure temperature of ∼900 K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Combined, these observations are consistent with cooling of the IVA irons in a metallic body with little or no silicate mantle.

^1,2Christopher Hamann,³Agnese Fazio,⁴Matthias Ebert,^1,2Lutz Hecht,⁵Richard Wirth,⁶Luigi Folco,⁷Alex Deutsch,^1,8,9Wolf Uwe Reimold
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12907]
¹Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany
³Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Jena, Germany
⁴Institut für Geo- und Umweltnaturwissenschaften, Albert-Ludwigs-Universität Freiburg, Freiburg im Breisgau, Germany
⁵Helmholtz-Zentrum Potsdam—Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany
⁶Dipartimento di Science della Terra, Università di Pisa, Pisa, Italy
⁷Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster, Germany
⁸Humboldt-Universität zu Berlin, Berlin, Germany
⁹Laboratório de Geocronologia, Instituto de Geociências, Universidade de Brasília, Brazil
Published by arrangement with John Wiley & Sons

We have investigated silicate emulsions in impact glasses and impact melt rocks from the Wabar (Saudi Arabia), Kamil (Egypt), Barringer (USA), and Tenoumer (Mauritania) impact structures, and in experimentally generated impact glasses and laser-generated glasses (MEMIN research unit) by scanning electron microscopy, electron microprobe analysis, and transmission electron microscopy. Textural evidence of silicate liquid immiscibility includes droplets of one glass disseminated in a chemically distinct glassy matrix; sharp phase boundaries (menisci) between the two glasses; deformation and coalescence of droplets; and occurrence of secondary, nanometer-sized quench droplets in Si-rich glasses. The compositions of the conjugate immiscible liquids (Si-rich and Fe-rich) are consistent with phase separation in two-liquid fields in the general system Fe₂SiO₄–KAlSi₃O₈–SiO₂–CaO–MgO–TiO₂–P₂O₅. Major-element partition coefficients are well correlated with the degree of polymerization (NBO/T) of the Si-rich melt: Fe, Ca, Mg, and Ti are concentrated in the poorly polymerized, Fe-rich melt, whereas K, Na, and Si prefer the highly polymerized, Si-rich melt. Partitioning of Al is less pronounced and depends on bulk melt composition. Thus, major element partitioning between the conjugate liquids closely follows trends known from tholeiitic basalts, lunar basalts, and experimental analogs. The characteristics of impact melt inhomogeneity produced by melt unmixing in a miscibility gap are then compared to impact melt inhomogeneity caused by incomplete homogenization of different (miscible or immiscible) impact melts that result from shock melting of different target lithologies from the crater’s melt zone, which do not fully homogenize and equilibrate due to rapid quenching. By taking previous reports on silicate emulsions in impact glasses into account, it follows that silicate impact melts of variable composition, cooling rate, and crystallization history might readily unmix during cooling, thereby rendering silicate liquid immiscibility a much more common process in the evolution of impact melts than previously recognized.

¹Marco Delbo’, ²Kevin Walsh, ¹Bryce Bolin, ³Chrysa Avdellidou, ¹Alessandro Morbidelli
Science 357, 1026-1029 Link to Article [DOI: 10.1126/science.aam6036]
¹Université Côte d’Azur, CNRS–Lagrange, Observatoire de la Côte d’Azur, CS 34229–F 06304 Nice Cedex 4, France.
²Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO 80302, USA.
³Scientific Support Office, Directorate of Science, European Space Agency, Keplerlaan 1, NL-2201 AZ Noordwijk ZH, Netherlands.
Reprinted with permission from AAAS

A quarter of known asteroids is associated with more than 100 distinct asteroid families, meaning that these asteroids originate as impact fragments from the family parent bodies. The determination of which asteroids of the remaining population are members of undiscovered families, or accreted as planetesimals from the protoplanetary disk, would constrain a critical phase of planetary formation by unveiling the unknown planetesimal size distribution. We discovered a 4-billion-year-old asteroid family extending across the entire inner part of the main belt whose members include most of the dark asteroids previously unlinked to families. This allows us to identify some original planetesimals, which are all larger than 35 kilometers, supporting the view of asteroids being born big. Their number matches the known distinct meteorite parent bodies.

¹Zack Gainsforth, ²Dante S. Lauretta, ³Nobumichi Tamura, ¹Andrew J. Westphal, ¹Christine E. Jilly-Rehak, ¹Anna L. Butterworth
American Mineralogist 102, 1881-1893 Link to Article [DOI
https://doi.org/10.2138/am-2017-5848]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, California 94720, U.S.A.
²Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, U.S.A.
³Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, U.S.A.
Copyright: The Mineralogical Society of America

Lauretta (2005) produced sulfide in the laboratory by exposing canonical nebular metal analogs to H2S gas under temperatures and pressures relevant to the formation of the Solar System. The resulting reactions produced a suite of sulfides and nanophase materials not visible at the microprobe scale, but which we have now analyzed by TEM for comparison with interplanetary dust samples and comet Wild 2 samples returned by the Stardust mission. We find the unexpected result that disequilibrium formation favors pyrrhotite over troilite and also produces minority schreibersite, daubréelite, barringerite, taenite, oldhamite, and perryite at the metal-sulfide interface. TEM identification of nanophases and analysis of pyrrhotite superlattice reflections illuminate the formation pathway of disequilibrium sulfide. We discuss the conditions under which such disequilibrium can occur, and implications for formation of sulfide found in extraterrestrial materials.

¹Dustin Ward, ¹Addi Bischoff, ²Julia Roszjar, ³Jasper Berndt, ⁴Martin J. Whitehouse
American Mineralogist 102, 1856-1880 Link to Article [DOI
https://doi.org/10.2138/am-2017-6056]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, 1010 Vienna, Austria
³Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
⁴Department of Geosciences, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden
Copyright: The Mineralogical Society of America

Most extraterrestrial samples feature the two accessory Ca-phosphates (apatite-group minerals and merrillite), which are important carrier phases of the rare earth elements (REE). The trace-element concentrations (REE, Sc, Ti, V, Cr, Mn, Co, As, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb, Th, and U) of selected grains were analyzed by LA-ICP-MS and/or SIMS (REE only). This systematic investigation includes 99 apatite and 149 merrillite analyses from meteorites deriving from various asteroidal bodies including 1 carbonaceous chondrite, 8 ordinary chondrites, 3 acapulcoites, 1 winonaite, 2 eucrites, 5 shergottites, 1 ureilitic trachyandesite, 2 mesosiderites, and 1 silicate-bearing IAB iron meteorite.

Although Ca-phosphates predominantly form in metamorphic and/or metasomatic reactions, some are of igneous origin. As late-stage phases that often incorporate the vast majority of their host’s bulk REE budget, the investigated Ca-phosphates have REE enrichments of up to two orders of magnitude compared to the host rock’s bulk concentrations. Within a single sample, each phosphate species displays a uniform REE-pattern, and variations are mainly restricted to their enrichment, therefore indicating similar formation conditions. Exceptions are brecciated samples, i.e., the Adzhi-Bogdo (LL3-6) ordinary chondrite. Despite this uniformity within single samples, distinct meteorite groups do not necessarily have unique REE-patterns. Four basic shapes dominate the REE patterns of meteoritic Ca-phosphates: (1) flat patterns, smoothly decreasing from La-Lu with prominent negative Eu anomalies (acapulcoites, eucrites, apatite from the winonaite and the ureilitic trachyandesite, merrillite from ordinary chondrites); (2) unfractionated patterns, with only minor or no anomalies (mesosiderites, enriched shergottites, IAB-iron meteorite); (3) LREE-enriched patterns, with either positive or slightly negative Eu anomalies (chondritic apatite); and (4) strongly LREE-depleted patterns, with negative Eu anomalies (depleted shergottites). The patterns do not correlate with the grade of metamorphism (petrologic type), specific adjacent mineral assemblages or with Ca-phosphate grain size. Neither the proportions of different REE, nor particular REE patterns themselves are universally correlated to a specific formation mechanism yet Eu (i.e., magnitude of the Eu anomaly) is a sensitive indicator to evaluate the timing of plagioclase and phosphate crystallization. Based on our data, U and Th abundances in apatite increase (almost linearly) with the grade of metamorphism, as well as with the differentiation of their host rock.

¹Sundar Srinivaan et al. (>10)
Planetary and Space Science (in Press) Link to Article [https://doi.org/10.1016/j.pss.2017.08.012]
¹Institute of Astronomy & Astrophysics, Academia Sinica, 11F, Astronomy-Mathematics Building, No. 1, Roosevelt Rd, Sec 4, Taipei 10617, Taiwan, Republic of China

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¹Simone De Angelis, ¹Paola Manzari, ¹Maria Cristina De Sanctis, ¹Francesca Altieri, ¹Cristian Carli, ²Giovanna Agrosì
Planetary and Space Science 144 1-15 Link to Article [https://doi.org/10.1016/j.pss.2017.06.005]
¹Institute for Space Astrophysics and Planetology, INAF-IAPS, via Fosso del Cavaliere, 100, 00133, Rome, Italy
²University of Bari “Aldo Moro”, Department of Earth and Geoenvironmental Sciences, via Orabona, 4, 70125, Bari, Italy

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

^1,2M.E.I.Riebe et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12936]
¹Institute of Geochemistry and Petrology, ETH Zürich, Zurich, Switzerland
²Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA
Published by arrangement with John Wiley & Sons

The Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides 10Be, 26Al, and 36Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3-5, CB, LL4/5, R-like). The cosmic-ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS-179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short-lived radionuclides in fragment MS-179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS-179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous 21Ne production-rate, or, perhaps most likely, MS-179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS-179 had an extraordinarily high activity of neutron-capture 36Cl, ~600 dpm kg−1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl-bearing mineral lawrencite.

Christopher D.K.Herd et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.037]
¹Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada
Copyright Elsevier

Northwest Africa (NWA) 8159 is an augite-rich shergottite, with a mineralogy dominated by Ca-, Fe-rich pyroxene, plagioclase, olivine, and magnetite. NWA 8159 crystallized from an evolved melt of basaltic composition under relatively rapid conditions of cooling, likely in a surface lava flow or shallow sill. Redox conditions experienced by the melt shifted from relatively oxidizing (with respect to known Martian lithologies, ∼QFM) on the liquidus to higher oxygen fugacity (∼QFM+2) during crystallization of the groundmass, and under subsolidus conditions. This shift resulted in the production of orthopyroxene and magnetite replacing olivine phenocryst rims. NWA 8159 contains both crystalline and shock-amorphized plagioclase (An50-62), often observed within a single grain; based on known calibrations we bracket the peak shock pressure experienced by NWA 8159 to between 15 and 23 GPa. The bulk composition of NWA 8159 is depleted in LREE, as observed for Tissint and other depleted shergottites; however, NWA 8159 is distinct from all other martian lithologies in its bulk composition and oxygen fugacity. We obtain a Sm-Nd formation age of 2.37 ± 0.25 Ga for NWA 8159, which represents an interval in Mars geologic time which, until recently, was not represented in the other martian meteorite types. The bulk rock 147Sm/144Nd value of 0.37 ± 0.02 is consistent with it being derived directly from its source and the high initial ε143Nd value indicates this source was geochemically highly depleted. Cr, Nd, and W isotopic compositions further support a unique mantle source. While the rock shares similarities with the 2.4-Ga NWA 7635 meteorite, there are notable distinctions between the two meteorites that suggest differences in mantle source compositions and conditions of crystallization. Nevertheless, the two samples may be launch-paired. NWA 8159 expands the known basalt types, ages and mantle sources within the Mars sample suite to include a second igneous unit from the early Amazonian.