¹David.V. Bekaert, ¹Guillaume Avice, ¹Bernard Marty, ²Bryana Henderson, ²Murthy.S. Gudipati
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.041]
¹Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, Université de Lorraine, UMR 7358, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès-Nancy, France
²Science Division, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr, Pasadena, 8 CA 91109, USA
Copyright Elsevier

Despite extensive effort during the last four decades, no clear signature of a lunar indigenous noble gas component has been found. In order to further investigate the possible occurrence of indigenous volatiles in the Moon, we have re-analyzed the noble gas and nitrogen isotopic compositions in three anorthosite samples. Lunar anorthosites 60025, 60215 and 65315 have the lowest exposure duration (∼2 Ma) among Apollo samples and consequently contain only limited cosmogenic (e.g. 124,126Xe) and solar wind (SW) noble gases. Furthermore, anorthosites have negligible contributions of fissiogenic Xe isotopes because of their very low Pu and U contents. As observed in previous studies (Lightner and Marti, 1974 ; Leich and Niemeyer, 1975), lunar anorthosite Xe presents an isotopic composition very close to that of terrestrial atmospheric Xe, previously attributed to “anomalous adsorption” of terrestrial Xe after sample return. The presumed atmospheric Xe contamination can only be removed by heating the samples at medium to high temperatures under vacuum, and is therefore different from common adsorption. To test this hypothesis, we monitored the adsorption of Xe onto lunar anorthositic powder using infrared reflectance spectroscopy. A clear shift in the anorthosite IR absorbance peaks is detected when comparing the IR absorbance spectra of the lunar anorthositic powder before and after exposure to a neutral Xe-rich atmosphere. This observation accounts for the chemical bonding (chemisorption) of Xe onto anorthosite, which is stronger than the common physical bonding (physisorption) and could account for the anomalous adsorption of Xe onto lunar samples.

Our high precision Xe isotope analyses show slight mass fractionation patterns across 128-136Xe isotopes with systematic deficits in the heavy Xe isotopes (mostly 136Xe and marginally 134Xe) that have not previously been observed. This composition could be the result of mixing between an irreversibly adsorbed terrestrial contaminant that is mostly released at high temperature and an additional signature. Solar Wind (SW) Xe contents, estimated from SW-Ne and SW-Ar concentrations and SW-Ne/Ar/Xe elemental ratios, do not support SW as the additional contribution. Using a χ2 test, the latter is best accounted for by cometary Xe as measured in the coma of Comet 67P/Churyumov-Gerasimenko (Marty et al., 2017) or by the primordial U-Xe composition inferred to be the precursor of atmospheric Xe (Pepin, 1994 ; Avice et al., 2017). It could have been contributed to the lunar budget by volatile-rich bodies that participated to the building of the terrestrial atmosphere inventory (Marty et al., 2017).

¹G. Hublet, ¹V. Debaille, ²J. Wimpenny, ²Q-Z. Yin
Geochimica et Cosmochimica Acta(in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.09.005]
¹Laboratoire G-Time, Université Libre de Bruxelles, CP 160/02, 50, Av. F.D. Roosevelt, 1050 Brussels, Belgium,
²Department of Earth and Planetary Sciences, University of California at Davis, Davis, CA 95616, USA
Copyright Elsevier

The ²⁶Al-²⁶Mg short-lived chronometer has been widely used for dating ancient objects in studying the early Solar System. Here, we use this chronometer to investigate and refine the geological history of the asteroid 4-Vesta. Ten meteorites widely believed to come from Vesta (4 basaltic eucrites, 3 cumulate eucrites and 3 diogenites) and the unique achondrite Asuka 881394 were selected for this study. All samples were analyzed for their δ²⁶Mg∗ and ²⁷Al/²⁴Mg ratios, in order to construct both whole rock and model whole rock isochrons. Mineral separation was performed on 8 of the HED’s in order to obtain internal isochrons. While whole rock Al-Mg analyses of HED’s plot on a regression that could be interpreted as a vestan planetary isochron, internal mineral isochrons indicate a more complex history. Crystallization ages obtained from internal ²⁶Al-²⁶Mg systematic in basaltic eucrites show that Vesta’s upper crust was formed during a short period of magmatic activity at  million years (Ma) after Calcium-Aluminum inclusions (after CAI). We also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of Vesta while ²⁶Al was still extant. Cumulate eucrites crystallized progressively from  to >7.25 Ma after CAI. Model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modelled at  Ma after CAI, i.e. contemporaneously from the onset of the basaltic eucritic crust. Based on their cumulate texture, we suggest cumulate eucrites were likely formed deeper in the crust of Vesta. Diogenites have a more complicated history and their ²⁶Al-²⁶Mg systematics show that they likely formed after the complete decay of ²⁶Al and thus are younger than eucrites. This refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4-Vesta from which the basaltic eucrites rapidly crystallized. In order to explain the younger age and the complex history of diogenites, we postulate that a second episode of magmatism was possibly triggered by a mantle overturn. We bring a refined chronology of the geological history of Vesta that shows that the asteroid has known a more-complex differentiation than previously thought.

^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