^1,2Ann N. Nguyen,^2,3Eve L. Berger,²Keiko Nakamura-Messenger,²Scott Messenger,²Lindsay P. Keller
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12905]
¹Jacobs, NASA Johnson Space Center, Houston, Texas, USA
²Robert M. Walker Laboratory for Space Science, EISD Directorate, Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas, USA
³GeoControl Systems – Jacobs JETS Contract, NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

We have discovered in a Stardust mission terminal particle a unique mineralogical assemblage of symplectically intergrown pentlandite ((Fe,Ni)9S8) and nanocrystalline maghemite (γ-Fe2O3). Mineralogically similar cosmic symplectites (COS) have only been found in the primitive carbonaceous chondrite Acfer 094 and are believed to have formed by aqueous alteration. The O and S isotopic compositions of the Wild 2 COS are indistinguishable from terrestrial values. The metal and sulfide precursors were thus oxidized by an isotopically equilibrated aqueous reservoir either inside the snow line, in the Wild 2 comet, or in a larger Kuiper Belt object. Close association of the Stardust COS with a Kool mineral assemblage (kosmochloric Ca-rich pyroxene, FeO-rich olivine, and albite) that likely originated in the solar nebula suggests the COS precursors also had a nebular origin and were transported from the inner solar system to the comet-forming region after they were altered.

^1,2Arshad Ali,¹Sobhi J. Nasir,²Iffat Jabeen,³Ahmed Al Rawas,²Neil R. Banerjee,^2,4Gordon R. Osinski
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12910]
¹Earth Sciences Research Centre, Sultan Qaboos University, Al-Khodh, Sultanate of Oman
²Department of Earth Sciences & Centre for Planetary Science and Exploration, Western University, London, Ontario, Canada
³Department of Physics, College of Science, Sultan Qaboos University, Al-Khodh, Sultanate of Oman
⁴Department of Physics and Astronomy, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

Mean bulk chemical data of recently found H5 and L6 ordinary chondrites from the deserts of Oman generally reflect isochemical features which are consistent with the progressive thermal metamorphism of a common, unequilibrated starting material. Relative differences in abundances range from 0.5–10% in REE (Eu = 14%), 6–13% in siderophile elements (Co = 48%), and >10% in lithophile elements (exceptions are Ba, Sr, Zr, Hf, U = >30%) between H5 and L6 groups. These differences may have accounted for variable temperature conditions during metamorphism on their parent bodies. The CI/Mg-normalized mean abundances of refractory lithophile elements (Al, Ca, Sm, Yb, Lu, V) show no resolvable differences between H5 and L6 suggesting that both groups have experienced the same fractionation. The REE diagram shows subtle enrichment in LREE with a flat HREE pattern. Furthermore, overall mean REE abundances are ~0.6 × CI with enriched La abundance (~0.9 × CI) in both groups. Precise oxygen isotope compositions demonstrate the attainment of isotopic equilibrium by progressive thermal metamorphism following a mass-dependent isotope fractionation trend. Both groups show a ~slope-1/2 line on a three-isotope plot with subtle negative deviation in ∆17O associated with δ18O enrichment relative to δ17O. These deviations are interpreted as the result of liberation of water from phyllosilicates and evaporation of a fraction of the water during thermal metamorphism. The resultant isotope fractionations caused by the water loss are analogous to those occurring between silicate melt and gas phase during CAI and chondrule formation in chondrites and are controlled by cooling rates and exchange efficiency.

¹A. J. Westphal et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12893]
¹Space Sciences Laboratory, U.C. Berkeley, Berkeley, California, USA
*Find the extensive, full author and affiliation list on the publishers website
Published by arrangement with John Wiley & Sons

Recent observations indicate that >99% of the small bodies in the solar system reside in its outer reaches—in the Kuiper Belt and Oort Cloud. Kuiper Belt bodies are probably the best-preserved representatives of the icy planetesimals that dominated the bulk of the solid mass in the early solar system. They likely contain preserved materials inherited from the protosolar cloud, held in cryogenic storage since the formation of the solar system. Despite their importance, they are relatively underrepresented in our extraterrestrial sample collections by many orders of magnitude (~1013 by mass) as compared with the asteroids, represented by meteorites, which are composed of materials that have generally been strongly altered by thermal and aqueous processes. We have only begun to scratch the surface in understanding Kuiper Belt objects, but it is already clear that the very limited samples of them that we have in our laboratories hold the promise of dramatically expanding our understanding of the formation of the solar system. Stardust returned the first samples from a known small solar system body, the Jupiter-family comet 81P/Wild 2, and, in a separate collector, the first solid samples from the local interstellar medium. The first decade of Stardust research resulted in more than 142 peer-reviewed publications, including 15 papers in Science. Analyses of these amazing samples continue to yield unexpected discoveries and to raise new questions about the history of the early solar system. We identify nine high-priority scientific objectives for future Stardust analyses that address important unsolved problems in planetary science.

^1,2Marianna Mészáros,¹Ingo Leya,^2,3Beda A. Hofmann
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12904]
¹Space Research and Planetary Sciences, University of Bern, Bern, Switzerland
²Natural History Museum Bern, Bern, Switzerland
³Institute of Geological Sciences, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

We measured the concentrations and isotopic compositions of the stable isotopes of He, Ne, Ar, Kr, and Xe in the two lunar impact-melt breccias Abar al’ Uj (AaU) 012 and Shişr 166 to obtain information on their cosmic-ray exposure histories and possible launch pairing; the latter was suggested because of their similar chemical composition. AaU 012 has higher gas concentrations than Shişr 166 and clearly contains implanted solar wind gases, indicating a shallow to moderate shielding for this meteorite in the lunar regolith. The maximum shielding depth of AaU 012 was most likely ≤310 g cm−2 and its lunar regolith residence time was ≥420 ± 70 Ma. Our results indicate that in Shişr 166 the trapped component is a mixture of air and solar wind. The low concentration of cosmogenic and solar wind gases indicate substantial diffusive gas loss and a shielding depth of <700 g cm−2 on the Moon for Shişr 166. All differences seen in the concentrations and isotopic compositions of the noble gases suggest that AaU 012 and Shişr 166 are most likely not launch pairs, although a different exposure history on the Moon does not exclude the possibility that the two meteorites were ejected by a single, large impact event.

^1,2Naoya Imae, ³Hiroshi Isobe
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.05.040]
¹Antarctic Meteorite Research Center, National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
²Department of Polar Science, School of Multidisciplinary Sciences, SOKENDAI (The Graduate University for Advanced Studies), 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan
³Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University, Chuo-ku, Kumamoto 860-8555, Japan
Copyright Elsevier

Chondrules, igneous objects of ∼1 mm in diameter, formed in the earliest solar system via a transient heating event, are divided into two types: main (type I, FeO-poor) and minor (type II, FeO-rich). Using various chondritic materials for different redox conditions and grain sizes, chondrule reproduction experiments were carried out at IW-2 to IW-3.8, with cooling rates mainly ∼100°C/h, with peak temperatures mainly at 1450 °C, and mainly at 100 Pa in a Knudsen cell providing near chemical equilibrium between the charge and the surrounding gas at the peak temperatures. Vapor pressures in the capsule were controlled using solid buffers. After and during the significant evaporation of the iron component from the metallic iron-poor starting materials in near equilibrium, crystallization occurred. This resulted in the formation of a product similar to the type I chondrules. Dusty olivine grains occurred in charges that had precursor type II chondrules containing coarse ferroan olivine, but such grains are not common in type I chondrules. Therefore fine-grained ferroan matrices rather than type II chondrules are main precursor for type I chondrules. The type I chondrules would have evolved via evaporation and condensation in the similar conditions to the present experimental system. Residual gas, which escaped in experiments, could have condensed to form matrices, leading to complementary compositions. Clusters of matrices and primordial chondrules could have been recycled to form main-generation chondrules originated from the shock heating.

^1,2B.J. Shaulis, ¹M. Rightera, T.J. Lapen, ³B.L. Jolliff, ⁴A.J. Irving
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.031]
¹Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007
²Department of Geosciences, University of Arkansas, Fayetteville, AR 72701
³Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130
⁴Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195
Copyright Elsevier

The Northwest Africa (NWA) 773 clan of meteorites is a group of paired and/or petrogenetically related stones that contain at least six different lithologies: magnesian gabbro, ferroan gabbro, anorthositic gabbro, olivine phyric basalt, regolith breccia, and polymict breccia. Uranium-lead dates of baddeleyite in the magnesian gabbro, ferroan gabbro, and components within breccia lithologies of paired lunar meteorites NWA 773, NWA 3170, NWA 6950, and NWA 7007 indicate a chronologic link among the meteorites and their components. A total of 50 baddeleyite grains were analyzed and yielded weighted average 207Pb-206Pb dates of 3119.4 ± 9.4 (n = 27), 3108 ± 20 (n = 13), and 3113 ± 15 (n = 10) Ma for the magnesian gabbro, ferroan gabbro, and polymict breccia lithologies, respectively. A weighted average date of 3115.6 ± 6.8 Ma (n = 47/50) was calculated from the baddeleyite dates for all lithologies. A single large zircon grain found in a lithic clast in the polymict breccia of NWA 773 yielded a U-Pb concordia date of 3953 ± 18 Ma, indicating a much more ancient source for some of the components within the breccia. A U-Pb concordia date of apatite and merrillite grains from the magnesian gabbro and polymict breccia lithologies in NWA 773 is 3112 ± 33 Ma, identical to the baddeleyite dates. Magnesian and ferroan gabbros, as well as the dated baddeleyite and Ca-phosphate-bearing detritus in the breccia lithologies, formed during the same igneous event at about 3115 Ma. These data also strengthen proposed petrogenetic connections between magnesian and ferroan gabbro lithologies, which represent some of the youngest igneous rocks known from the Moon.

^1,2Graeme M. Poole, ^1,3Mark Rehkämper, ¹Barry J. Coles, ^1,4Tatiana Goldberg, ³Caroline L. Smith
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.05.001]
¹Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK<
²School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
³Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK
⁴Institute of Applied Geosciences, TNO, Utrecht, The Netherlands
Copyright Elsevier

We have investigated nucleosynthetic Mo isotope anomalies in 38 different bulk iron meteorites from 11 groups, to produce by far the largest and most precise dataset available to date for such samples. All magmatic iron groups were found to display deficits in s-process Mo isotopes, with essentially constant anomalies within but significant variations between groups. Only meteorites of the non-magmatic IAB/IIICD complex revealed terrestrial Mo isotopic compositions.

The improved analytical precision achieved in this study enables two isotopically distinct suites of iron meteorites to be identified. Of these, the r=p suite encompasses the IC, IIAB, IIE, IIIAB, IIIE and IVA groups and exhibits relatively modest but ‘pure’ s-process deficits, relative to Earth. The second r>p suite includes groups IIC, IIIF and IVB. These iron meteorites show larger s-process deficits than the r=p suite, coupled with an excess of r-process relative to p-process components.

Comparison of the results with data for other elements (e.g., Cr, Ni, Ru, Ti, Zr) suggests that the Mo isotope variability is most likely produced by thermal processing and selective destruction of unstable presolar phases. An updated model is proposed, which relates the iron meteorite suites to different extents of thermal processing in the solar nebula, as governed by heliocentric distance. In detail, the r=p suite of iron meteorite parent bodies is inferred to have formed closer to the Sun, where the extent of thermal processing was similar to that experienced by terrestrial material, so that the meteorites exhibit only small s-process deficits relative to Earth. In contrast, the r>p suite formed at greater heliocentric distance, where more subtle thermal processing removed a smaller proportion of r- and p-process host phases, thereby generating larger s-process deficits relative to the terrestrial composition. In addition, the thermal conditions enabled selective destruction of p- versus r-isotope carrier phases, to produce the observed divergence of r- and p-process Mo isotope abundances.

¹Donald D. Clayton, ¹Bradley S. Meyer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.027]
¹Clemson University, Department of Physics and Astronomy, Clemson, SC 29634
Copyright Elsevier

Our goal is to compute the abundances of carbon atomic complexes that emerge from the C+O cores of core-collapse supernovae. We utilize our chemical reaction network in which every atomic step of growth employs a quantum-mechanically guided reaction rate. This tool follows step-by-step the growth of linear carbon chain molecules from C atoms in the oxygen-rich C+O cores. We postulate that once linear chain molecules reach a sufficiently large size, they isomerize to ringed molecules, which serve as seeds for graphite grain growth. We demonstrate our technique for merging the molecular reaction network with a parallel program that can follow 1017 steps of C addition onto the rare seed species. Due to radioactivity within the C+O core, abundant ambient oxygen is unable to convert C to CO, except to a limited degree that actually facilitates carbon molecular ejecta. But oxygen severely minimizes the linear-carbon-chain abundances. Despite the tiny abundances of these linear-carbon-chain molecules, they can give rise to a small abundance of ringed-carbon molecules that serve as the nucleations on which graphite grain growth builds. We expand the C+O-core gas adiabatically from 6000K for 109 s when reactions have essentially stopped. These adiabatic tracks emulate the actual expansions of the supernova cores. Using a standard model of 1056 atoms of C+O core ejecta having O/C=3, we calculate standard ejection yields of graphite grains of all sizes produced, of the CO molecular abundance, of the abundances of linear-carbon molecules, and of Buckminsterfullerene. None of these except CO was expected from the C+O cores just a few years past.

¹Conel M. O’D. Alexander
Philosophical Transactions of the Royal Society A 375, 2094 Link to Article [https://doi.org/10.1098/rsta.2015.0384]
¹Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA

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¹L. J. Hallis
Philosophical Transactions of the Royal Society 373, 2094 Link to Article [DOI: 10.1098/rsta.2015.0390]
¹School of Geographical and Earth Sciences, Gregory Building, University of Glasgow, Glasgow G12 8QQ, UK

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