^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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¹D. H. Wooden, ²H. A. Ishii, ³M. E. Zolensky
Philosophical Transactions of the Royal Society A 375, 2097 Link to Article [https://doi.org/10.1098/rsta.2016.0260]
¹NASA Ames Research Center, Moffett Field, CA 94035-0001, USA
²University of Hawaii, Hawai’i Institute of Geophysics and Planetology, Honolulu, HI 96822, USA
³NASA Johnson Space Center, ARES, X12 2010 NASA Parkway, Houston, TX 77058-3607, USA
e-mail: diane.wooden@nasa.gov

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^1,2Hidenori Genda, ²Tsuyoshi Iizuka, ^3,4Takanori Sasaki, ^1,4Yuichiro Ueno, ^2,5Masahiro Ikoma
Earth and Planetary Science Letters 470, 87-95 Link to Article [https://doi.org/10.1016/j.epsl.2017.04.035]
¹Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
²Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁴Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
⁵Research Center for the Early Universe, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Copyright Elsevier

The Earth was born in violence. Many giant collisions of protoplanets are thought to have occurred during the terrestrial planet formation. Here we investigated the giant impact stage by using a hybrid code that consistently deals with the orbital evolution of protoplanets around the Sun and the details of processes during giant impacts between two protoplanets. A significant amount of materials (up to several tens of percent of the total mass of the protoplanets) is ejected by giant impacts. We call these ejected fragments the giant-impact fragments (GIFs). In some of the erosive hit-and-run and high-velocity collisions, metallic iron is also ejected, which comes from the colliding protoplanets’ cores. From ten numerical simulations for the giant impact stage, we found that the mass fraction of metallic iron in GIFs ranges from ∼1 wt% to ∼25 wt%. We also discussed the effects of the GIFs on the dynamical and geochemical characteristics of formed terrestrial planets. We found that the GIFs have the potential to solve the following dynamical and geochemical conflicts: (1) The Earth, currently in a near circular orbit, is likely to have had a highly eccentric orbit during the giant impact stage. The GIFs are large enough in total mass to lower the eccentricity of the Earth to its current value via their dynamical friction. (2) The concentrations of highly siderophile elements (HSEs) in the Earth’s mantle are greater than what was predicted experimentally. Re-accretion of the iron-bearing GIFs onto the Earth can contribute to the excess of HSEs. In addition, Iron-bearing GIFs provide significant reducing agent that could transform primitive CO2–H2O atmosphere and ocean into more reducing H2-bearing atmosphere. Thus, GIFs are important for the origin of Earth’s life and its early evolution.

¹S.L. Simpson, ²A.J. Boyce, ³P. Lambert, ¹P. Lindgren, ¹M.R. Lee
Earth and Planetary Science Letters 460, 192-200 Link to Article [https://doi.org/10.1016/j.epsl.2016.12.023]
¹University of Glasgow, School of Geographical and Earth Sciences, Lilybank Gardens, G12 8QQ, Glasgow, UK
²Scottish Universities Environmental Research Centre, Rankine Ave, East Kilbride, G75 0QF, Glasgow, UK
³Sciences et Applications, 218 Boulevard Albert 1er, 33800 Bordeaux, France
Copyright Elsevier

The highly eroded 23 km diameter Rochechouart impact structure, France, has extensive evidence for post-impact hydrothermal alteration and sulphide mineralisation. The sulphides can be divided into four types on the basis of their mineralogy and host rock. They range from pyrites and chalcopyrite in the underlying coherent crystalline basement to pyrites hosted in the impactites. Sulphur isotopic results show that δ34S values vary over a wide range, from −35.8‰ to +0.4‰. The highest values, δ34S −3.7‰ to +0.4‰, are recorded in the coherent basement, and likely represent a primary terrestrial sulphur reservoir. Sulphides with the lowest values, δ34S −35.8‰ to −5.2‰, are hosted within locally brecciated and displaced parautochthonous and autochthonous impactites. Intermediate δ34S values of −10.7‰ to −1.2‰ are recorded in the semi-continuous monomict lithic breccia unit, differing between carbonate-hosted sulphides and intraclastic and clastic matrix-hosted sulphides. Such variable isotope values are consistent with a biological origin, via bacterial sulphate reduction, for sulphides in the parautochthonous and autochthonous units; these minerals formed in the shallow subsurface and are probably related to the post impact hydrothermal system. The source of the sulphate is likely to have been seawater, penecontemporaneous to the impact, as inferred from the marginal marine paleogeography of the structure. In other eroded impact craters that show evidence for impact-induced hydrothermal circulation, indirect evidence for life may be sought isotopically within late-stage (≤120 °C) secondary sulphides and within the shocked and brecciated basement immediately beneath the transient crater floor.