The most popular papers on Cosmochemistry Papers in May were:

1-Liu N et al. (2015) Correlated Strontium and Barium Isotopic Compositions of Acid-cleaned Single Mainstream Silicon Carbides from Murchison. Astrophysical Journal 803,12 Link to Article [doi:10.1088/0004-637X/803/1/12]

2-Rubin AE (2015) Maskelynite in asteroidal, lunar and planetary basaltic meteorites: An indicator of shock pressure during impact ejection from their parent bodies. Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2015.05.010]

3-Tomeoka K, Ohnishi I (2015) Redistribution of chondrules in a carbonaceous chondrite parent body: A model. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.05.012]

4-Hartmann WK (2014) The giant impact hypothesis: past, present (and future?). Philosophical Transactions of the Royal Society A 13,372, 2024 Link to Article [doi: 10.1098/rsta.2013.0249]

5-Dauphas N, Pourmand A (2015) Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.03.037]

^1,2Xiaojia Zeng, ¹Xiongyao Li, ¹Shijie Wang, ¹Shijie Li, ³Nicole Spring, ¹Hong Tang, ¹Yang Li, ¹Junming Feng
¹Lunar and Planetary Science Research Center, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 55002, China
²University of Chinese Academy of Sciences, Beijing 10049, China
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK

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Reference
Zeng X, Li X, Wang S, Li S, Spring N, Tang H, Li Y, Feng J (2015) JMSS-1: a new Martian soil simulant.
Earth, Planets and Space 67:72
Link to Article [doi:10.1186/s40623-015-0248-5]

¹E. Ammannito
¹University of California Los Angeles, Earth Planetary and Space Sciences, Los Angeles, CA-90095, USA
*Find the extensive, full author and affiliation list on the publishers website

We present and describe the maps of spectral parameters such as pyroxene band centers and depths, reflectance at 1.4 μm and 2.8μm band depth in the Rheasilvia quadrangle. We found a broad anti-correlation between pyroxene band centers and depths while the reflectance is not correlated with the pyroxene spectral parameters. In addition, we found that the Rheasilvia quadrangle is free of OH absorption signatures. We also derived lithological maps with improvements in the spatial resolution with respect to previous lithological maps of the same region. We confirm that the central mound is dominated by eucritic/howarditic pyroxene while diogenitic lithology has been found mainly in a region delineated by Tarpeia, Severina and Mariamne craters. We found small scale variations in the composition of pyroxene. These variations identify lithological units that extend for tens of km, although small units of less than 1 km have also been found. We consider this fact as an indication of a high level of compositional heterogeneity within the Vestan crust.

Reference
Ammannito E et al. (2015) Compositional variations in the Vestan Rheasilvia basin. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.017]

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¹Lidia Pittarello, ²Fabrizio Nestola, ³Cecilia Viti, ⁴Alvaro Penteado Crósta, ^1,5Christian Koeberl
¹ Department of Lithospheric Research, University of Vienna, Vienna, Austria
² Department of Geosciences, University of Padova, Padova, Italy
³ Department of Physics, Earth and Environmental Sciences, University of Siena, Siena, Italy
⁴ Institute of Geosciences, Campinas SP, Brazil
⁵ Natural History Museum, Vienna, Austria

Shatter cones are one of the most widely recognized pieces of evidence for meteorite impact events on Earth, but the process responsible for their formation is still debated. Evidence of melting on shatter cone surfaces has been rarely reported in the literature from terrestrial impact craters but has been recently observed in impact experiments. Although several models for shatter cones formation have been proposed, so far, no one can explain all the observed features. Shatter cones’ from the Vista Alegre impact structure, Brazil, formed in fine-grained basalt of the Jurassic-Cretaceous Serra Geral Formation (Paraná large igneous province). A continuous quenched melt film, consisting of a crystalline phase, mica, and amorphous material, decorates the striated surface. Ultracataclasites, containing subrounded pyroxene clasts in an ultrafine-grained matrix, occur subparallel to the striated surface. Several techniques were applied to characterize the crystalline phase in the melt, including Raman spectroscopy and transmission electron microscopy. Results are not consistent with any known mineral, but they do suggest a possible rare or new type of clinopyroxene. This peculiar evidence of melting and cataclasis in relation with shatter cone surfaces is interpreted as the result of tensile fracturing at the tip of a fast propagating shock-induced rupture, which led to the formation of shatter cones at the tail of the shock front, likely during the early stage of the impact events.

References
Pittarello L, Nestola F, Viti C, Crósta AP, Koeberl C (2015) Melting and cataclastic features in shatter cones in basalt from the Vista Alegre impact structure, Brazil. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12466]

Published by arrangement with John Wiley&Sons

¹Alan E. Rubin
¹Department of Earth, Planetary, and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more-pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact-induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near-surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4-level shock-damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.

Reference
Rubin AE (2015) Shock and annealing in aubrites: Implications for parent-body history. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12464]
Published by arrangement with John Wiley&Sons

^1,2,3Katherine H. Joy, ⁴Channon Visscher, ⁵Michael E. Zolensky, ⁶Takashi Mikouchi, ⁷Kenji Hagiya, ⁸Kazumasa Ohsumi, ^1,2David A. Kring
¹Center for Lunar Science and Exploration, The Lunar and Planetary Institute—USRA, Houston, Texas, USA
² NASA Solar System Exploration Research Virtual Institute
³School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
⁴ Dordt College, Iowa, USA
⁵ARES, NASA Johnson Space Center, Houston, Texas, USA
⁶Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo, Japan
⁷Graduate School of Science, University of Hyogo, Ako-gun, Hyogo, Japan
⁸Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo, Japan

Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe-oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95-97), pyroxene (En74-75, Fs21-22,Wo3-4), and olivine (Fo66-67) are distributed in and adjacent to the Fe-oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X-ray absorption near edge spectroscopy (XANES) and X-ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe-magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization-driven mineral replacement of Fe-metal or desulphurisation from Fe-sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H₂O steam/liquid or CO₂ gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet Impact.

Reference
Joy KH, Visscher C, Zolensky ME, Mikouchi T, Hagiya K, Ohsumi K, Kring DA (2015) Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12462]

Published by arrangement with John Wiley&Sons

¹Klaus Keil et al. (>10)*
¹Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Mānoa, Honolulu, Hawaii, USA
*Find the extensive, full author and affiliation list on the publishers Website

The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm⁻³); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (δ¹⁷O = 3.768 ± 0.042‰, δ¹⁸O = 5.359 ± 0.042‰, Δ¹⁷O = 0.981 ± 0.020‰); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe⁰/total iron (0.105) versus total Fe/Mg (1.164), and of Ni/Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr₂O₃ content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped ¹³²Xe_tr (0.317 × 10⁻⁸cm³STP g⁻¹), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is ~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 μm in size, secondary fayalite grains, defined as olivine with Fa_>75, large enough to allow in situ measurement of oxygen and Mn-Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass-dependent fractionation line with a slope of ~0.5 and Δ¹⁷O of 4.0 ± 0.3‰, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a Δ¹⁷O of ~4‰, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low-temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the ⁵³Mn-⁵³Cr chronology age of secondary fayalite, took place Ma after formation of CV CAIs when anchored to the quenched angrite D’Orbigny.

Reference
Keil K et al. (2015) The Vicência meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2) LL chondrite. Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12456]
Published by arrangement with John Wiley&Sons

^1,2Ulli Raschke, ¹Ralf Thomas Schmitt, ³Iain McDonald, ^1,4Wolf Uwe Reimold, ⁵Dieter Mader, ^5,6Christian Koeberl
¹Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
² Freie Universität Berlin, Institut für Geologische Wissenschaften, Berlin, Germany
³School of Earth & Ocean Sciences, Cardiff University, Cardiff, UK
⁴Humboldt-Universität zu Berlin, Berlin, Germany
⁵Department of Lithospheric Research, Center for Earth Sciences, University of Vienna, Vienna, Austria
⁶Natural History Museum, Vienna, Austria

The complex impact structure El’gygytgyn (age 3.6 Ma, diameter 18 km) in northeastern Russia was formed in ~88 Ma old volcanic target rocks of the Ochotsk-Chukotsky Volcanic Belt (OCVB). In 2009, El’gygytgyn was the target of a drilling project of the International Continental Scientific Drilling Program (ICDP), and in summer 2011 it was investigated further by a Russian–German expedition. Drill core material and surface samples, including volcanic target rocks and impactites, have been investigated by various geochemical techniques in order to improve the record of trace element characteristics for these lithologies and to attempt to detect and constrain a possible meteoritic component. The bedrock units of the ICDP drill core reflect the felsic volcanics that are predominant in the crater vicinity. The overlying suevites comprise a mixture of all currently known target lithologies, dominated by felsic rocks but lacking a discernable meteoritic component based on platinum group element abundances. The reworked suevite, directly overlain by lake sediments, is not only comparatively enriched in shocked minerals and impact glass spherules, but also contains the highest concentrations of Os, Ir, Ru, and Rh compared to other El’gygytgyn impactites. This is—to a lesser extent—the result of admixture of a mafic component, but more likely the signature of a chondritic meteoritic component. However, the highly siderophile element contribution from target material akin to the mafic blocks of the ICDP drill core to the impactites remains poorly constrained.

Reference
Raschke U, Schmitt RT, McDonald I, Reimold WU, Mader D, Koeberl C (2015) Geochemical studies of impact breccias and country rocks from the El’gygytgyn impact structure, Russia. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12455]

Published by arrangement with John Wiley&Sons

¹E. P. Gurov, ¹S. B. Shekhunova, ¹V. V. Permyakov
¹Institute of Geological Sciences, National Academy of Sciences of Ukraine, Kiev, Ukraine

Electron microprobe analyses of accessory and opaque minerals from the impact melt rocks of the Boltysh structure, in the central part of the Ukrainian Shield, are presented in this report. Our study establishes a variety of minerals represented by native metals, alloys, oxides, sulfides, phosphates, and silicates, formed during several stages of cooling and solidification of the thick impact melt sheet. Baddeleyite was determined to be the earliest high-temperature mineral to occur in the impact melt rocks. Iron and titanium oxides crystallized earlier or simultaneously with the microliths of orthopyroxene and feldspars. High concentrations of TiO2, Al2O3, and Cr2O3 were identified in some hematite varieties. Cu- and Ni-bearing pyrrhotites occur in impact melt rocks with a glassy matrix. Native metals—copper, platinum, and silver—were likely formed due to the hydrothermal alteration of the upper unit of the impact melt sheet. Zircon is the only accessory mineral found in impact melt rocks that is preserved from the basement granites of the Boltysh structure.

Reference
Gurov EP, Shekhunova SB, Permyakov VV (2015) Accessory and opaque minerals in impact melt rocks of the Boltysh structure, Ukraine. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12457]

Published by arrangement with John Wiley&Sons

¹A. Ody,²F. Poulet, C. ¹Quantin, ²J.-P. Bibring, ³J.L. Bishop, ⁴M.D. Dyar
¹LGLTPE, université Lyon1, bat Geode, campus de la Doua
²rue Raphael Dubois, 69622 Villerbanne CEDEX. 2. IAS, Centre universitaire d’Orsay, bat 121, 91405 Orsay CEDEX
³SETI Institute, Carl Sagan Center, 189 Bernardo Ave., Mountain View, CA 94043, USA
⁴Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075

The objective of this study is to identify and map spectral analogues of some key Martian meteorites (basaltic shergottites Los Angeles, Shergotty, QUE 94201, lherzolitic shergottite ALH A77005, Nakhla, Chassigny and the orthopyroxenite ALH 84001) in order to localize terrain candidates for their source regions. We develop a best fit procedure to reproduce the near-infrared (NIR) spectral properties of the Martian surface as seen by the hyperspectral imaging spectrometer OMEGA/MEx from the NIR spectra of the Martian meteorites. The fitting process is tested and validated, and Root Mean Square (RMS) global maps for each meteorite are obtained. It is found that basaltic shergottites have NIR spectral properties the most representative of the Martian surface with best spectral analogues found in early Hesperian volcanic provinces. Sites with spectral properties similar to those of ALH A77005 are scarce. They are mainly localized in olivine-bearing regions such as Nili Fossae and small Noachian/early Hesperian terrains. The only plausible source region candidate for Chassigny is the Nili Patera caldera dated to 1.6Ga. Widespread spectral analogues for the ALH 84001 meteorite are found northeast of Syrtis major and northwest of the Hellas basin. While this distribution is in agreement with the low-calcium-pyroxene-rich composition and old age (4.1 Ga) of this meteorite, the modal mineralogy of these candidates is not consistent with that of this meteorite. No convincing spectral analogue is found for the Amazonian-aged Nakhla meteorite suggesting that its olivine/high-calcium-pyroxene-rich composition could be representative of the Amazonian terrains buried under dust. Finally, some young rayed craters are proposed as possible candidate for source crater of the studied Martian meteorites.

References
Ody A, Poulet F, Quantin C, Bibring J-P, Bishop JL, Dyard MD (2015) Candidates source regions of Martian Meteorites as identified by OMEGA/MEx. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.05.019]

Copyright Elsevier