¹Frans J.M. Rietmeijer
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA

In Stardust tracks C2044,0,38, C2044,0,39, and C2044,0,42 (Brennan et al. 2007) and Stardust track 10 (this work) gold is present in excess of its cosmochemical abundance. Ultra-thin sections of allocation FC6,0,10,0,26 (track 10) show a somewhat wavy, compressed silica aerogel/silica glass interface which challenges exact location identification, i.e., silica glass, compressed silica aerogel, or areas of overlap. In addition to domains of pure silica ranging from SiO2 to SiO3 glass, there is MgO-rich silica glass with a deep metastable composition, MgO = 14 ± 6 wt%, due to assimilation of Wild 2 Mg-silicate matter in silica melt. This magnesiosilica composition formed when temperatures during hypervelocity capture reached >2000 °C followed by ultrafast quenching of the magnesiosilica melt when it came into contact with compressed aerogel at ~155 °C. The compressed silica aerogel in track 10 has a continuous Au background as result of the melting point depression of gold particles

Reference
Rietmeijer FJM (2016) At the interface of silica glass and compressed silica aerogel in Stardust track 10: Comet Wild 2 is not a goldmine. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12608]
Published by arrangement with John Wiley & Sons

¹Aurore Hutzler et al.(>10)*
¹ Aix-Marseille Université, CNRS, CEREGE UM34, Technopôle de l’Environnement Arbois-Méditerranée, Aix-en-Provence, France
*Find the extensive, full author and affiliation list on the publishers website

We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The “El Médano collection” was recovered by systematic on-foot search in El Médano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The “private collection” has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Médano collection and gives 251 and 168 meteorites larger than 10 g km−2, before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic-ray exposure ages on the order of 1–10 Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2 Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.

Reference
Hutzler A et al. (2016) Description of a very dense meteorite collection area in western Atacama: Insight into the long-term composition of the meteorite flux to Earth. Meteoritics & Planetary Sciences (in Press)
Link to Article [DOI: 10.1111/maps.12607]
Published by arrangement with John Wiley & Sons

¹Mary Sue Bell
¹Jacobs@NASA/Johnson Space Center, Houston, Texas, USA

A study of pure, single crystal calcite shocked to pressures from 9.0 to 60.8 GPa was conducted to address contradictory data for carbonate shock behavior. The recovered materials were analyzed optically and by transmission electron microscopy (TEM), as well as by thermogravimetry (TGA), X-ray diffraction (XRD), and Raman-spectroscopy. In thin section, progressive comminution of calcite is observed although grains remain birefringent to at least 60.8 GPa. TGA analysis reveals a positive correlation between percent of mass loss due to shock and increasing shock pressure (R = 0.77) and suggests that shock loading leads to the modest removal of structural volatiles in this pressure range. XRD patterns of shocked Iceland spar samples produce peaks that are qualitatively and quantitatively less intense, more diffuse, and shift to lower o2θ. However, the regularity observed in these shocked powder patterns suggests that structures with very uniform unit cell separations persist to shock pressures as high as 60.8 GPa. Raman spectral analyses indicate no band asymmetry and no systematic peak shifting or broadening. TEM micrographs display progressively diminishing crystallite domain sizes. Selected area electron diffraction (SAED) patterns reveal no signatures of amorphous material. These data show that essentially intact calcite is recovered at shock pressures up to 60.8 GPa with only slight mass loss (~7%). This work suggests that the amount of CO2 gas derived from shock devolatilization of carbonate by large meteorite impacts into carbonate targets has been (substantially) overestimated.

Reference
Bell MS (2016) CO2 release due to impact devolatilization of carbonate: Results of shock experiments. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12613]
Published by arrangement with John Wiley & Sons

^1,2Christian M. Schrader, ¹Barbara A. Cohen, ³John J. Donovan, ⁴Edward P. Vicenzi
¹Marshall Space Flight Center, NASA, Huntsville, Alabama, USA
²Geology Department, Bowdoin College, Brunswick, Maine, USA
³Department of Chemistry, University of Oregon, Eugene, Oregon, USA
⁴Smithsonian Institution, Museum Conservation Institute, Suitland, Maryland, USA

Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the unusual composition of its shock melt glass, particularly its enrichment in sulfur relative to the host shergottite. It has been hypothesized that Martian regolith was incorporated into the melt or, conversely, that the S-enrichment stems from preferential melting of sulfide minerals in the host rock during shock. We present results from an electron microprobe study of EET 79001 including robust measurements of major and trace elements in the shock melt glass (S, Cl, Ni, Co, V, and Sc) and minerals in the host rock (Ni, Co, and V). We find that both S and major element abundances can be reconciled with previous hypotheses of regolith incorporation and/or excess sulfide melt. However, trace element characteristics of the shock melt glass, particularly Ni and Cl abundances relative to S, cannot be explained either by the incorporation of regolith or sulfide minerals. We therefore propose an alternative hypothesis whereby, prior to shock melting, portions of EET 79001 experienced acid-sulfate leaching of the mesostasis, possibly groundmass feldspar, and olivine, producing Al-sulfates that were later incorporated into the shock melt, which then quenched to glass. Such activity in the Martian near-surface is supported by observations from the Mars Exploration Rovers and laboratory experiments. Our preimpact alteration model, accompanied by the preferential survival of olivine and excess melting of feldspar during impact, explains the measured trace element abundances better than either the regolith incorporation or excess sulfide melting hypothesis does.

Reference
Schrader CM, Cohen BA, Donovan JJ, Vicenzi EP (2016) Ni/S/Cl systematics and the origin of impact-melt glasses in Martian meteorite Elephant Moraine 79001. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12612]
Published by arrangement with John Wiley & Sons

¹E. Dobrică, ²R. C. Ogliore
¹Department of Earth and Planetary Sciences MSC03-2040, 1 University of New Mexico, Albuquerque 87131-0001, NM, USA
²Department of Physics, Washington University in St. Louis, St. Louis 63117, MO, USA

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Reference
Dobrică E, Ogliore RC (2016) Adhering grains and surface features on two Itokawa particles. Earth, Planets and Space 68:21
Link to Article [doi:10.1186/s40623-016-0391-7]

^1,2Derek W.G. Sears, ^1,2Hazel Sears, ^1,2Daniel R. Ostrowski, ^1,2Kathryne L. Bryson,
¹Jessie Dotson, ³Megan Syal, ³Damian C Swift
¹NASA Ames Research Center, Mountain View, CA 94035, USA
²Bay Area Environmental Research Institute, NASA Ames Research Center, Mountain View, CA 94035, USA
³Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA

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Reference
Sears DWG, Sears H, Ostrowski DR, Bryson KL, Dotson J, Syal M, Swift DC (2016) A meteorite perspective on asteroid hazard mitigation. Planetary and Space Science (in Press)
Link to Article [doi:10.1016/j.pss.2016.01.016]

¹Yang Liu, ²Jeffrey G. Catalano
¹Southwest Research Institute, San Antonio, Texas, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri, USA

Using visible and near infrared data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we identified a sequence of hydrated sulfates and Fe/Mg-smectites in southwest Melas Chasma. Specifically, these hydrated sulfate and semectite deposits are interbedded and have been highly deformed. Equilibrium thermodynamic calculations of coupled basalt weathering and fluid evaporation predict that sequential formation of Fe/Mg-smectites and sulfate evaporites in similar quantities, as observed in the interbedded smectite-sulfate sequences, is chemically plausible. The Fe/Mg-smectite-sulfate deposits may have thus formed through in situ basalt weathering and fluid evaporation, although an origin involving repeated cycles of transport and deposition of detrital clays by a neutral fluid containing Mg and SO4 and subsequent evaporation cannot be ruled out. If the Fe/Mg-smectites are authigenic, the Hesperian (or younger) age Melas Chasma would have experienced prolonged periods with near-surface water, providing potentially habitable conditions.

Reference
Liu Y, Catalano JG (2016) Implications for the Aqueous History of Southwest Melas Chasma, Mars as Revealed by Interbedded Hydrated Sulfate and Fe/Mg-Smectite Deposits. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2016.02.015]
Copyright Elsevier

¹Tsuyoshi Iizuka, ²Yi-Jen Lai, ^2,3Waheed Akram, ⁴Yuri Amelin, ^2,3Maria Schönbächler
¹Department of Earth and Planetary Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan
²Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
³School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁴Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia

Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb–Zr isochrons for three basaltic achondrites with known U–Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb–Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb–Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb–Zr and Pb–Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb–Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of (1.4±0.5)×10−5(1.4±0.5)×10−5 at 4557.93±0.36 Ma4557.93±0.36 Ma, which corresponds to a 92Nb/93Nb ratio of (1.7±0.6)×10−5(1.7±0.6)×10−5 at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb–Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.

Reference
Iizuka I, Lai Y-J, Akram W, Amelin Y, Schönbächler M (2016) The initial abundance and distribution of 92Nb in the Solar System. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2016.02.005]
Copyright Elsevier

¹G. W. Evatt, ¹M. J. Coughlan, ²K. H. Joy, ³A. R. D. Smedley, ³P. J. Connolly,
¹I. D. Abrahams
¹UK School of Mathematics, University of Manchester, Manchester M13 9PL, UK
²The School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK
³Centre for Atmospheric Science, The School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK

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Reference
Evatt GW, Coughlan MJ, Joy KH, Smedley ARD, Connolly PJ, Abrahams ID (2016) A potential hidden layer of meteorites below the ice surface of Antarctica. Nature Communications 7, Article number: 10679
Link to Article [doi:10.1038/ncomms10679]

¹Michael A. Velbel
¹Division of Meteorites, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th and Constitution Avenues NW, Washington, DC 20560-0119

Several nakhlites (clinopyroxenite meteorites from Mars) contain olivine phenocrysts with corrosion features identical in size, shape and distribution to the smaller etch-pits of well-characterized weathered terrestrial olivine. Miller Range (MIL) 03346 is an Antarctic nakhlite find, recovered after long exposure to Antarctic conditions. The distribution of discrete olivine etch-pits almost exclusively within a few hundred microns of allocation MIL 03346,171’s documentably exposed surface suggests that they formed by terrestrial weathering in Antarctica. The small size of olivine etch-pits in MIL 03346,171 relative to commonly much larger etch-pits in even incipiently weathered terrestrial examples suggests that the duration of its exposure to weathering conditions was short, or the weathering conditions to which it was exposed did not favor olivine corrosion (in the form of etch-pit formation), or both. Time-scales for the formation of etch-pits, estimated from experimentally determined dissolution rates of olivine over a range of pHs, are comparable to the measured terrestrial age of the meteorite and short relative to the time available for possible similar corrosion on Mars. Etch-pits of the observed size on MIL 03346 olivine phenocrysts would be relatively easy to form supraglacially under brief episodic acidic Antarctic conditions, but the terrestrial age of MIL 03346 is long enough that its olivine might have been weathered to the observed state by englacial films of alkaline Antarctic water. The paucity of similar etch-pits in olivine from the interior of MIL 03346 suggests that olivine in this Mars meteorite was exposed to even less aqueous alteration after iddingsitization during its 1.3 billion years on Mars than its exterior was subjected to during its Pleistocene-Holocene exposure to Antarctic weathering conditions.

Reference
Velbel MA (2016) Aqueous Corrosion of Olivine in the Mars Meteorite Miller Range (MIL) 03346 During Antarctic Weathering: Implications for Water on Mars. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.036]
Copyright Elsevier