¹Maria Eugenia Varela,²Ernst Zinner
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), San Juan, Argentina
²Laboratory for Space Sciences and the Physics Department, Washington University, St. Louis, Missouri, USA

Glass-bearing inclusions hosted by different mineral phases in SNC meteorites provide important information on the conditions that prevailed during formation of early phases and/or on the composition of the primary trapped liquids/melts of these rocks. Although extensive previous work has been reported on such inclusions, several questions are still unresolved. We performed a chemical and petrographic study of the constituents (glasses and mineral assemblage) of glassy and multiphase inclusions in Shergotty and Chassigny. We focused on obtaining accurate trace element contents of glasses and co-existing minerals and discussing their highly variable REE contents. Our results reveal an unusual geochemistry of trace element contents that appear to be independent of their major element compositions. Chemical equilibrium between phases inside inclusions as well as between glasses and host minerals could not be established. The LREE contents of glasses in glass inclusions can vary by up to two orders of magnitude. The depletion in trace element abundances shown by glasses seem to be inconsistent with these phases being residual melts. The light lithophile element contents of glasses are highly variable with enrichment in incompatible elements (e.g., Be, Sr, Ba, and LREE) indicating some processes involving percolation of fluids. All of these features are incompatible with glass-bearing inclusions in the host minerals acting as closed systems preserving unmodified primary liquids/melts. Glass-bearing inclusions in Shergotty and Chassigny appear to have been altered (as was the rock itself) by different postformational processes (e.g., shock, metamorphism, metasomatic [?] fluids) that affected these meteorites with different degree of intensity. Our results indicate that these inclusions could not preserve a reliable sample of the primary trapped melt.

Reference
Varela ME, Zinner E (2015) Glass-bearing inclusions in Shergotty and Chassigny: Consistent samples of a primary trapped melt? Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12566]
Publsihed by arrangement with John Wiley & Sons

¹Julie E. M. McGeoch,²Malcolm W. McGeoch

¹Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts, USA
²PLEX LLC, Fall River, Massachusetts, USA

It has been proposed that exothermic gas phase polymerization of amino acids can occur in the conditions of a warm dense molecular cloud to form hydrophobic polymer amide (HPA) (McGeoch and McGeoch 2014). In a search for evidence of this presolar chemistry Allende and Murchison meteorites and a volcano control were diamond burr-etched and Folch extracted for potential HPA yielding 85 unique peaks in the meteorite samples via matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI TOF/MS). The amino acids after acid hydrolysis in Allende were below the level of detection but many of the Allende peaks via the more sensitive MALDI/TOF analysis could be fitted to a polymer combination of glycine, alanine, and alpha-hydroxyglycine with high statistical significance. A similar significant fit using these three amino acids could not be applied to the Murchison data indicating more complex polymer chemistry.

Reference
McGeoch JEM, McGeoch MW (2015) Polymer amide in the Allende and Murchison meteorites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12558]

Published by arrangement with John Wiley & Sons

^1,2Penelope J. Wozniakiewicz, ³Hope A. Ishii, ^1,2Anton T. Kearsley, ³John P. Bradley, ¹Mark. C. Price, ¹Mark J. Burchell, ⁴Nick Teslich,¹Mike J. Cole
¹School of Physical Science, Centre for Astrophysics and Planetary Sciences, University of Kent, Canterbury, UK
²Department of Earth Sciences, Impacts & Astromaterials Research Centre (IARC), Natural History Museum, London, UK
³Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mānoa, Honolulu, Hawai’i, USA
⁴Lawrence Livermore National Laboratory, Livermore, California, USA

Comet 81P/Wild 2 samples returned by NASA’s Stardust mission provide an unequalled opportunity to study the contents of, and hence conditions and processes operating on, comets. They can potentially validate contentious interpretations of cometary infrared spectra and in situ mass spectrometry data: specifically the identification of phyllosilicates and carbonates. However, Wild 2 dust was collected via impact into capture media at ~6 km s−1, leading to uncertainty as to whether these minerals were captured intact, and, if subjected to alteration, whether they remain recognizable. We simulated Stardust Al foil capture conditions using a two-stage light-gas gun, and directly compared transmission electron microscope analyses of pre- and postimpact samples to investigate survivability of lizardite and cronstedtite (phyllosilicates) and calcite (carbonate). We find the phyllosilicates do not survive impact as intact crystalline materials but as moderately to highly vesiculated amorphous residues lining resultant impact craters, whose bulk cation to Si ratios remain close to that of the impacting grain. Closer inspection reveals variation in these elements on a submicron scale, where impact-induced melting accompanied by reducing conditions (due to the production of oxygen scavenging molten Al from the target foils) has resulted in the production of native silicon and Fe- and Fe-Si-rich phases. In contrast, large areas of crystalline calcite are preserved within the calcite residue, with smaller regions of vesiculated, Al-bearing calcic glass. Unambiguous identification of calcite impactors on Stardust Al foil is therefore possible, while phyllosilicate impactors may be inferred from vesiculated residues with appropriate bulk cation to Si ratios. Finally, we demonstrate that the characteristic textures and elemental distributions identifying phyllosilicates and carbonates by transmission electron microscopy can also be observed by state-of-the-art scanning electron microscopy providing rapid, nondestructive initial mineral identifications in Stardust residues.

Reference
Wozniakiewicz PJ, Ishii HA, Kearsley AT, Bradley JP, Price MC, Burchell MJ, Teslich N, Cole MJ (2015) The survivability of phyllosilicates and carbonates impacting Stardust Al foils: Facilitating the search for cometary water. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12568]

Published by arrangement with John Wiley & Sons

¹Ebbe N. Bak, ²Svend J. Knak Jensen, ¹Per Nørnberg, ^1,3Kai Finster
¹Department of Bioscience, Aarhus University, Ny Munkegade 116, Building 1540, 8000 Aarhus C, Denmark
²Department of Chemistry, Aarhus University, Langelandsgade 140, Building 1511, 8000 Aarhus C, Denmark
³Stellar Astrophysics Center, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Building 1520, 8000 Aarhus C, Denmark

The only organic compounds that have been detected in the Martian soil are simple chlorinated compounds released from heated surface material. However, the sources of the organic carbon are in dispute. Wind abraded silicates, which are widespread on the Martian surface, can sequester atmospheric methane which generates methylated silicates and thus could provide a mechanism for accumulation of reduced carbon in the surface soil. In this study we show that thermal volatilization of methylated silicates in the presence of perchlorate leads to the production of chlorinated methane. Thus, methylated silicates could be a source of the organic carbon released as chlorinated methane upon thermal volatilization of Martian soil samples. Further, our experiments show that the ratio of the different chlorinated compounds produced is dependent on the mass ratio of perchlorate to organic carbon in the soil.

Reference
Bak EN, Knak Jensen SJ, Nørnberg P, Finster K (2015) Methylated silicates may explain the release of chlorinated methane from Martian soil. Earth & Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2015.10.044]
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