¹Aiden A. Martin, ²Ting Lin, ¹Milos Toth, ³Andrew J. Westphal, ⁴Edward P. Vicenzi, ⁵Jeffrey Beeman,²Eric H. Silver
¹School of Physics and Advanced Materials, University of Technology, Sydney, Ultimo, New South Wales, Australia
²Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA
³Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA
⁴Smithsonian Institution, Museum Conservation Institute, Suitland, California, USA
⁵Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA

In 2006, NASA’s Stardust spacecraft delivered to Earth dust particles collected from the coma of comet 81P/Wild 2, with the goal of furthering the understanding of solar system formation. Stardust cometary samples were collected in a low-density, nanoporous silica aerogel making their study technically challenging. This article demonstrates the identification, exposure, and elemental composition analysis of particles analogous to those collected by NASA’s Stardust mission using in-situ SEM techniques. Backscattered electron imaging is shown by experimental observation and Monte Carlo simulation to be suitable for locating particles of a range of sizes relevant to Stardust (down to submicron diameters) embedded within silica aerogel. Selective removal of the silica aerogel encapsulating an embedded particle is performed by cryogenic NF3-mediated electron beam–induced etching. The porous, low-density nature of the aerogel results in an enhanced etch rate compared with solid material, making it an effective, nonmechanical method for the exposure of particles. After exposure, elemental composition of the particle was analyzed by energy-dispersive X-ray spectroscopy using a high spectral resolution microcalorimeter. Signals from fluorine contamination are shown to correspond to nonremoved silica aerogel and only in residual concentrations.

Reference
Martin AA, Lin T, Toth M, Westphal AJ, Vicenzi EP, Beeman J,Silver EH (2016) Exposure and analysis of microparticles embedded in silica aerogel keystones using NF3-mediated electron beam–induced etching and energy-dispersive X-ray spectroscopy. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12655]
Published by arrangement with John Wiley & Sons

¹Kadlag, Y., ¹Becker, H
¹Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany

187Re-187Os systematics, abundances of highly siderophile elements (HSE: Re, PGE, and Au), chalcogen elements (Te, Se, and S), and some major and minor elements were determined in physically separated components of the Allende (CV3) and Murchison (CM2) carbonaceous chondrites. Substantial differences exist in the absolute and relative abundances of elements in the components, but the similarity of calculated and literature bulk rock abundances of HSE and chalcogens indicate that chemical complementarity exists among the components, with CI chondrite-like ratios for many elements. Despite subsequent alteration and oxidation, the overall cosmochemical behavior of most moderately to highly siderophile elements during high-temperature processing has been preserved in components of Allende at the sampling scale of the present study. The 187Re-187Os systematics and element variations of Allende are less disturbed compared with Murchison, which reflects different degrees of oxidation and alteration of these meteorites. The HSE systematics (with the exception of Au) is controlled by two types of materials: Pd-depleted condensates and CI chondrite-like material. Enrichment and heterogeneous distribution of Au among the components is likely the result of hydrothermal alteration. Chalcogen elements are depleted compared with HSE in all components, presumably due to their higher volatility. Small systematic variations of S, Se, and Te in components bear the signature of fractional condensation/partial evaporation and metal–sulfide–silicate partitioning.

Reference
Kadlag Y, Becker H (2016) Highly siderophile and chalcogen element constraints on the origin of components of the Allende and Murchison meteorites. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12653]
Published by arrangement with John Wiley & Sons

¹Martin R. Lee, ¹Paula Lindgren, ²Ashley J. King, ³Richard C. Greenwood, ³Ian A. Franchi, ⁴Robert Sparkes
¹School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
²Department of Earth Science, Natural History Museum (London), Cromwell Rd, London SW7 5BD
³Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁴Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL

Elephant Moraine (EET) 96029 is a CM carbonaceous chondrite regolith breccia with evidence for unusually mild aqueous alteration, a later phase of heating and terrestrial weathering. The presence of phyllosilicates and carbonates within chondrules and the fine-grained matrix indicates that this meteorite was aqueously altered in its parent body. Features showing that water-mediated processing was arrested at a very early stage include a matrix with a low magnesium/iron ratio, chondrules whose mesostasis contains glass and/or quench crystallites, and a gehlenite-bearing calcium- and aluminium-rich inclusion. EET 96029 is also rich in Fe,Ni metal relative to other CM chondrites, and more was present prior to its partial replacement by goethite during Antarctic weathering. In combination, these properties indicate that EET 96029 is one of the least aqueously altered CMs yet described (CM2.7) and so provides new insights into the original composition of its parent body. Following aqueous alteration, and whilst still in the parent body regolith, the meteorite was heated to ∼400–600 °C by impacts or solar radiation. Heating led to the amorphisation and dehydroxylation of serpentine, replacement of tochilinite by magnetite, loss of sulphur from the matrix, and modification to the structure of organic matter that includes organic nanoglobules. Significant differences between samples in oxygen isotope compositions, and water/hydroxyl contents, suggests that the meteorite contains lithologies that have undergone different intensities of heating. EET 96029 may be more representative of the true nature of parent body regoliths than many other CM meteorites, and as such can help interpret results from the forthcoming missions to study and return samples from C-complex asteroids.

Reference
Lee MR, Lindgren P, King AJ, Greenwood RC, Franchi IA, Sparkes R (2016) Elephant Moraine 96029, a very mildly aqueously altered and heated CM carbonaceous chondrite: implications for the drivers of parent body processing. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.05.008]
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