Ellen R. Harju^a, Alan E. Rubin^b, Insu Ahn^c, Byeon-Gak Choi^d, Karen Ziegler^a,1, John T. Wasson^a,b,e

^aDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
^bInstitute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA
^cKorea Polar Research Institute, Incheon, 406-840, Korea
^dDepartment of Earth Science Education, Seoul National University, Seoul, 151-748, Korea
^eDepartment of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1567, USA

The wide range in the degree of aqueous alteration of CR chondrites prompted us to formulate a numerical sequence for these rocks that ranges from petrologic type 2.0 to 2.8. (Hypothetical CR3.0 chondrites should be completely free of aqueous alteration effects.) About 70% of CR chondrites are slightly altered, type-2.8 rocks that exhibit heterogeneous alteration; these meteorites contain moderately abundant metallic Fe-Ni, no magnetite, and generally, a few chondrules with clear glassy mesostases. None of the chondrules in these rocks shows evidence of alteration of mafic silicate phenocrysts, but several chondrules are surrounded by phyllosilicate-rich rims that appear “smooth” when viewed by back-scattered-electron imaging. Matrix regions in slightly altered CR chondrites contain high S (~3 wt.%), but some matrix patches in the same thin sections record alteration effects and contain appreciably less S (<1.5 wt.%). In CR chondrites that have been more-significantly altered (e.g., Renazzo and Al Rais), metallic Fe-Ni has been partially replaced by magnetite±sulfide; mafic silicates have been partly altered to phyllosilicates, particularly along edges, fractures and twin boundaries. One of the most-altered CR chondrites (type-2.0 GRO 95577) contains abundant magnetite, additional oxide phases, iron carbonate, only very rare metallic Fe-Ni and essentially no mafic silicate grains. The whole-rock O-isotopic compositions of CR chondrites correlate with the degree of aqueous alteration: Δ¹⁷O ranges from ~-2.6‰ in type-2.8 samples to ~-0.4‰ in type 2.0.

Reference
Harju ER, Rubin AE, Ahn I, Choi B-G, Ziegler K and Wasson JT (in press) Progressive aqueous alteration of CR carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.048]
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Alessandro Maturilli^a, Jörn Helbert^a, James M. St. John^b, James W. Head III^c, William M. Vaughan^c, Mario D’Amore^a, Matthias Gottschalk^d, Sabrina Ferrari^a

^aInstitute for Planetary Research, German Aerospace Center DLR, Rutherfordstr. 2, Berlin–Adlershof, Germany
^bSchool of Earth Sciences, The Ohio State University at Newark, Newark, OH 43055, USA
^cDepartment of Geological Sciences, Brown University, Providence, RI 02912, USA
^dDepartment of Chemistry and Physics of Earth Materials, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany

The elemental composition of Mercury’s surface, which has been recently measured by the NASA MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, suggests a mineralogy dominated by magnesium-rich orthopyroxene and feldspar. The most magnesium-rich and aluminium-poor regions of Mercury’s surface (which are presumably orthopyroxene-rich) have compositions, and possibly mineralogies, analogous to terrestrial boninites and basaltic komatiites. Unfortunately, little is known about the spectral properties of komatiites, especially at the high surface temperatures of Mercury. We therefore have collected three terrestrial komatiites with different compositions plus a synthetic komatiitic sample, and measured their reflectances in the visible and thermal infrared spectral ranges. Samples divided into four grain size ranges (when enough material was available) were measured fresh and after thermal processing in vacuum (10 Pa) at 500 °C, comparable to Mercury peak surface temperatures. Our measurements show that spectral changes between fresh and thermally processed samples occur in both spectral channels, but are stronger in the visible range, with reddening affecting all the samples, while darkening is more selective. It is important to note that darkening and reddening after thermally processing the samples are independent of the komatiites ferrous iron content. In fact the synthetic sample which is nearly iron-free is most strongly affected. From our study it turns out that thermally processing the samples in vacuum at Mercury surface temperature produces the removal of samples’ colour centres. The results of our study show also that the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument on MESSENGER orbiting Mercury currently cannot distinguish between different compositions of komatiites, while the future MErcury Radiometer and Thermal infrared Imaging Spectrometer (MERTIS) on the upcoming ESA BepiColombo mission will resolve their differences in the 7-14 µm spectral range.

Reference
Maturilli A, Helbert J, St. John JM, Head III JW, Vaughan WM, D’Amore M, Gottschalk M and Ferrari S (2014) Komatiites as Mercury surface analogues: Spectral measurements at PEL. Earth and Planetary Science Letters Volume 398:58–65.
[doi:10.1016/j.epsl.2014.04.035]
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I. Ramírez¹, A. T. Bajkova², V. V. Bobylev^2,3, I. U. Roederer⁴, D. L. Lambert¹, M. Endl¹, W. D. Cochran¹, P. J. MacQueen¹ and R. A. Wittenmyer^5,6

¹McDonald Observatory and Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, Texas 78712-1205, USA
²Central (Pulkovo) Astronomical Observatory of RAS, 65/1, Pulkovskoye Chaussee, St. Petersburg 196140, Russia
³Sobolev Astronomical Institute, St. Petersburg State University, Bibliotechnaya pl. 2, St. Petersburg 198504, Russia
⁴Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁵School of Physics, UNSW Australia, Sydney 2052, Australia
⁶Australian Centre for Astrobiology, University of New South Wales, UNSW Kensington Campus, Sydney 2052, Australia

Dynamical information along with survey data on metallicity and in some cases age have been used recently by some authors to search for candidates of stars that were born in the cluster where the Sun formed. We have acquired high-resolution, high signal-to-noise ratio spectra for 30 of these objects to determine, using detailed elemental abundance analysis, if they could be true solar siblings. Only two of the candidates are found to have solar chemical composition. Updated modeling of the stars’ past orbits in a realistic Galactic potential reveals that one of them, HD 162826, satisfies both chemical and dynamical conditions for being a sibling of the Sun. Measurements of rare-element abundances for this star further confirm its solar composition, with the only possible exception of Sm. Analysis of long-term high-precision radial velocity data rules out the presence of hot Jupiters and confirms that this star is not in a binary system. We find that chemical tagging does not necessarily benefit from studying as many elements as possible but instead from identifying and carefully measuring the abundances of those elements that show large star-to-star scatter at a given metallicity. Future searches employing data products from ongoing massive astrometric and spectroscopic surveys can be optimized by acknowledging this fact.

Reference
Ramírez I, Bajkova AT, Bobylev VV, Roederer IU, Lambert DL, Endl M, Cochran WD, MacQueen PJ and Wittenmyer RA (2014) Elemental Abundances of Solar Sibling Candidates. The Astrophysical Journal 787:154.
[doi:10.1088/0004-637X/787/2/154]

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