Glenn J. MacPherson¹ and Alexander N. Krot²

¹Department of Mineral Sciences, U.S. National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA

CV (Vigarano type) carbonaceous chondrites, comprising Allende-like (CV_oxA) and Bali-like (CV_oxB) oxidized and reduced (CV_red) subgroups, experienced differing degrees of fluid-assisted thermal and shock metamorphism. The abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups: (1) ferroan olivine and diopside–hedenbergite solid solution pyroxenes are common in all CVs; (2) nepheline and sodalite are abundant in CV_oxA, rare in CV_red, and absent in CV_oxB; (3) phyllosilicates and nearly pure fayalite are common in CV_oxB, rare in CV_red, and virtually absent in CV_oxA; (4) andradite, magnetite, and Fe-Ni-sulfides are common in oxidized CVs, but rare in reduced CVs; the latter contain kirschsteinite instead. Thus, a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the Ca-, Fe-rich silicates (pyroxenes, andradite, kirschsteinite) among the CVox and CVred meteorites. The extent of secondary mineralization was controlled by the distribution of water ices, permeability, and porosity, which in turn were controlled by impacts on the asteroidal parent body. More intense shock metamorphism in the region where the reduced CVs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids. The mineralogy, petrography, and bulk chemical compositions of both the reduced and oxidized CV chondrites indicate that mobile elements were redistributed between Ca,Al-rich inclusions, dark inclusions, chondrules, and matrices only locally; there is no evidence for large-scale (>several cm) fluid transport. Published ⁵³Mn-⁵³Cr ages of secondary fayalite in CV, CO, and unequilibrated ordinary chondrites, and carbonates in CI, CM, and CR chondrites are consistent with aqueous alteration initiated by heating of water ice-bearing asteroids by decay of ²⁶Al, not shock metamorphism.

Reference
MacPherson GJ and Krot AN (in press) The formation of Ca-, Fe-rich silicates in reduced and oxidized CV chondrites: The roles of impact-modified porosity and permeability, and heterogeneous distribution of water ices. Meteoritics & Planetary Science
[doi:10.1111/maps.12316]
Published by arrangement with John Wiley & Sons

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Masayuki Uesugi¹ et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

¹JAXA Space Exploration Center, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa, Japan

We report the investigation of cutting methods for Hayabusa samples. The purpose of our study is to explore the possibility of applying multiple analyses to a single particle effectively. We investigated the cutting performance of a blade dicing saw, laser, focused ion beam (FIB), and physical breaking by microindenter. Cutting performance was examined by estimating the aspect ratio of the cut slit, i.e., depth over width of the slit. We also investigated the possible contamination and sample damage by cutting. The result of the investigation shows that we can cut the samples from <50 μm to 500 μm using those methods with aspect ratios from 10 to 20, although they would introduce some contamination or damage to the samples. Our investigations also provide an important basis for the analysis of samples obtained by future sample return missions.

Reference
Uesugi et al. (in press) Investigation of cutting methods for small samples of Hayabusa and future sample return missions. Meteoritics & Planetary Science
[doi:10.1111/maps.12322]
Published by arrangement with John Wiley & Sons

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F. Poulet^a, O. Ruesch^b, Y. Langevin^a, H. Hiesinger^b

^aInstitut d’Astrophysique Spatiale, CNRS/Université Paris Sud, 91405 Orsay Cedex
^bInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Münster

The observations of the surface of 4 Vesta by the Visible and Infrared Mapping Spectrometer (VIR) onboard the Dawn spacecraft reveals that its composition is dominated by pyroxenes with olivine in very localized spots. To derive new constraints on the surface composition of the asteroid, we apply a scattering model to VIR reflectance spectra. The model is first calibrated by performing a non-linear deconvolution of laboratory spectra of mineral mixtures and howardite eucrite diogenite (HED) meteorites. Abundance estimates of minerals are accurate to within 15–25% for the analyzed samples, while the estimated particle sizes are within the intervals of actual sizes. Grain size effects complicate spectral deconvolution and estimation of modal abundances of samples (both HED and mineral mixtures) that contain olivine. The magnesium-rich olivine detection threshold is 10-20% for large grain sizes (100’s μm) and several 10’s% for small grain sizes (<50 μm). Major expected minerals (low-calcium pyroxenes, high-calcium pyroxenes, plagioclase and olivine) can provide satisfactory fits of VIR spectra with excellent residuals ⩽1%. Terrains with the strongest low-calcium pyroxene signatures are well representative of diogenites. The best fits of any unit are obtained by including Fo70 olivine at an abundance level of 10% to 20%, with an uncertainty of ∼10%. Olivine is therefore likely to be ubiquitous over the whole surface of Vesta. Olivine is coarser grained (a few hundred μm) than other minerals such as orthopyroxene and clinopyroxene (grain sizes typically smaller than 100 μm). Both the grain size variance and the modal mineralogy are consistent with the lithologic size and mineral distributions of howardites containing olivine phenocryst-bearing melt. These howardites are the best petrologic analogues of Vesta. Such a surface assemblage could be the result of successive melting and mixing processes due to impacts. The compositional view confirms that Vesta underwent major homogenization processes, resulting in a relatively uniform modal mineralogy and explaining the lack of specific olivine enrichment in the Rheasilvia ejecta.

Reference
Poulet F, Ruesch O, Langevin Y and Hiesinger H (in press) Modal mineralogy of the surface of Vesta: Evidence for ubiquitous olivine and identification of meteorite analogue. Icarus
[doi:10.1016/j.icarus.2014.06.002]
Copyright Elsevier

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L. Neslušan¹ and M. Hajduková jr.²

¹Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia
²Astronomical Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 84504 Bratislava, Slova

Aims. In our overall work, we attempt to predict some new meteor showers associated with as many as possible known periodic comets and to find the generic relationship of some already known showers with these comets. In this paper, we focus our attention on the meteor-shower complex of the long-period comet C/1917 F1 (Mellish), which is the known parent body of the December Monocerotids. Some other showers have also been suggested to be associated with this comet. We map its whole complex here.
Methods. For five perihelion passages of the parent comet in the past, we model associated theoretical streams, with each consisting of 10 000 test particles and follow their dynamical evolution until the present. Subsequently, we analyze the orbital characteristics of the parts of found streams that approach the Earth’s orbit.
Results. We confirm the generic relationship between the studied parent comet and December Monocerotids. The comet is probably also the parent body of the April ρ-Cygnids. The evolution of meteoroids to the orbits of April ρ-Cygnids is very long at about 20 millennia. If we follow even a longer evolutionary period, which is up to 50 millennia, then two diffuse showers with the radiant situated symmetrically to both the December Monocerotids and April ρ-Cygnids showers with respect to the apex of the Earth’s motion occur. Our simulation does not confirm any relationship between C/1917 F1 and the November Orionids, although this shower was found in all three databases of observed orbits.

Reference
Neslušan L and Hajduková jr. M (2014)The meteor-shower complex of comet C/1917 F1 (Mellish). Astronomy & Astrophysics 566:A33.
[doi:10.1051/0004-6361/201423382]
Reproduced with permission © ESO

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