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]
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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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Beverley J. Tkalcec and Frank E. Brenker

Institute of Geoscience, Goethe University, Frankfurt am Main, Germany

Numerous petrologic and geochemical studies so far on the howardite, eucrite, and diogenite (HED) meteorites have produced various crystallization scenarios for their parent body, believed to be the differentiated asteroid 4 Vesta. Structural analyses of diogenites can reveal important insights into postcrystallization deformation on the parent body. Recently published results (Tkalcec et al. 2013) of structural analysis on the olivine-rich diogenite NWA 5480 reveal that it underwent solid-state plastic deformation, although not at the base of a magma chamber. Dynamic mantle downwelling has been proposed as a plausible deformation mechanism (Tkalcec et al. 2013). The purpose of this study is to investigate whether the plastic deformation found in NWA 5480 is an isolated case. We expand the structural analysis on NWA 5480 and extend it to NWA 5784 and MIL 07001,6, two other samples of rare olivine-rich diogenites, using electron-backscattered-diffraction (EBSD) techniques. Our EBSD results show that the diogenites analyzed in this study underwent solid-state plastic deformation, confirming that the observed deformation of NWA 5480 was not an isolated case on the diogenite parent body. The lattice-preferred orientations (LPOs) of olivine in NWA 5784 and NWA 5480 are clearly distinct from that typical for cumulate rocks at the base of magma chambers, indicating a different stress environment and a different deformation mechanism. The LPO of olivine in MIL 07001 is less conclusive. The structural results of this study suggest that plastic deformation occurred on the diogenite parent body at high temperatures (1273 < T ≤ 1573 K) in the solid state, i.e., after crystallization of the diogenites themselves, in a dynamic environment with active stress fields.

Reference
Tkalcec BJ and Brenker FE (in press) Plastic deformation of olivine-rich diogenites and implications for mantle processes on the diogenite parent body. Meteoritics & Planetary Science
[doi:10.1111/maps.12324]
Published by arrangement with John Wiley & Sons

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Jon M. Friedrich^1,2, Alan E. Rubin³, Sky P. Beard⁴, Timothy D. Swindle^4,5, Clark E. Isachsen⁴, Mark L. Rivers⁶ and Robert J. Macke⁷

¹Department of Chemistry, Fordham University, Bronx, New York, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
³Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
⁴Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona, USA
⁵Department of Geosciences, The University of Arizona, Tucson, Arizona, USA
⁶Consortium for Advanced Radiation Sources, University of Chicago, Argonne, Illinois, USA
⁷Vatican Observatory, Vatican City State, Rome

We use a combination of 2D and 3D petrographic examination and ⁴⁰Ar-³⁹Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OCs studied here indicate closure of the ⁴⁰Ar-³⁹Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the ⁴⁰Ar-³⁹Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of ²⁶Al, or both processes.

Reference
Friedrich JM, Rubin AE, Beard SP, Swindle TD, Isachsen CE, Rivers ML and Macke RJ (in press) Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids. Meteoritics & Planetary Science
[doi:10.1111/maps.12328]
Published by arrangement with John Wiley & Sons

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János Kodolányi^a,b, Peter Hoppe^a, Elmar Gröner^a, Christoph Pauly^c, Frank Mücklich^c

^aMax Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany
^bVrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
^cChair of Functional Materials, Saarland University, Campus, 66041 Saarbrücken, Germany

We report O and Mg isotope compositions of presolar silicate grains which likely formed around asymptotic giant branch stars. Our grains represent the most abundant Mg-rich presolar grain group and their Mg isotope composition provides thus far missing information about the contribution of isotopically anomalous presolar dust to the Mg isotope inventory of the early Solar System.
Presolar silicate grains were identified in situ, using the NanoSIMS, in the matrix of the ungrouped carbonaceous chondrite Acfer 094. O isotope compositions suggest that the presolar grains of the present study formed in the stellar winds of low mass (M = < ∼2.2 × M_solar) asymptotic giant branch stars of close-to-solar metallicity and thus belong to the most abundant presolar silicate grain group.
In order to minimise matrix contributions during spatially poorly resolved Mg isotope analyses (spatial resolution comparable to average grain size), meteorite matrix in the presolar grains’ vicinity was removed using a focussed Ga ion beam. To monitor accuracy, we prepared and analysed O-isotopically regular (Solar System) matrix grains the same way as the presolar grains. The ²⁵Mg/²⁴Mg ratios of all seven successfully analysed presolar silicate grains are identical to that of the Solar System at the precision of our measurements. The ²⁶Mg/²⁴Mg ratios of five grains are also solar but two grains have significant positive anomalies in ²⁶Mg/²⁴Mg. On average, however, ²⁵Mg/²⁴Mg and ²⁶Mg/²⁴Mg ratios are higher than solar by a few %. All grain compositions are consistent with Galactic chemical evolution and, possibly, isotope fractionation caused by interstellar or Solar System processing (sputtering and/or recondensation). The grain with the strongest enrichment in ²⁶Mg relative to ²⁵Mg (δ²⁵Mg = 34 ± 25 ‰, δ²⁶Mg = 127 ± 25 ‰; where δ^xMg = 1000 × [(^xMg/²⁴Mg)_grain/(^xMg/²⁴Mg)_{meteorite matrix})−1] with x = 25 or 26; the reported uncertainty corresponds to 1 σ), probably incorporated ²⁶Al during grain condensation. Our and previously reported Mg isotope data on presolar oxide and silicate grains indicate that the isotopically anomalous O-rich dust component of the Solar System’s parent molecular cloud was heterogeneous with respect to Mg isotope compositions and probably had a higher ²⁶Mg/²⁴Mg ratio on average than that of the present-day Solar System.

Reference
Kodolányi J, Hoppe P,Gröner E, Pauly C and Mücklich F (in press) The Mg isotope composition of presolar silicate grains from red giant stars. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.05.053]
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C. Pinte^1,2 and G. Laibe^3,4

¹UMI-FCA, CNRS/INSU France (UMI 3386), and Departamento de Astronomía, Universidad de Chile, Casilla 36-D Santiago, Chile
²Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France CNRS, IPAG, 38000 Grenoble, France
³Monash Centre for Astrophysics and School of Mathematical Sciences, Monash University, Clayton, Vic 3800, Australia
⁴School of Physics and Astronomy, University of Saint Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK

The growth of dust particles into planet embryos needs to circumvent the “radial-drift barrier”, i.e. the accretion of dust particles onto the central star by radial migration. The outcome of the dust radial migration is governed by simple criteria between the dust-to-gas ratio and the exponents p and q of the surface density and temperature power laws. The transfer of radiation provides an additional constraint between these quantities because the disc thermal structure is fixed by the dust spatial distribution. To assess which discs are primarily affected by the radial-drift barrier, we used the radiative transfer code MCFOST to compute the temperature structure of a wide range of disc models, stressing the particular effects of grain size distributions and vertical settling. We find that the outcome of the dust migration process is very sensitive to the physical conditions within the disc. For high dust-to-gas ratios (≳0.01) and/or flattened disc structures (H/R ≲ 0.05), growing dust grains can efficiently decouple from the gas, leading to a high concentration of grains at a critical radius of a few AU. Decoupling of grains from gas can occur at a large fraction (>0.1) of the initial radius of the particle, for a dust-to-gas ratio greater than ≈0.05. Dust grains that experience migration without significant growth (millimetre and centimetre-sized) are efficiently accreted for discs with flat surface density profiles (p < 0.7) while they always remain in the disc if the surface density is steep enough (p > 1.2). Between (0.7 < p < 1.2), both behaviours may occur depending on the exact density and temperature structures of the disc. Both the presence of large grains and vertical settling tend to favour the accretion of non-growing dust grains onto the central object, but it slows down the migration of growing dust grains. If the disc has evolved into a self-shadowed structure, the required dust-to-gas ratio for dust grains to stop their migration at large radius become much smaller, of the order of 0.01. All the disc configurations are found to have favourable temperature profiles over most of the disc to retain their planetesimals.

Reference
Pinte C and Laibe G (2014) Diversity in the outcome of dust radial drift in protoplanetary discs. Astronomy & Astrophysics 565:A129.
[doi:10.1051/0004-6361/201220545]
Reproduced with permission © ESO

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Julia Roszjar^a,c, Martin J. Whitehouse^b and Addi Bischoff^c

^aInstitut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Carl-Zeiss-Promenade 10, DE-07745 Jena, Germany
^bDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^cInstitut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, DE-48149 Münster, Germany

In common with the remarkable variation in the bulk rock Zr content of distinct meteorite groups, ranging from <1 ppm to >800 ppm, the occurrence and abundance of accessory zircon is also highly diverse and limited to certain meteorite classes. A detailed literature study on the occurrence of meteoritic zircon, along with other Zr-bearing phases reveals that lunar rocks, eucrites and mesosiderites are the prime sources of meteoritic zircon. Rare zircon grains occur in chondrites, silicate-bearing iron meteorites and Martian meteorites, with grain sizes of >5 μm allowing chemical and chronological studies at high spatial resolution using secondary ion mass spectrometry (SIMS) technique. Grain sizes, crystal habits, structural and chemical characteristics of zircon grains derived from various meteorite types, including their REE abundances, minor element concentrations, and Zr/Hf values is diverse. Superchondritic Zr/Hf values (47 ± 8; s.d. with n = 97), i.e., typical for zircon in eucrites and mesosiderites, indicate crystallization from a fractionated, incompatible-element-rich (residual) melt. Differences in REE abundances, occurrence or absence of Ce- and Eu-anomalies, and overall REE patterns that are often fractionated with a depletion in LREE, might be primarily controlled by variable formation conditions of individual grains and/or differences in the residual melt compositions on a small, local scale within single samples. Subsequent fractionation/modification of the chemical fingerprint of meteoritic zircon can involve high-temperature annealing processes during thermal metamorphic reactions and/or impact events along with mixing of lithic fragments since many samples are breccias.

Reference
Roszjar J, Whitehouse MJ and Bischoff A (in press) Meteoritic zircon – Occurrence and chemical characteristics. Chemie der Erde – Geochemistry
[doi:10.1016/j.chemer.2014.05.002]
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Stefano Orsini, Valeria Mangano, Alessandro Mura, Diego Turrini, Stefano Massetti, Anna Milillo, Christina Plainaki

INAF-IAPS, Istituto di Astrofisica e Planetologia Spaziali, Roma, Italy

Mercury, due to its close location to the Sun, is surrounded by an environment whose conditions may be considered as ‘extreme’ in the entire Solar System. Both solar wind and radiation are stronger with respect to other Solar System bodies, so that their interactions with the planet cause high emission of material from its surface. Moreover, the meteoritic precipitation plays a significant role in surface emission processes. This emitted material is partially lost in space. Although under the present conditions the surface particles loss rate does not seem to be able to produce significant erosion of the planetary mass and volume, the long-term effects over billions of years should be carefully considered to properly understand the evolution of the planet. In the early stages, under even more extreme conditions, some of these processes were much more effective in removing material from the planet’s surface. This study attempts to provide a rough estimation of the material loss rate as a function of time, in order to evaluate whether and how this environmental effect can be applied to understand the Hermean surface evolution. We show that the most potentially effective Sun-induced erosion process in early times is a combination of ion sputtering, photon stimulated desorption and enhanced diffusion, which could have caused the loss of a surface layer down to a depth of 20 m, as well as a relevant Na depletion.

Reference
Orsini S, Mangano V, Mura A, Turrini D, Massetti S, Milillo A and Plainaki C (in press) The Influence of Space Environment on the Evolution of Mercury. Icarus
[doi:10.1016/j.icarus.2014.05.031]
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