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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Andrea Milani^a, Alberto Cellino^b, Zoran Knežević^c, Bojan Novaković^d, Federica Spoto^a and Paolo Paolicchi^e

^aDipartimento di Matematica, Università di Pisa, Largo Pontecorvo 5, 56127 Pisa, Italy
^bINAF–Osservatorio Astrofisico di Torino, 10025 Pino Torinese, Italy
^cAstronomical Observatory, Volgina 7, 11060 Belgrade 38, Serbia
^dDepartment of Astronomy, Faculty of Mathematics, University of Belgrade, Studenski trg 16, 11000 Belgrade, Serbia
^eDipartimento di Fisica, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy

The number of asteroids with accurately determined orbits increases fast, and this increase is also accelerating. The catalogs of asteroid physical observations have also increased, although the number of objects is still smaller than in the orbital catalogs. Thus it becomes more and more challenging to perform, maintain and update a classification of asteroids into families. To cope with these challenges we developed a new approach to the asteroid family classification by combining the Hierarchical Clustering Method (HCM) with a method to add new members to existing families. This procedure makes use of the much larger amount of information contained in the proper elements catalogs, with respect to classifications using also physical observations for a smaller number of asteroids.
Our work is based on a large catalog of high accuracy synthetic proper elements (available from AstDyS), containing data for  numbered asteroids. By selecting from the catalog a much smaller number of large asteroids, we first identify a number of core families; to these we attribute the next layer of smaller objects. Then, we remove all the family members from the catalog, and reapply the HCM to the rest. This gives both satellite families which extend the core families and new independent families, consisting mainly of small asteroids. These two cases are discriminated by another step of attribution of new members and by merging intersecting families. This leads to a classification with 128 families and currently 87095 members. The number of members can be increased automatically with each update of the proper elements catalog; changes in the list of families are not automated.
By using information from absolute magnitudes, we take advantage of the larger size range in some families to analyze their shape in the proper semimajor axis vs. inverse diameter plane. This leads to a new method to estimate the family age, or ages in cases where we identify internal structures. The analysis of the plot above evidences some open problems but also the possibility of obtaining further information of the geometrical properties of the impact process. The results from the previous steps are then analyzed, using also auxiliary information on physical properties including WISE albedos and SDSS color indexes. This allows to solve some difficult cases of families overlapping in the proper elements space but generated by different collisional events.
The families formed by one or more cratering events are found to be more numerous than previously believed because the fragments are smaller. We analyze some examples of cratering families (Massalia, Vesta, Eunomia) which show internal structures, interpreted as multiple collisions. We also discuss why Ceres has no family.

Reference
Milani A, Cellino A, Knežević Z, Novaković B, Spoto F and Paolicchi P (in press) Asteroid families classification: Exploiting very large data sets. Icarus
[doi:10.1016/j.icarus.2014.05.039]
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Andreas Breslau, Manuel Steinhausen, Kirsten Vincke and Susanne Pfalzner

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Most stars do not form in isolation, but as part of a star cluster or association. These young stars are initially surrounded by protoplanetary discs. In these cluster environments tidal interactions with other cluster members can alter the disc properties. Besides the disc frequency, its mass, angular momentum, and energy, the disc’s size is particularly prone to being changed by a passing star. So far the change in disc size has only been investigated for a small number of very specific encounters. Several studies investigated the effect of the cluster environment on the sizes of planetary systems like our own solar system, based on a generalisation of information from this limited sample. We performed numerical simulations covering the wide parameter space typical of young star clusters, to test the validity of this approach. Here the sizes of discs after encounters are presented, based on a size definition that is comparable to the one used in observational studies. We find that, except for encounters between equal-mass stars, the usually applied estimates are insufficient. They tend to severely overestimate the remaining disc size. We show that the disc size after an encounter can be described by a relatively simple dependence on the periastron distance and the mass ratio of the encounter partners. This knowledge allows us, for example, to pin down the types of encounter possibly responsible for the structure of today’s solar system.

Reference
Breslau A, Steinhausen M, Vincke K and Pfalzner S (2014) Sizes of protoplanetary discs after star-disc encounters. Astronomy & Astrophysics 565:A130.
[doi:10.1051/0004-6361/201323043]
Reproduced with permission © ESO

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L.J. Bridgestock^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aDepartment of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK

Zinc isotope compositions (δ  ⁶⁶Zn) and concentrations were determined for metal samples of 15 iron meteorites across groups IAB, IIAB, and IIIAB. Also analyzed were troilite and other inclusions from the IAB iron Toluca. Furthermore, the first Zn isotope data are presented for metal–silicate partitioning experiments that were conducted at 1.5 GPa and 1650 K. Three partitioning experiments with run durations of between 10 and 60 min provide consistent Zn metal–silicate partition coefficients of ∼0.7 and indicate that Zn isotope fractionation between molten metal and silicate is either small (at less than about ±0.2‰) or absent. Metals from the different iron meteorite groups display distinct ranges in Zn contents, with concentrations of 0.08–0.24 μg/g for IIABs, 0.8–2.5 μg/g for IIIABs, and 12–40 μg/g for IABs. In contrast, all three groups show a similar range of δ  ⁶⁶Zn values (reported relative to ‘JMC Lyon Zn’) from +0.5‰ to +3.0‰, with no clear systematic differences between groups. However, distinct linear trends are defined by samples from each group in plots of δ  ⁶⁶Zn vs. 1/Zn, and these correlations are supported by literature data. Based on the high Zn concentration and δ  ⁶⁶Zn ≈ 0 determined for a chromite-rich inclusion of Toluca, modeling is employed to demonstrate that the Zn trends are best explained by segregation of chromite from the metal phase. This process can account for the observed Zn–δ  ⁶⁶Zn–Cr systematics of iron meteorite metals, if Zn is highly compatible in chromite and Zn partitioning is accompanied by isotope fractionation with Δ⁶⁶Zn_chr-met≈−1.5‰. Based on these findings, it is likely that the parent bodies of the IAB complex, IIAB and IIIAB iron meteorites featured δ  ⁶⁶Zn values of about −1.0 to +0.5‰, similar to the Zn isotope composition inferred for the bulk silicate Earth and results obtained for chondritic meteorites. Together, this implies that most solar system bodies formed with similar bulk Zn isotope compositions despite large differences in Zn contents.

Reference
Bridgestock et al. (2014) Unlocking the zinc isotope systematics of iron meteorites. Earth and Planetary Science Letters 400:153.
[doi:10.1016/j.epsl.2014.05.029]
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J.A. Cartwright^a,b, U. Ott^c,a, S. Herrmann^a, C.B. Agee^d,e

^aMax Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
^bCalifornia Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
^cUniversity of West Hungary, Savaria Campus, 9700 Szombathely, Hungary
^dInstitute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
^eDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA

The NWA 7034 Martian basaltic breccia, dated at ~4.4 Ga, represents an entirely new type of Martian meteorite. However, due to the unique make-up of NWA 7034 compared to other Martian meteorite types (including its anomalous oxygen isotope ratios), noble gas analyses – a key tool for Martian meteorite identification – are important to confirm its Martian origin. Here, we report the first noble gas results for NWA 7034, which show the presence of a trapped component that resembles the current Martian atmosphere. This trapped component is also similar in composition to trapped gases found in the much younger shergottites (~150–600 Ma). Our formation ages for the sample suggest events at ~1.6 Ga (K–Ar), and ~170 Ma (U–Th/He), which are considerably younger than those observed by Rb–Sr (2.1 Ga), and Sm–Nd (4.4 Ga; zircons ~4.4 Ga). However, our K–Ar age is similar to a disturbance in the U–Pb zircon data at ~1.7 Ga, which could hint that both chronometers have been subjected to disturbance by a common process or event. The U–Th/He age of ~170 Ma could relate to complete loss of radiogenic ⁴He at this time, and is a similar age to the crystallisation age of most shergottites. While this may be coincidental, it could indicate that a single event is responsible for both shergottite formation and NWA 7034 thermal metamorphism. As for cosmic ray exposure ages, our favoured age is ~5 Ma, which is outside the ranges for other Martian meteorite groups, and may suggest a distinct ejection event. NWA 7034 shows evidence for neutron capture on Br, which has caused elevations in Kr isotopes ⁸⁰Kr and ⁸²Kr. These elevated abundances indicate significant shielding, and could relate to either a large meteoroid size, and/or shielding in relation to a regolithic origin. We have also applied similar neutron capture corrections to Ar and Xe data, which further refine the likelihood of a modern atmospheric component, though such corrections remain speculative. Cosmogenic production rates and noble gas data are consistent with a meteoroid radius of >50 cm. Fission contributions are clear in the Xe data, with evidence to suggest that NWA 7034 contains both ²³⁸U and ²⁴⁴Pu derived fission Xe components. If the gas in NWA 7034 was trapped at its ancient igneous formation, this would suggest little evolution of the Martian atmosphere between ~4.4 Ga and present day. However, as NWA 7034 is a regolith breccia with multiple lithologies and a strong compositional similarity to Gusev soils, the timing and incorporation of trapped atmospheric gases is unclear. With hints of resetting events at ~1.5–2.1 Ga, the atmospheric component may have been incorporated during breccia formation – possibly in the Amazonian, though it could also have been incorporated on ejection from the surface.

Reference
Cartwright JA, Ott U, Herrmann S and Agee CB (2014) Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034. Earth and Planetary Science Letters 400:77.
[doi:10.1016/j.epsl.2014.05.008]
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