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]
Copyright Elsevier

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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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