Gregor J. Golabek^1,2, Bernard Bourdon² and Taras V. Gerya¹

¹ETH Zurich, Institute of Geophysics, Zurich, Switzerland
²Laboratoire de Géologie de Lyon, ENS Lyon, CNRS and Université Claude Bernard de Lyon, Lyon Cedex 07, France

The acapulcoite-lodranite meteorites are members of the primitive achondrite class. The observation of partial melting and resulting partial removal of Fe-FeS indicates that this meteorite group could be an important link between achondrite and iron meteorites, on the one hand, and chondrite meteorites, on the other. Thus, a better understanding of the thermomechanical evolution of the parent body of this meteorite group can help improve our understanding of the evolution of early planetesimals. Here, we use 2-D and 3-D finite-difference numerical models to determine the formation time, initial radius of the parent body of the acapulcoite-lodranite meteorites, and their formation depth inside the body by applying available geochronological, thermal, and textural constraints to our numerical data. Our results indicate that the best fit to the data can be obtained for a parent body with 25–65 km radius, which formed around 1.3 Ma after calcium-aluminum-rich inclusions. The 2-D and 3-D results considering various initial temperatures and the effect of porosity indicate possible formation depths of the acapulcoite-lodranite meteorites of 9–19 and 14–25 km, respectively. Our data also suggest that other meteorite classes could form at different depths inside the same parent body, supporting recently proposed models (Elkins-Tanton et al. 2011; Weiss and Elkins-Tanton2013).

Reference
Golabek GJ, Bourdon B and Gerya TV (in press) Numerical models of the thermomechanical evolution of planetesimals: Application to the acapulcoite-lodranite parent body. Meteoritics & Planetary Science
[doi:10.1111/maps.12302]
Published by arrangement with John Wiley & Sons

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Steven J. Jaret^1,†, Linda C. Kah¹ and R. Scott Harris^2,3

¹Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
²Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
³Georgia Department of Transportation, Office of Materials and Testing, Forest Park, Georgia, USA
^††Department of Geosciences, Stony Brook University, Stony Brook, New York, USA

The Tenoumer impact structure is a small, well-preserved crater within Archean to Paleoproterozoic amphibolite, gneiss, and granite of the Reguibat Shield, north-central Mauritania. The structure is surrounded by a thin ejecta blanket of crystalline blocks (granitic gneiss, granite, and amphibolite) and impact-melt rocks. Evidence of shock metamorphism of quartz, most notably planar deformation features (PDFs), occurs exclusively in granitic clasts entrained within small bodies of polymict, glass-rich breccia. Impact-related deformation features in oligoclase and microcline grains, on the other hand, occur both within clasts in melt-breccia deposits, where they co-occur with quartz PDFs, and also within melt-free crystalline ejecta, in the absence of co-occurring quartz PDFs. Feldspar deformation features include multiple orientations of PDFs, enhanced optical relief of grain components, selective disordering of alternate twins, inclined lamellae within alternate twins, and combinations of these individual textures. The distribution of shock features in quartz and feldspar suggests that deformation textures within feldspar can record a wide range of average pressures, starting below that required for shock deformation of quartz. We suggest that experimental analysis of feldspar behavior, combined with detailed mapping of shock metamorphism of feldspar in natural systems, may provide critical data to constrain energy dissipation within impact regimes that experienced low average shock pressures.

Reference
Jaret SJ, Kah LC and Harris RS (in press) Progressive deformation of feldspar recording low-barometry impact processes, Tenoumer impact structure, Mauritania. Meteoritics & Planetary Science
[doi:10.1111/maps.12310]
Published by arrangement with John Wiley & Sons

Link to Article

Nan Liu^1,2,3, Roberto Gallino⁴, Sara Bisterzo^4,5, Andrew M. Davis^1,2,6, Michael R. Savina^2,3, and Michael J. Pellin^1,2,3,6

¹Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
²Chicago Center for Cosmochemistry, Chicago, IL 60637, USA
³Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁴Dipartimento di Fisica, Università di Torino, Torino I-10125, Italy
⁵INAF-Osservatorio Astrofisico di Torino-Strada Osservatorio 20, Pino Torinese I-10025, Italy
⁶Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA

We present postprocess asymptotic giant branch (AGB) nucleosynthesis models with different ¹³C-pocket internal structures to better explain zirconium isotope measurements in mainstream presolar SiC grains by Nicolussi et al. and Barzyk et al. We show that higher-than-solar ⁹²Zr/⁹⁴Zr ratios can be predicted by adopting a ¹³C-pocket with a flat ¹³C profile, instead of the previous decreasing-with-depth ¹³C profile. The improved agreement between grain data for zirconium isotopes and AGB models provides additional support for a recent proposal of a flat ¹³C profile based on barium isotopes in mainstream SiC grains by Liu et al.

Reference
Liu N, Gallino R, Bisterzo S, Davis AM, Savina MR and Pellin MJ (in press) The 13C-Pocket Structure in AGB Models: Constraints from Zirconium Isotope Abundances in Single Mainstream SiC Grains. The Astrophysical Journal Letters 788:163.
[doi:10.1088/0004-637X/788/2/163]

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Jaime E. Pineda¹, Sascha P. Quanz¹, Farzana Meru¹, Gijs D. Mulders², Michael R. Meyer¹, Olja Panić³ and Henning Avenhaus¹

¹Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
²Lunar and Planetary Laboratory, The University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA
³Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

The disk around the Herbig Ae/Be star HD 100546 has been extensively studied and it is one of the systems for which there are observational indications of ongoing and/or recent planet formation. However, up until now, no resolved image of the millimeter dust emission or the gas has been published. We present the first resolved images of the disk around HD 100546 obtained in Band 7 with the ALMA observatory. The CO (3-2) image reveals a gas disk that extends out to 350 au radius at the 3σ level. Surprisingly, the 870 μm dust continuum emission is compact (radius <60 au) and asymmetric. The dust emission is well matched by a truncated disk with an outer radius of 50 au. The lack of millimeter-sized particles outside 60 au is consistent with radial drift of particles of this size. The protoplanet candidate, identified in previous high-contrast NACO/VLT L‘ observations, could be related to the sharp outer edge of the millimeter-sized particles. Future higher angular resolution ALMA observations are needed to determine the detailed properties of the millimeter emission and the gas kinematics in the inner region (<2”). Such observations could also reveal the presence of a planet through the detection of circumplanetary disk material.

Reference
Pineda JE, Quanz SP, Meru F, Mulders GD, Meyer MR, Panić O and Avenhaus H (in press) Resolved Images of the Protoplanetary Disk around HD 100546 with ALMA. The Astrophysical Journal Letters
[doi:10.1088/2041-8205/788/2/L34]

Link to Article