Patrick Lajeunesse^1,*, Guillaume St-Onge², Jacques Locat³, Mathieu J. Duchesne⁴, Michael D. Higgins⁵, Richard Sanfaçon⁶, Joseph Ortiz⁷

¹Centre d’études nordiques and Département de géographie, Université Laval, Québec City, Québec, Canada
²Canada Research Chair in Marine Geology, Institut des sciences de la mer de Rimouski and GEOTOP, Université du Québec à Rimouski, Rimouski, Québec, Canada
³Département de géologie et de génie géologique, Université Laval, Québec City, Québec, Canada
⁴Natural Resources Canada, Geological Survey of Canada, Québec City, Québec, Canada
⁵Sciences de la Terre, Université du Québec à Chicoutimi, Saguenay, Québec, Canada
⁶Canadian Hydrographic Service, Institut Maurice-Lamontagne, Mont-Joli, Québec, Canada
⁷Department of Geology, Kent State University, Kent, Ohio, USA

We report on a 4.1 (±0.2) km diameter and 185 m deep circular submarine structure exposed on the seabed in >40 m water depths in the northwestern Gulf of St. Lawrence (Eastern Canada) from the analysis of high-resolution multibeam bathymetric and seismic data. The presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures. Also, other origins, such as kimberlites, intrusions, karsts, or diapirs, can be eliminated on geological criteria. A single 4 cm long breccia fragment recovered from the central uplift has numerous glassy droplets of fluorapatite composition, assumed to be impact melts, and a single quartz grain with planar intersection features thought to be shock-induced planar deformation features (PDFs). The absolute age of this possible impact structure is unknown, but its geological setting indicates that it was formed long after the Mid-Ordovician and before regional pre-Quaternary sea-level lowstands. Present results outline the need for further examination to confirm an impact origin and to precisely date the formation of the structure.

Reference
Lajeunesse P, St-Onge G, Locat J, Duchesne MJ, Higgins MD, Sanfaçon R and Ortiz J (in press) The Corossol structure: A possible impact crater on the seafloor of the northwestern Gulf of St. Lawrence, Eastern Canada. Meteoritics & Planetary Science
[doi:10.1111/maps.12224]
Published by arrangement with John Wiley & Sons

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O. Pravdivtseva^1,*, A. Meshik¹, C. M. Hohenberg¹, G. Kurat^2,†

¹McDonnell Center for the Space Sciences and Physics Department of Washington University, Saint Louis, Missouri, USA
²Department of Lithospheric Sciences, University of Vienna, Vienna, Austria
^†Deceased

Using in situ laser analyses of a polished thin section from the IAB iron meteorite Campo del Cielo, we identified two silicate grains rich in radiogenic ^129*Xe, Cr-diopside, and oligoclase, excavated them from the metal, and irradiated them with thermal neutrons for I-Xe dating. The release profiles of ^129*Xe and ^128*Xe are consistent with these silicates being diopside and oligoclase, with activation energies, estimated using Arrhenius plots, of ∼201 and ∼171 kcal mole⁻¹, respectively. The 4556.4 ± 0.4 Ma absolute I-Xe age of the more refractory diopside isyounger than the 4558.0 ± 0.7 Ma I-Xe age of the less refractory oligoclase. We suggest that separate impact events at different locations and depths on a porous initial chondritic IAB parent body led to the removal of the melt and recrystallization of diopside and oligoclase at the times reflected by their respective I-Xe ages. The diopside and oligoclase grains were later brought into the studied inclusion by a larger scale catastrophic collision that caused breakup and reassembly of the debris, but did not reset the I-Xe ages dating the first events. The metal melt most probably was <1250 °C when it surrounded studied silicate grains. This reassembly could not have occurred earlier than the I-Xe closure in diopside at 4556.4 ± 0.4 Ma.

Reference
Pravdivtseva O, Meshik A, Hohenberg CM and Kurat G (in press) I-Xe ages of Campo del Cielo silicates as a record of the complex early history of the IAB parent body. Meteoritics & Planetary Science
[doi:10.1111/maps.12233]
Published by arrangement with John Wiley & Sons

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A. Garufi¹, S. P. Quanz¹, H. Avenhaus¹, E. Buenzli^{2, 3}, C. Dominik⁴, F. Meru¹, M. R. Meyer¹, P. Pinilla^{5, 6}, H. M. Schmid¹ and S. Wolf⁷

¹Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²Department of Astronomy and Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
³Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴Sterrenkundig Instituut Anton Pannekoek, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁶Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstrasse 15, 24098 Kiel, Germany

Context. Transitional disks represent a short stage of the evolution of circumstellar material. Studies of dust grains in these objects can provide pivotal information on the mechanisms of planet formation. Dissimilarities in the spatial distribution of small (μm−size) and large (mm−size) dust grains have recently been pointed out.
Aims. Constraints on the small dust grains can be obtained by imaging the distribution of scattered light at near-infrared wavelengths. We aim at resolving structures in the surface layer of transitional disks (with particular emphasis on the inner 10−50 AU), thus increasing the scarce sample of high-resolution images of these objects.
Methods. We obtained VLT/NACO near-IR high-resolution polarimetric differential imaging observations of SAO 206462 (HD 135344B). This technique allows one to image the polarized scattered light from the disk without any occulting mask and to reach an inner working angle of ~0.1″.
Results. A face-on disk is detected in H and K_s bands between 0.1″ and 0.9″. No significant differences are seen between the H and K_s images. In addition to the spiral arms, these new data allow us to resolve for the first time an inner disk cavity for small dust grains. The cavity size (≃28 AU) is much smaller than what is inferred for large dust grains from (sub-)mm observations (39 to 50 AU). This discrepancy cannot be ascribed to any resolution effect.
Conclusions. The interaction between the disk and potential orbiting companion(s) can explain both the spiral arm structure and the discrepant cavity sizes for small and large dust grains. One planet may be carving out the gas (and, thus, the small grains) at 28 AU, and generating a pressure bump at larger radii (39 AU), which holds back the large grains. We analytically estimate that, in this scenario, a single giant planet (with a mass between 5 and 15 M_J) at 17 to 20 AU from the star is consistent with the observed cavity sizes.

Reference
Garufi A, Quanz SP, Avenhaus H, Buenzli E, Dominik C, Meru F, Meyer MR, Pinilla P, Schmid HM and Wolf S (2013) Small vs. large dust grains in transitional disks: do different cavity sizes indicate a planet? SAO 206462 (HD 135344B) in polarized light with VLT/NACO. Astronomy & Astrophysics 560:A105.
[doi:10.1051/0004-6361/201322429]
Reproduced with permission © ESO

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