¹C. A. Lorenz, ¹M. A. Ivanova, ^2,3N. A. Artemieva, ¹D. A. Sadilenko, ⁴H. Chennaoui Aoudjehane, ¹I. A. Roschina, ¹A.V. Korochantsev,⁵M. Humayun
¹Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia
²Institute for Dynamics of Geospheres, Moscow, Russia
³Planetary Science Institute, Tucson, Arizona, USA
⁴Hassan II University Casablanca, Faculty of Sciences, GAIA Laboratory, Casablanca, Morocco
⁵National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, Florida, USA

A relic impact structure was recognized within the strewn field of the Agoudal iron meteorite. The heavily eroded structure has preserved shatter cones in a limestone basement, and remnants of autochthonous and allochthonous breccias. Fragments of iron incorporated into the allochthonous breccia have a chemical composition (Ni = 5.16 wt%, Ir = 0.019 ppm) similar to that of the Agoudal meteorite, supporting a syngenetic origin of the strewn field and the impact structure. The total recovered mass of Agoudal meteorite fragments is estimated at approximately 500 kg. The estimated size of the SE–NW-oriented strewn field is 6 × 2 km. Model calculations with minimal preatmospheric size show that a similar meteorite strewn field plus one small crater with observed shock effects could be formed by fragmentation of a meteoroid approximately 1.4 m in diameter with an impact angle of approximately 60° from the horizontal. However, the most probable is an impact of a larger, 3–4 m diameter meteoroid, resulting a strewn field with approximately 10 craters, 10–30 m in diameter each, plus numerous meteorite fragments. The calculated scattering area of meteorite shrapnel ejected from these impact craters could completely cover the observed strewn field of the Agoudal meteorite.

Reference
Lorenz CA, Ivanova MA, Artemieva NA, Sadilenko DA, Aoudjehane HC, Roschina IA, Korochantsev AV, Humayun M (2014) Formation of a small impact structure discovered within the Agoudal meteorite strewn field, Morocco. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12406]

Published by arrangement with John Wiley&Sons

During the last days of this year, Cosmochemistry Papers will be on Christmas break (more or less). Normal service will commence on January, 2nd, after we have recovered.

Merry Christmas & Happy New Year to everyone and see you in 2015 !

¹Kazuhide Nagashima, ¹Alexander N. Krot, ¹Gary R. Huss
¹Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96826, USA

To investigate a possible relationship between chondrules and matrix, we studied mineralogy, mineral chemistry, and in situ O-isotope compositions of chondrules, clastic matrix grains, and amoeboid olivine aggregates (AOAs) in the Kakangari K-grouplet chondrite. Most olivines and low-Ca pyroxenes in the Kakangari chondrules, matrix, and AOAs have similar magnesium-rich compositions, Fo∼95–97 (∼0.3−0.5 wt% MnO) and En∼90–96, respectively. These rather uniform chemical compositions of the different chondritic components are likely due to partial Fe-Mg-Mn equilibration during thermal metamorphism experienced by the host meteorite. Oxygen-isotope compositions of olivine and low-Ca pyroxene grains in chondrules and matrix plot along a slope-1 line on a three O-isotope diagram and show a range from 16O-enriched composition similar to that of the Sun to 16O-depleted composition similar to the terrestrial O-isotope composition. Most olivines and low-Ca pyroxenes in chondrules are 16O-poor and plot on or close to the terrestrial mass-fractionation line (mean Δ17O values ± 2 standard deviations: 0.0±0.8‰ and +0.2±0.9‰ for olivine and pyroxene, respectively), consistent with the previously reported compositions of bulk chondrules (Δ17O = –0.16±0.70‰). In addition to these 16O-poor grains, a coarse-grained igneous rim surrounding a porphyritic chondrule contains abundant 16O-rich relict olivines (Δ17O ∼ –24‰). Oxygen-isotope compositions of olivines and low-Ca pyroxenes in matrix show a bimodal distribution: 12 out of 13 olivine and 4 out of 17 pyroxene grains measured are similarly 16O-rich (Δ17O ∼ –23.5±2.9‰), others are similarly 16O-poor (Δ17O ∼ –0.1±1.7‰). Due to slow oxygen self-diffusion, olivines and low-Ca pyroxenes largely retained their original oxygen-isotope compositions. The nearly identical O-isotope compositions between the chondrule phenocrysts and the 16O-poor matrix grains suggest both chondrules and matrix of Kakangari sampled isotopically the same reservoirs. In addition, the presence of abundant 16O-rich grains in matrix and the chondrule igneous rim suggests both components acquired similar precursor inventories. These observations imply that chondrules and matrix in Kakangari are genetically related in the sense that material that formed matrix was one of the precursors of chondrules and chondrules and some fraction of matrix experienced the same thermal processing event. The 16O-enriched bulk matrix value compared to the bulk chondrules reported previously is likely due to presence of abundant 16O-rich grains in the Kakangari matrix.

Reference
Nagashima K, Krot AN, Huss GR (2014) Oxygen-isotope compositions of chondrule phenocrysts and matrix grains in Kakangari K-grouplet chondrite: Implication to a chondrule-matrix genetic relationship. Geochimica et Cosmochimica (in Press)
Link to Article [doi:10.1016/j.gca.2014.12.012]

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