^1,2Masaaki Miyahara, ^1,3Eiji Ohtani, ⁴Ahmed El Goresy, ⁵Yangting Lin, ⁵Lu Feng, ⁵Jian-Chao Zhang, ⁶Philippe Gillet, ⁷Toshiro Nagase, ¹Jun Muto, ⁸Masahiko Nishijima
¹Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
²Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, 739-8526, Japan
³V. S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Science, 630090 Novosibirsk, Russia
⁴Bayerisches Geoinstitut, Universität Bayreuth, D-95440, Bayreuth, Germany
⁵Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029, China
⁶Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL) Station 1, 1015 Lausanne, Switzerland
⁷Center for Academic Resources and Archives, Tohoku University, Sendai 980-8578, Japan
⁸Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

The Almahata Sitta MS-170 ureilite (a piece of a breccia originating from the asteroid, 2008 TC3) consists mainly of olivine, with many diamond and graphite grains existing between the olivine grains. The occurrences of the diamonds are unique; i.e., i) some diamonds exhibit sub-euhedral habits and ii) some diamonds have large grain-size (up to about 40 μm). Several diamonds are segmented into many fragments by fractures. Individual fragments have similar crystallographic orientation, which implies that the adjacent diamond segments were originally a single crystal. Large diamond assemblages occur besides such individual diamond grains. In one of the largest assemblages (almost about 100 m in size) has also the same crystallographic orientation. They can be regarded as the pieces of a previously unique single diamond, which provides evidence for large single-crystals diamond in meteorites. Almahata Sitta MS-170 is a meteorite fragment from the 2008 TC3 asteroid that underwent less shock than other ureilitic meteorites. It is unlikely that such large diamonds were formed from graphite through a shock-induced phase transformation during planetesimal collision, despite this idea being now widely accepted as the diamond formation mechanism of ureilites. Fine-scale heterogeneous distribution of impurities (hydrogen, nitrogen, and oxygen) exists in single crystal diamonds, indicative of sluggish growth. This distribution is reminiscent of sector zoning growth. Its grain size, the shock features of MS-170, and the C- and N- isotopic composition signatures allow us to revive classical and but not widely accepted models for diamond formation in ureilites; i.e., a diamond formed from partially melted magma or a C–O–H fluid in the deep interior of the ureilite parent-body or, alternatively, through a chemical vapor deposition (CVD) process in the solar nebula. Considering present mineralogical and isotopic features, the former scenario is more favorable.

Reference
Miyahara M, Ohtani E, El Goresy A, Lin Y, Feng L, Zhang J-C, Gillet P, Nagase T, Muto J, Nishijima M (2015)
Unique large diamonds in a ureilite from Almahata Sitta 2008 TC3 Asteroid. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.035]

Copyright Elsevier

¹Ryan C. Ogliore, ¹Kazuhide Nagashima, ¹Gary R. Huss, ²Andrew J. Westphal, ²Zack Gainsforth, ²Anna L. Butterworth
¹Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA

Individual particles from comet 81P/Wild 2 collected by NASA’s Stardust mission vary in size from small sub-μm fragments found in the walls of the aerogel tracks, to large fragments up to tens of μm in size found towards the termini of tracks. The comet, in an orbit beyond Neptune since its formation, retains an intact a record of early-Solar-System processes that was compromised in asteroidal samples by heating and aqueous alteration. We measured the O isotopic composition of seven Stardust fragments larger than ∼2 μm extracted from five different Stardust aerogel tracks, and 63 particles smaller than ∼2 μ m from the wall of a Stardust track. The larger particles show a relatively narrow range of O isotopic compositions that is consistent with 16O16O-poor phases commonly seen in meteorites. Many of the larger Stardust fragments studied so far have chondrule-like mineralogy which is consistent with formation in the inner Solar System. The fine-grained material shows a very broad range of O isotopic compositions (-70-70‰< Δ17OΔ17O<+60<+60‰) suggesting that Wild 2 fines are either primitive outer-nebula dust or a very diverse sampling of inner Solar System compositional reservoirs that accreted along with a large number of inner-Solar-System rocks to form comet Wild 2.

Reference
Ogliore RC, Nagashima K, Huss GR, Westphal AJ, Gainsforth Z, Butterworth AL (2015) Oxygen Isotopic Composition of coarse- and fine-grained material from Comet 81P/Wild 2. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.028]

Copyright Elsevier

^1,2Alex Ruzicka, ¹Richard Hugo, ^1,2Melinda Hutson
¹Portland State University, Department of Geology, 1721 SW Broadway, Portland, OR, U.S.A
²Cascadia Meteorite Laboratory, Portland State University, 1721 SW Broadway, Portland, OR, U.S.A

We show that olivine microstructures in seven metamorphosed ordinary chondrites of different groups studied with optical and transmission electron microscopy can be used to evaluate the post-deformation cooling setting of the meteorites, and to discriminate between collisions affecting cold and warm parent bodies. The L6 chondrites Park (shock stage S1), Bruderheim (S4), Leedey (S4), and Morrow County (S5) were affected by variable shock deformation followed by relatively rapid cooling, and probably cooled as fragments liberated by impact in near-surface settings. In contrast, Kernouvé (H6 S1), Portales Valley (H6/7 S1), and MIL 99301 (LL6 S1) appear to have cooled slowly after shock, probably by deep burial in warm materials. In these chondrites, post-deformation annealing lowered apparent optical strain levels in olivine. Additionally, Kernouvé, Morrow County, Park, MIL 99301, and possibly Portales Valley, show evidence for having been deformed at an elevated temperature (⩾800-1000 °C). The high temperatures for Morrow County can be explained by dynamic heating during intense shock, but Kernouvé, Park, and MIL 99301 were probably shocked while the H, L and LL parent bodies were warm, during early, endogenically-driven thermal metamorphism. Thus, whereas the S4 and S5 chondrites experienced purely shock-induced heating and cooling, all the S1 chondrites examined show evidence for static heating consistent with either syn-metamorphic shock (Kernouvé, MIL 99301, Park), post-deformation burial in warm materials (Kernouvé, MIL 99301, Portales Valley), or both. The results show the pitfalls in relying on optical shock classification alone to infer an absence of shock and to construct cooling stratigraphy models for parent bodies. Moreover, they provide support for the idea that “secondary” metamorphic and “tertiary” shock processes overlapped in time shortly after the accretion of chondritic planetesimals, and that impacts into warm asteroidal bodies were common.

Reference
Ruzicka A, Hugo R, Hutson M (2015) Deformation and thermal histories of ordinary chondrites: Evidence for post-deformation annealing and syn-metamorphic shock. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.04.030]

Copyright Elsevier