^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]

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