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Chondrule flattening by shock metamorphism in unequilibrated chondrites
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  • Masaaki Miyahara,
  • Junnosuke Edanaga,
  • Akira Yamaguchi,
  • Takamichi Kobayashi,
  • Toshimori Sekine,
  • Ayaka Nakamura
Masaaki Miyahara
Hiroshima University

Corresponding Author:miyahara@hiroshima-u.ac.jp

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Junnosuke Edanaga
Hiroshima University
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Akira Yamaguchi
National Institute of Polar Research
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Takamichi Kobayashi
National Institute for Materials Science
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Toshimori Sekine
The University of Osaka
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Ayaka Nakamura
Hiroshima University
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Shock recovery experiments using ALH-78084 H3 and Y-793375 L3 chondrites are conducted in the shock pressure range of 11–43 GPa to reproduce shock-induced melting and chondrule flattening. Shock experiments prove that shock-induced melting occurs beyond 11 GPa at least. The melting occurs at the boundaries between chondrules and matrices. The melts include fine-grained silicate minerals, glasses, and amoeba or spherical metallic Fe-Ni or metallic Fe-Ni­–iron-sulfide with a eutectic texture, which coincides with shock-induced melts in shocked natural chondrites. Shock experiments also prove that shock-induced flattening of chondrules occurs and the flattening degree increases with increasing shock pressure. Taking account of not only the shock experiments of ordinary chondrites but also carbonaceous chondrites, the flattening degree does not depend significantly on the densities, porosities, and chondrule/matrix ratios of chondrites. Considering the shock experiments of the Allende CV3 and Murchison CM2 carbonaceous chondrites along with present shock experiments using H/L3 ordinary chondrites, the aspect ratios of chondrules in unequilibrated chondrites (Rcho) can be expressed as follows: Rcho = 0.011 (±1) × Pressure (GPa) + 1.18 (±3). The long axes of chondrules in shocked ALH-78084 H3 and Y-793375 L3 chondrites have preferred orientations and the degree increases with increasing shock pressure. Natural L/LL3 ordinary chondrites with shock-induced melts have higher aspect ratios and preferred orientations than those without shock-induced melts although it is difficult to determine quantitatively shock pressure using the empirical formula between the aspect ratios of chondrules and shock pressure.