11:30 〜 11:45
[PPS07-14] Shock effects in silica minerals in eucrites
キーワード：ユークライト隕石、シリカ鉱物、衝撃変成作用
Eucrite is a group of differentiated meteorites probably originated from the Vestan crust [1]. Recently, we defined the progressive shock degrees (A-E) of eucrites on the basis of petrographic and X-ray diffraction methods [2]. Here, we studied shock effects in eucrites of shock degrees C-E focusing on shock textures of silica minerals.
We studied polished thin sections of five eucrites, Y 980433 (shock degree: D), Y-792510 (C), A-881747 (C), Cachari (D), and A-87272 (E) using a micro-Raman spectrometer (Renishaw inVia), a Chroma CL (Gatan), and an energy dispersive spectrometer (Oxford Aztek Energy) attached to the JEOL JSM-7100 field emission scanning electron microscope at NIPR.
Eucrites studied here contain significant amounts (~10 vol%) of silica minerals. We identified tridymite, quartz, and silica glass in the host basaltic rocks, and quartz, coesite, stishovite in shock melt veins (Table 1). In eucrites with shock degree C, characterized by the absence of maskelynite, silica minerals do not show clear shock textures.
Eucrites with shock degree D, are characterized by the presence of maskelynite near shock veins, but these eucrites do not contain maskelynite in the basaltic host (unmelted portions). Maskelynite near shock veins was produced by shock under the heat from shock melt veins [e.g., 3]. Tridymite in the basaltic host in Cachari and Y 980433 is converted to glass (~50 vol%). Patches of remnant tridymite are heterogeneously distributed in silica glass. The tridymite (partly converted to glass) preserves original lath shape. In contrast, Y-790266 (D) contains quartz and cristobalite, and is not converted into glass. The facts indicate that the amorphization pressure of tridymite is lower than those of quartz, cristobalite, and calcic plagioclase. A-87272 (shock with shock degree: E) is characterized by the abundant maskelynite (>50 vol%). Most of the silica minerals (~90 vol%) in the host are converted to glass. We found traces of quartz and tridymite.
Shock veins are commonly observed in eucrites with shock degrees C or higher[2]. We found coesite and stishovite in Y-790266, quartz, stishovite, and coesite in Cachari, and silica glass in Y 980433. The occurrences of these silica minerals are controlled by local pressure-temperature paths in or near the shock melt veins [e.g., 4].
References
[1] Binzel R.P. and Xu S. (1993) Science 260, 186-191.
[2] Kanemaru R. et al. (2020) Polar Science 26, 100605.
[3] Yamaguchi A. et al. (2002) High-Pressure Shock Compression of Solids V. Springer, 29–44 chap. 2.
[4] Miyahara M. et al. (2014) PNAS 111, 30, 10939-10942.
We studied polished thin sections of five eucrites, Y 980433 (shock degree: D), Y-792510 (C), A-881747 (C), Cachari (D), and A-87272 (E) using a micro-Raman spectrometer (Renishaw inVia), a Chroma CL (Gatan), and an energy dispersive spectrometer (Oxford Aztek Energy) attached to the JEOL JSM-7100 field emission scanning electron microscope at NIPR.
Eucrites studied here contain significant amounts (~10 vol%) of silica minerals. We identified tridymite, quartz, and silica glass in the host basaltic rocks, and quartz, coesite, stishovite in shock melt veins (Table 1). In eucrites with shock degree C, characterized by the absence of maskelynite, silica minerals do not show clear shock textures.
Eucrites with shock degree D, are characterized by the presence of maskelynite near shock veins, but these eucrites do not contain maskelynite in the basaltic host (unmelted portions). Maskelynite near shock veins was produced by shock under the heat from shock melt veins [e.g., 3]. Tridymite in the basaltic host in Cachari and Y 980433 is converted to glass (~50 vol%). Patches of remnant tridymite are heterogeneously distributed in silica glass. The tridymite (partly converted to glass) preserves original lath shape. In contrast, Y-790266 (D) contains quartz and cristobalite, and is not converted into glass. The facts indicate that the amorphization pressure of tridymite is lower than those of quartz, cristobalite, and calcic plagioclase. A-87272 (shock with shock degree: E) is characterized by the abundant maskelynite (>50 vol%). Most of the silica minerals (~90 vol%) in the host are converted to glass. We found traces of quartz and tridymite.
Shock veins are commonly observed in eucrites with shock degrees C or higher[2]. We found coesite and stishovite in Y-790266, quartz, stishovite, and coesite in Cachari, and silica glass in Y 980433. The occurrences of these silica minerals are controlled by local pressure-temperature paths in or near the shock melt veins [e.g., 4].
References
[1] Binzel R.P. and Xu S. (1993) Science 260, 186-191.
[2] Kanemaru R. et al. (2020) Polar Science 26, 100605.
[3] Yamaguchi A. et al. (2002) High-Pressure Shock Compression of Solids V. Springer, 29–44 chap. 2.
[4] Miyahara M. et al. (2014) PNAS 111, 30, 10939-10942.