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Chinese Science Bulletin, Volume 64, Issue 5-6: 579-587(2019) https://doi.org/10.1360/N972018-00972

Petrography, mineral chemistry and shock metamorphism of the Mangui meteorite

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  • ReceivedSep 26, 2018
  • AcceptedNov 12, 2018
  • PublishedDec 4, 2018

Abstract

The Mangui meteorite, also known as the Xishuangbanna meteorite, fell in Menghai, Xishuangbanna Dai Nationality Autonomous Prefecture, Yunnan Province, China at ~9:45 pm on June 1, 2018. More than 500 fragments of Mangui were collected a total mass of ~50 kg. The mass of the largest fragment is ~1228 g. Mangui has a black fusion crust, a light gray exposure surface, and contains many black shock melt veins and pockets. In this work, the classification of Mingui and its shock history are investigated by studying its petrology, mineral chemistry, and the nature of its shock assemblages. Mangui is mainly composed of olivine, pyroxene, feldspar, Fe-Ni metal, troilite, chromite, merrillite and apatite, showing typical petrologic features of an ordinary chondrite. Relict chondrules are observed in hand specimens but their boundaries are less clear under FE-SEM. Electron Probe Micro Analysis (EPMA) shows that the chemical compositions of olivine (Fa25.1±0.3 mol%(n=71)), low-Ca pyroxene (Fs21.1±0.3Wo1.5±0.2 mol% (n=58)) and plagioclase Ab84.1±0.7Or7.3±0.4 mol%(n=49)) in Mangui are relatively homogeneous. The petrographic type of Mangui is classified as type 6 by the coarse grains of secondary plagioclase, the homogeneous compositions of olivine and pyroxene, the recrystallization of matrix and blurry boundaries of relict chondrules. The chemical group of Mangui is L based on the average Fa content of olivine (Fe/(Fe+Mg)×100, atomic percentage, 25.1±0.3, n=71) and the average Fs content of pyroxene (Fe/ (Fe+Mg+Ca)×100, atomic percentage, 21.1±0.3, n=58). Therefore, the Mangui meteorite is classified as an L6 ordinary chondrite.

The Mangui parent body experienced strong shock metamorphism with the widespread distribution of shock melt veins with widths up to 600 μm. Shock induced melt pockets were also observed. Micron-sized olivine and pyroxene in the center of the shock melt veins display chemical zonation, suggesting that they recrystallized from the shock induced melt. Plagioclase in the shock melt veins has transformed into maskelynite. Two high pressure polymorphs, jadeite and majorite, occur in the shock melt veins and melt pockets as measured by Raman analysis. Jadeite usually displays granular textures in these assemblages as euhedral to sub-euhedral crystals. Majorite was only found in one jadeite assemblage, occurring as inclusions in outer rim of an assemblage. The co-occurrence of majorite and jadeite indicate shock pressures from 17–20 GPa at temperatures ranging from 1900–2000°C. However, ringwoodite, whitlockite and other high pressure polymorphs have not been identified in the shock melt veins pockets. The occurrences of maskelynite, jadeite and majorite, and widespread distribution of shock melt veins and pockets suggest the shock stage of Mangui as S5. The absence of ringwoodite and tuite probably attribute to slow post-shock cooling rates, suggesting that the Mangui parent body experienced strong shock metamorphism with a subsequent slow post-shock cooling history. TEM and XRD work will be performed to better constrain the formation mechanisms of jadeite and majorite and their P-T-t histories. Furthermore, we aim on modelling the impact conditions on the Mangui parent body such as impact velocity, and parent body-impactor sizes by further study of the high-pressure minerals in shock-melt veins of the Mangui meteorite.


Funded by

国家自然科学基金(41430105)

国家自然科学基金(41490631)

国家自然科学基金(41573057)

中国科学院前沿科学重点研究项目(QYZDJ-SSW-DQC001)


Acknowledgment

致谢 研究样品由陨石爱好者蒋维捐赠. 感谢两位匿名审稿人的意见和建议, 以及中国科学院地质与地球物理研究所Changela博士对英文摘要的润色.


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  • Figure 1

    (Color online) Hand specimen image of Mangui (a) and fragments images of Mangui sample (b). SMV, shock-melt vein

  • 图 2

    (网络版彩色)曼桂陨石显微结构. (a) 冲击熔脉; (b) 为(a)图的黄色方框放大区域, 可见不规则的硬玉; (c) 残余球粒, 轮廓较为模糊; (d) 主岩矿物-橄榄石等硅酸盐矿物破裂严重. SMV, 冲击熔脉; Met, 金属; Ol, 橄榄石; Px, 辉石; Jd, 硬玉

  • Figure 3

    (Color online) BSE images of maskelynite in Mangui. (a), (c) Local BSE images of shock-melt vein; (b), (d) enlarged view of the area shown by the yellow rectangular in (a) and (c) respectively. SMV, shock-melt vein; Met, metal; Ol, olivine; Px, pyroxene; Jd, jadeite; Mas, maskelynite; red spot, Raman analyse spot

  • Figure 4

    (Color online) BSE images of jadeite in Mangui. (a), (c), (e) Local BSE images of shock-melt vein; (b), (d), (f) enlarged view of the area shown by the yellow rectangular in (a), (c) and (e), respectively. SMV, shock-melt vein; Met, metal; Ol, olivine; Px, pyroxene; Jd, jadeite; Maj, majorite; red spot, Raman analyses spot

  • Figure 5

    Raman spectra of maskelynite, jadeite and majorite in Mangui. Mas, maskelynite; Jd, jadeite; Maj, majorite

  • Figure 6

    (Color online) Corresponding relation of olivine (Fa) and pyroxene (Fs) in ordinary chondrites. Other data come from Galimov et al.[3], Zucolotto et al.[4] and NASA Antarctic Meteorite Classification Database

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