Back‐transformation mechanisms of ringwoodite and majorite in an ordinary chondrite
Back‐transformation mechanisms of ringwoodite and majorite in an ordinary chondrite
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DOI：
10.1111/maps.13543
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发表时间：
2020-07
影响因子：
2.2
通讯作者：
K. Fukimoto;M. Miyahara;T. Sakai;H. Ohfuji;N. Tomioka;Y. Kodama;E. Ohtani;A. Yamaguchi
中科院分区：
文献类型：
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作者：
K. Fukimoto;M. Miyahara;T. Sakai;H. Ohfuji;N. Tomioka;Y. Kodama;E. Ohtani;A. Yamaguchi
我们研究了在进入大气加热过程中，大和75267 H6普通球粒陨石的冲击熔融脉（SMV）中发生的林伍德石和镁橄榄石的逆转变机制。在熔融地壳附近的冲击熔体中，林伍德石和镁橄榄石分别转化为橄榄石和顽火辉石。林伍德石（Fa~18）以细粒多晶组合出现在SMV中。接近熔融地壳时，细粒多晶橄榄石取代灵伍德石成为主要矿物。从灵伍德石到橄榄石的逆转变通过非相干成核和界面控制的生长机制进行：成核发生在灵伍德石的晶界上，随后橄榄石晶粒生长。主要成分（Fs 16 - 17 En 82 - 83 Wo 1）以细粒多晶组合的形式出现在SMV中。接近熔壳时，主要颗粒变成玻璃化。更接近熔融地壳，斜/正顽火辉石颗粒出现在玻璃化的镁铝榴石中。从镁铝榴石到顽火辉石的逆转变是由玻璃化引发的，并且生长通过随后在玻璃化镁铝榴石中的成核而继续。
We investigated the back‐transformation mechanisms of ringwoodite and majorite occurring in a shock‐melt vein (SMV) of the Yamato 75267 H6 ordinary chondrite during atmospheric entry heating. Ringwoodite and majorite in the shock melt near the fusion crust have back‐transformed into olivine and enstatite, respectively. Ringwoodite (Fa~18) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, fine‐grained polycrystalline olivine becomes dominant instead of ringwoodite. The back‐transformation from ringwoodite to olivine proceeds by incoherent nucleation and by an interface‐controlled growth mechanism: nucleation occurs on the grain boundaries of ringwoodite, and subsequently olivine grains grow. Majorite (Fs16–17En82–83Wo1) occurs in the SMV as a fine‐grained polycrystalline assemblage. Approaching the fusion crust, the majorite grains become vitrified. Approaching the fusion crust even more, clino/orthoenstatite grains occur in the vitrified majorite. The back‐transformation from majorite to enstatite is initiated by the vitrification, and growth continues by the subsequent nucleation in the vitrified majorite.