Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body
Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body
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西北非洲 7203 红榴石独特的火成岩结构和冲击变质作用：对红榴石母体结晶过程和进化历史的影响
DOI：
10.1111/maps.13776
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发表时间：
2021
影响因子：
2.2
通讯作者：
Goodrich Cryena
中科院分区：
文献类型：
--
作者：
Hayashi Hideyuki;Mikouchi Takashi;Kim Nak Kyu;Park Changkun;Sano Yuji;Takenouchi Atsushi;Yamaguchi Akira;Kagi Hiroyuki;Bizzarro Martin;Goodrich Cryena
西北非洲（NWA）7203是一种淬冷角闪石，其矿物学特征通常不存在于其他淬冷角闪石中。NWA 7203显示出晶粒尺寸从细晶粒（<10 μm）到粗晶粒（~3 mm）变化的织构，而其他淬火的角铁矿仅显示出单一尺寸的织构。细粒和粗粒岩石具有几乎相同的体积组成。细粒岩石的冷却速率估计为~80 °C h− 1，粗粒岩石的冷却速率估计为~1 °C h− 1。富镁橄榄石（~ Fo 64）仅在细粒岩性中发现。NWA 7203的结晶开始于细粒岩石中，富镁橄榄石颗粒充当结晶晶种。随后在缓慢冷却的条件下形成粗粒岩性。NWA 7203显示出明显的冲击变质纹理，与除NWA 1670外的其他淬火角闪石不同。我们确认，NWA 7203的氧同位素比值的绘图上的角闪石分馏线内的不确定性。而NWA 7203的Pb-Pb年龄为4543 ± 19 Ma，比其它淬冷角闪石的年龄要小，可能是受到冲击变质作用干扰的结果。NWA 7203的冲击变质作用的发现表明，一些安格利特可能来自于在后期重撞击期间仍然很大（直径>10公里）的小行星。
Northwest Africa (NWA) 7203 is a quenched angrite, showing mineralogical features typically not present in other quenched angrites. NWA 7203 exhibits textures whose grain size varies from fine grains (<10 μm) to coarse grains (~3 mm), while other quenched angrites show only single‐sized textures. Fine‐grained and coarse‐grained lithologies have nearly the same bulk compositions. Cooling rates were estimated to be ~80 °C h−1for fine‐grained lithologies and ~1 °C h−1for coarse‐grained lithologies. Mg‐rich olivines (~Fo64) were found only in fine‐grained lithologies. Crystallization of NWA 7203 started in the fine‐grained lithologies with Mg‐rich olivine grains acting as seeds for crystallization. Coarse‐grained lithologies were subsequently formed under conditions of slower cooling. NWA 7203 shows clear shock metamorphic textures unlike other quenched angrites except for NWA 1670. We confirm that the oxygen isotopic ratios of NWA 7203 plot on the angrite fractionation line within uncertainty. However, the obtained Pb‐Pb age of NWA 7203 is 4543 ± 19 Ma, younger than the ages of other quenched angrites, which might be a result of disturbance by shock metamorphism. The finding of shock metamorphism of NWA 7203 suggests that some angrites might be derived from asteroids that remained large (>10 km in diameter) during the late heavy bombardment.