Shock-induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650
Shock-induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650
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西北非 2650 锂辉石陨石中斜长石冲击引起的相变
DOI：
10.1111/maps.13286
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发表时间：
2019
影响因子：
2.2
通讯作者：
Li Yang
中科院分区：
文献类型：
--
作者：
Chen De-Liang;Zhang Ai-Cheng;Pang Run-Lian;Chen Jia-Ni;Li Yang
钙长石是小天体中玄武质无球粒陨石的重要组成矿物。它在冲击陨石中的高压相变尚未得到系统的研究。在这项研究中，我们报告了在一个冲击eucrite西北非洲（NWA）2650，其中还包含柯石英，stishovite，富空位单斜辉石，超石榴子石和reidite钙长石的各种相变行为。NWA 2650中的钙长石已转化为钙长石玻璃（钙长石玻璃状脉，maskelynite和具有纹影结构和囊泡的玻璃），组织岩并解离成三相组合钙铝榴石+蓝晶石+硅玻璃。不同产状的钙长石玻璃可能分别通过剪切熔融、固态转变和冲击后热熔融机制形成。天青石可能是在高压斜长石熔体中结晶出来的残余辉石碎片和早期形成的富空位单斜辉石可能促进了组织岩的成核。钙铝榴石、蓝晶石和石英玻璃的三相组合应该是在高压和高温条件下由钙长石熔体形成的。maskelynite和reidite的存在可能表明最小峰值冲击压力高达20 GPa，而其他高压相表明冲击熔体脉结晶过程中的冲击压力可能在>8 GPa至>2 GPa之间变化，温度分布不均匀。
Anorthite is an important constituent mineral in basaltic achondrites from small celestial bodies. Its high‐pressure phase transformation in shocked meteorites has not been systematically studied. In this study, we report the diverse phase transformation behaviors of anorthite in a shocked eucrite Northwest Africa (NWA) 2650, which also contains coesite, stishovite, vacancy‐rich clinopyroxene, super‐silicic garnet, and reidite. Anorthite in NWA 2650 has transformed into anorthite glass (anorthite glassy vein, maskelynite, and glass with a schlieren texture and vesicles), tissintite and dissociated into three‐phase assemblage grossular + kyanite + silica glass. Different occurrences of anorthite glass might have formed via the mechanism involving shear melting, solid‐state transformation, and postshock thermally melting, respectively. Tissintite could have crystallized from a high‐pressure plagioclase melt. The nucleation of tissintite might be facilitated by relict pyroxene fragments and the early formed vacancy‐rich clinopyroxene. The three‐phase assemblage grossular, kyanite, and silica glass should have formed from anorthitic melt at high‐pressure and high‐temperature conditions. The presence of maskelynite and reidite probably suggests a minimum peak shock pressure up to 20 GPa, while the other high‐pressure phases indicate that the shock pressure during the crystallization of shock melt veins might vary from >8 GPa to >2 GPa with a heterogeneous temperature distribution.