Shock effects in plagioclase feldspar from the Mistastin Lake impact structure, Canada
Shock effects in plagioclase feldspar from the Mistastin Lake impact structure, Canada
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DOI：
10.1111/maps.12495
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发表时间：
2015-09
影响因子：
2.2
通讯作者：
A. Pickersgill;G. Osinski;R. Flemming
中科院分区：
文献类型：
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作者：
A. Pickersgill;G. Osinski;R. Flemming
由于长石晶体结构的复杂性、相对容易风化和化学变化，超高速撞击引起的激波变质作用是长石中一个鲜为人知的过程，这使得对激波长石的光学研究具有挑战性。了解长石，尤其是斜长石中的激波变质作用，对于了解月球、火星和许多其他行星体的历史至关重要。我们在这里提出了一项综合研究，研究了来自加拿大拉布拉多Mistastin湖撞击结构的安地山和拉布拉多岩的冲击效应。研究了来自不同环境的样品，从原位中央隆起物质到各种角砾岩和冲击熔融岩的碎屑。激波变质作用的证据包括不溶石消光、偏置孪晶、扭结孪晶、交替孪晶变形以及部分或完全转变为折弯斜长石玻璃。在某些情况下，观察到交替的双片各向同性。平面变形特征（pdf）在斜长石中明显不存在，即使在邻近的石英颗粒中也存在。值得注意的是，各种微晶、双平面和组成不同的片层很容易被误认为pdf，因此必须小心。在一些样品中，假晶沸石相（levyne‐Ca）被鉴定为复相长石玻璃的替代矿物，在某些情况下，也可能被误认为是pdf。我们认为，斜长石中缺乏pdf可能是由于与不同长石的晶体结构有关的结构控制和/或存在以孪晶和解理面形式存在的现有薄弱面。
Shock metamorphism, caused by hypervelocity impact, is a poorly understood process in feldspar due to the complexity of the crystal structure, the relative ease of weathering, and chemical variations, making optical studies of shocked feldspars challenging. Understanding shock metamorphism in feldspars, and plagioclase in particular, is vital for understanding the history of Earth's moon, Mars, and many other planetary bodies. We present here a comprehensive study of shock effects in andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada. Samples from a range of different settings were studied, from in situ central uplift materials to clasts from various breccias and impact melt rocks. Evidence of shock metamorphism includes undulose extinction, offset twins, kinked twins, alternate twin deformation, and partial to complete transformation to diaplectic plagioclase glass. In some cases, isotropization of alternating twin lamellae was observed. Planar deformation features (PDFs) are notably absent in the plagioclase, even when present in neighboring quartz grains. It is notable that various microlites, twin planes, and compositionally different lamellae could easily be mistaken for PDFs and so care must be taken. A pseudomorphous zeolite phase (levyne‐Ca) was identified as a replacement mineral of diaplectic feldspar glass in some samples, which could, in some instances, also be potentially mistaken for PDFs. We suggest that the lack of PDFs in plagioclase could be due to a combination of structural controls relating to the crystal structure of different feldspars and/or the presence of existing planes of weakness in the form of twin and cleavage planes.