Prevalence of growth twins among anhedral plagioclase microlites
Prevalence of growth twins among anhedral plagioclase microlites
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DOI：
10.2138/am-2015-4809
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发表时间：
2015-02
影响因子：
3.1
通讯作者：
C. Brugger;J. Hammer
中科院分区：
文献类型：
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作者：
C. Brugger;J. Hammer
摘要火山岩的晶体结构记录了岩浆储存和喷发上升的过程。同喷发结晶是指基质晶体根据热力学过冷等环境因素形成和生长，强烈影响喷发岩浆的结构。这一阶段很难在天然岩石中分离出来进行研究，但很适合进行实验室实验，因为可以模拟喷发过程的化学成分和结晶时间。这项研究考察了含水流纹岩岩浆在减压驱动脱气过程中斜长石结晶的早期阶段。用电子背散射衍射(EBSD)分析了近平衡和远离平衡条件下晶体生长的实验样品，以确定单个晶体的晶格取向。晶体取向研究证实了晶体数密度织构研究中的一个常见假设：具有平行面的连续晶体在结晶学上是连续的，而具有非平行面的连续晶体具有无关的晶格取向，因此代表单独的晶体。在高度过冷的样品中，约87%的晶体存在孪晶；在近平衡样品中，38%的晶体存在孪晶。我们发现，观察到的孪晶不太可能是形变、转变或结合的结果，而是结晶初期引入的生长缺陷的结果。我们认为，内部结构缺陷(孪晶)控制宏观形态缺陷(包裹体、燕尾和熔体夹杂物)，这是孪晶面边界高能的结果。斜长石结晶初期形成的孪晶是导致岩浆减压过程中生长的长石微石无面体形态的最重要因素，也是某些斜长石熔体包裹体发育的重要因素。
Abstract Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.