Pilot Study: Effect of Temperature Cycling on Crystal Size and Alignment in Magmas

这个EaGER将支持初步实验，旨在测试温度循环是否对火成岩中的晶体尺寸分布和晶体排列有显著影响。这项研究的动机来自于观察到深成岩中的晶体尺寸关系通常不符合相平衡实验的预测，也不符合晶体尺寸分布理论。例如，钾长石通常出现在巨大的晶体中（长度为10厘米），尽管它与石英一起最后在花岗岩中共晶。晶体大小的关系表明，这些大晶体是通过吞食小晶体而生长的。在各种材料科学领域（例如，食品加工）的实验表明，如果温度场振荡，大晶体可以以牺牲小晶体为代价快速生长。初步研究将采用三方面的方法来研究温度循环对晶体尺寸关系的影响：(1)在硫氰酸铵-氯化钴岩浆模拟系统中进行实验；(2)在一大气压气体混合炉中进行天然镁基岩浆温度循环实验；(3)采用冷密封压力容器进行花岗岩-水体系温度循环试验。将继续在岩浆模拟系统中进行实验，以建立晶体尺寸发展的定量数据集，并在1-atm和气体混合炉中进行实验，以检查不同温度对晶体生长的作用。这些实验的一个重要方面是，振荡温度可能会显著提高晶体生长速率，正如在其他材料中看到的那样；如果是这样，在未来的研究中，可能会部分避免困扰高硅体系实验的动力学问题。这项工作的智力价值主要在于发展了对控制深部岩石晶体尺寸和晶体相互关系特征（即质地）的过程的理解。构造是人们在野外容易观察到的少数东西之一，通常是绘制深成地体地质图的基础，但最近的许多工作对标准教科书对深成构造含义的解释提出了质疑。在熔融存在的系统中，振荡的温度深刻地影响着许多材料的结构，这项研究将验证在变化的温度场中结晶对火成岩的结构也有很强的控制作用的假设。特别是晶体尺寸和晶体排列，这两者对于岩浆岩的火成岩和构造解释至关重要，可能受到振荡温度的强烈影响。这项工作的广泛影响是多方面的。这项工作将涉及岩石学、构造地质学、构造学和火山学等领域。如果深成岩中的晶体排列可以由热梯度中的结晶引起，那么用于理解岩浆侵位的特征（例如晶体排列）必须重新评估。该项目可能为困扰高硅体系实验的令人烦恼的动力学问题提供部分解决方案，因为模拟实验表明，振荡温度大大加速了晶体生长。该项目将促进使用JEOL Hyperprobe的微量分析新技术的发展，JEOL Hyperprobe最近安装在费耶特维尔州立大学（FSU）。预计该项目将为FSU的学生提供在北卡罗来纳大学主校区工作和学习的途径。结晶过程的影片将在网上发布，以供课堂教学使用。该项目具有很强的教育成分，有几篇本科论文和一篇博士论文计划。

This EaGER will support the preliminary experiments designed to test if temperature cycling has a marked effect on the crystal size distribution and crystal alignment in igneous rocks. The motivation for this study comes from the observation that crystal size relationships in plutonic rocks commonly do not match the predictions of phase equilibrium experiments nor of crystal-size distribution theory. For example, K-feldspar commonly occurs in huge crystals (10 cm in length) even though it co-crystallizes last in granites along with quartz. Crystal size relationships suggest that these large crystals grow by cannibalism of smaller crystals. Experiments in various materials science fields (e.g., food processing) show that large crystals can grow rapidly at the expense of smaller ones if the temperature field oscillates. The preliminary study will use a three-fold approach to studying the effects of temperature cycling on crystal size relationships: (1) experiments in the ammonium thiocyanate-cobalt chloride magma analog system; (2) temperature cycling experiments in natural mafic magmas in a one-atmosphere gas-mixing furnace; and (3) temperature cycling experiments in the granite-water system using cold-seal pressure vessels. Experiments in the magma analog system will be continued in order to develop a quantitative dataset on crystal size development, and experiments at 1-atm and in gas-mixing furnaces will examine the role of varying temperature of crystal growth. An important facet of these experiments is the possibility that oscillating temperature will dramatically increase crystal growth rates, as is seen in other materials; if so, kinetic problems that plague experiments in high-silica systems may be partially avoided in future studies.The intellectual merit of this work lies mainly in developing an understanding of the processes that govern the crystal size and crystal interrelationship characteristics (i.e., texture) of plutonic rocks. Texture is one of the few things one can easily observe in the field and is often the basis for drawing geologic maps in plutonic terranes, but much recent work casts doubt on the standard textbook interpretations of what plutonic texture means. Oscillating temperature in a melt-present system profoundly affects the texture of many materials, and the study will test the hypothesis that crystallization in a varying temperature field exerts a strong control on the texture of igneous rocks as well. In particular, crystal size and crystal alignment, both of which are critical to igneous and structural interpretation of magmatic rocks, may be strongly affected by oscillating temperature. The broader impacts of this work are many-fold. This work will bear on the fields of petrology, structural geology, tectonics, and volcanology. If crystal alignment in plutonic rocks can result from crystallization in a thermal gradient, then features used to understand magma emplacement (e.g., crystal alignment) must be reevaluated. The project may provide a partial solution to the vexing kinetic problems that plague experiments in high-silica systems, because analog experiments demonstrate that crystal growth is greatly accelerated by oscillating temperature. The project will facilitate the development of new techniques of microanalysis using the JEOL Hyperprobe that was recently been installed at Fayetteville State University (FSU). It is expected that the project will provide a pathway for students from FSU to work and study at the main campus of the University of North Carolina. Movies of crystallization processes will be posted online for use in classroom instruction. The project has a strong educational component, with several undergraduate theses as well as one Ph.D. thesis planned.