Collaborative Research: Deformation Thermometry and Water Weakening of Quartz Tectonites - Case Studies from the Himalaya and the Caledonides of NW Scotland

定量确定地壳中不同构造环境下岩石变形的环境温度，对于约束地球构造板块的热力学模拟及其力学和热演化具有重要意义。环境变形温度控制着控制变形的物理过程和变形发生的速率。岩石的压力-温度-时间（PTt）历史可以通过反应平衡关系从矿物组成和矿物组合中得到，但这些决定与矿物反应的PT条件有关，而不一定是变形的PT条件。直接约束变形条件的温度计的发展是重要的，因为变形特征所指示的温度和平衡矿物组合所指示的温度可能不同，记录了岩石PTt历史的不同间隔。在富含石英的大陆地壳中，两种类型的变形温度计（基于石英再结晶过程和晶体结构）在过去二十年的广泛构造研究中被用作分析工具，假设温度是再结晶机制和结构发育的主要控制因素。然而，再结晶机制和织物发育也可能受到微量水的影响，水对变形机制和织物发育的影响可能与温度相当重要。我们建议测试、改进和验证含石英岩石的变形温度计，并检查需要评估含水量或变形率变化的条件。我们之所以选择喜马拉雅和西北苏格兰地区作为案例研究，是因为我们中的一个人已经在NSF之前的资助下从这些地区收集了大量合适的定向富含石英的构造岩。这些样品已经在广泛的构造环境下变形，变形温度测量以及更有限的基于成分的温度测量已经完成了许多这些样品。通过石英钛测温仪对单个样品的再结晶状态和晶体结构所指示的变形温度进行独立比较/测试，而水对再结晶和晶体结构的潜在影响将通过红外光谱和透射电子显微镜进行研究。地球科学和材料科学在过去半个世纪中发展起来的理论概念和分析技术的整合，使我们对岩石变形/流动的机制以及温度等环境因素对地壳流动的影响的理解取得了重大进展。然而，已知用于确定现在暴露在地球表面的岩石变形温度的最常用分析技术，也对构成岩石的矿物颗粒变形期间可能发生的化学诱导的弱化的波动很敏感。因此，使用这些温度计计算的变形温度可能是错误的，而使用这些温度数据模拟地壳流动的热力学数值模型可能会给出不切实际的结果。这个项目的目的是测试这些温度计的有效性，使用最近开发的温度计，考虑到这些化学过程。在以前的NSF资助下，从喜马拉雅和苏格兰的古山带收集的岩石样本被选为案例研究，化学弱化在这些样本中的潜在作用也将使用互补分析技术进行评估。这些研究对板块构造学具有重要的应用价值，其结果可能改变我们对板块力学和热特性的认识。板块的厚度和强度最初被认为仅仅取决于地球的温度和地温梯度。我们的研究解决了这一概念，并测试了除了温度之外，板的内部强度是否也可能受到含水量的影响。该项目是弗吉尼亚理工大学、德克萨斯农工大学和伦斯勒理工学院的研究人员共同努力的结果。除了该项目的科学目标之外，这项研究还有助于培养弗吉尼亚理工大学和德克萨斯农工大学的博士生，并为RPI的早期博士后研究员提供支持。所有参与机构的本科生都将参与这项研究。

Quantitative determination of the ambient temperatures at which rocks deform under different tectonic settings in the earth's crust is of critical importance for constraining thermo-mechanical modeling of the earth's tectonic plates, and their mechanical and thermal evolution. Ambient deformation temperatures control the physical processes that govern deformation and the rates at which deformation occurs. Pressure-Temperature-time (PTt) histories of rocks can be obtained from the compositions of minerals and mineral assemblages, through reaction equilibrium relations, but these determinations bear on the PT conditions of the mineral reactions, not necessarily those of deformation. Development of thermometers that directly constrain conditions of deformation is important because temperatures indicated by deformation features and those indicated by equilibrium mineral assemblages may differ, recording different intervals of a rock's PTt history. In quartz-rich continental crust, two types of deformation thermometers (based on quartz recrystallization processes and crystallographic fabrics) have been used as analytical tools over the last two decades in a wide range of tectonic studies, assuming that temperature is the primary controlling factor in recrystallization regime and fabric development. However, recrystallization regimes and fabric development may also be influenced by trace levels of water, and the effects of water on deformation mechanisms and fabric development may be of comparable importance to temperature. We propose to test, refine, and validate the deformation thermometers for quartz-bearing rocks and to examine those conditions under which water contents or variations in deformation rate need to be evaluated. We have chosen Himalayan and NW Scotland field areas as case studies because one of us has already collected extensive suites of appropriate oriented quartz-rich tectonites from these areas under prior NSF funding. These samples have been deformed under a wide range of tectonic settings and deformation thermometry, together with more restricted compositionally based thermometry, have already been completed on many of these samples. Deformation temperatures indicated by recrystallization regime and crystallographic fabrics in individual samples will be independently compared/tested by titanium-in-quartz thermometry, while the potential influence of water on recrystallization and crystallographic fabrics will be investigated by infrared spectroscopy and transmission electron microscopy. Integration of theoretical concepts and analytical techniques developed in Geosciences and Materials Science over the last half century has led to major advances in our understanding of both the mechanisms by which rocks deform/flow and the influence of environmental factors such as temperature on flow in the earth's crust. However the most commonly applied analytical techniques for determining the temperatures at which rocks now exposed at the earth's surface have been deformed are known to also be sensitive to fluctuations in chemically-induced weakening that may have occurred during deformation in the mineral grains making up the rock. Thus, deformation temperatures calculated using these thermometers may be in error, and thermomechanical numerical models developed to simulate flow in the crust using such temperature data may give unrealistic results. This project is designed to test the validity of these thermometers using a recently developed thermometer that takes such chemical processes in to account. Rock samples collected under previous NSF funding from ancient mountain belts in the Himalaya and Scotland have been chosen as case studies, and the potential role of chemical weakening in these samples will also be evaluated using complementary analytical techniques. These studies have important applications to plate tectonics and the results may change our understanding of the mechanical and thermal character of plates. Plate thickness and strength were originally considered to be due solely to temperature and the geothermal gradient in the earth. Our study addresses this concept and tests whether the internal strength of plates may also be influenced by water content, in addition to temperature. The project is a collaborative effort between researchers at Virginia Tech, Texas A&M University and Rensselaer Polytechnic Institute. In addition to the scientific goals of the project, this research is contributing to the training of Ph.D. students at Virginia Tech and Texas A&M, and providing support for an early career post-doctoral researcher at RPI. Undergraduate students at all of the participating institutions will be involved in the research.