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

定量确定地壳不同构造背景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型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）历史可以通过矿物和矿物组合的成分，通过反应平衡关系获得，但这些确定取决于矿物反应的PT条件，而不一定是变形条件。 温度计，直接约束变形条件的发展是很重要的，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，记录岩石的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（基于石英重结晶过程和晶体组构）已被用作分析工具，在过去的二十年中，在广泛的构造研究，假设温度是重结晶制度和组构发展的主要控制因素。 然而，重结晶制度和组构发展也可能受到微量水的影响，水对变形机制和组构发展的影响可能与温度相当重要。我们建议测试，完善，并验证含石英岩石的变形温度计，并检查这些条件下的水含量或变形率的变化需要进行评估。我们选择喜马拉雅和苏格兰西北地区作为案例研究，因为我们中的一个人已经收集了大量的适当的导向富含石英的构造岩套从这些地区在以前的NSF资金。 这些样品已经在广泛的构造环境和变形测温下变形，加上更严格的成分测温，已经完成了对许多这些样品。 将通过石英中的钛温度测量法独立地比较/测试各个样品中的再结晶制度和晶体结构所指示的变形温度，而将通过红外光谱和透射电子显微镜研究水对再结晶和晶体结构的潜在影响。在过去的半个世纪里，地球科学和材料科学中发展起来的理论概念和分析技术的结合，使我们对岩石变形/流动的机制以及环境因素（如温度）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已经变形的温度的最常用的分析技术也对化学诱导的弱化的波动敏感，所述化学诱导的弱化可能在构成岩石的矿物颗粒的变形期间发生。 因此，使用这些温度计计算的变形温度可能是错误的，并且使用这种温度数据来模拟地壳中的流动而开发的热机械数值模型可能给出不切实际的结果。 该项目旨在使用最近开发的温度计来测试这些温度计的有效性，该温度计考虑了此类化学过程。 根据先前NSF资助从喜马拉雅山和苏格兰的古老山脉地带收集的岩石样本已被选为案例研究，这些样本中化学弱化的潜在作用也将使用补充分析技术进行评估。 这些研究对板块构造有重要的应用，其结果可能会改变我们对板块力学和热性质的理解。 板块的厚度和强度最初被认为仅仅是由于地球的温度和地热梯度。 我们的研究解决了这一概念，并测试板的内部强度是否也可能受到水含量的影响，除了温度。该项目是弗吉尼亚理工大学、德克萨斯A M大学和伦斯勒理工学院的研究人员合作的成果。除了该项目的科学目标外，这项研究还有助于培养博士生。学生在弗吉尼亚理工大学和得克萨斯州A M，并提供支持的早期职业博士后研究员在RPI。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.