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环境下岩石变形的环境温度，对于约束地球构造板块的热机械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)历史可以根据矿物组成和矿物组合，通过反应平衡关系获得，但这些测定与矿物反应的PT条件有关，而不一定与变形的PT条件有关。开发直接限制变形条件的温度计很重要，因为由变形特征指示的温度和由平衡矿物组合指示的温度可能不同，记录了岩石PTT历史的不同间隔。在富含石英的陆壳中，两种形变温度计(基于石英重结晶过程和晶体组构)在过去20年的广泛构造研究中被用作分析工具，假设温度是重结晶制度和组构发展的主要控制因素。然而，再结晶制度和织物发展也可能受到微量水水平的影响，水对变形机制和织物发展的影响可能与温度同等重要。我们建议测试、改进和验证含石英岩石的变形温度计，并检查需要评估水分含量或变形速率变化的条件。我们选择喜马拉雅和苏格兰西北部地区作为案例研究，是因为我们中的一人已经在之前的NSF资助下从这些地区收集了大量适当定向的富石英构造岩套件。这些样品在广泛的构造环境下发生了变形，其中许多样品已经完成了形变测温，以及更受限制的基于成分的测温。由再结晶制度和结晶组构指示的变形温度将通过石英中钛测温仪进行独立的比较/测试，而水对再结晶和晶体组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。在过去的半个世纪里，地球科学和材料科学发展起来的理论概念和分析技术的结合，使我们在理解岩石变形/流动的机制以及温度等环境因素对地壳流动的影响方面取得了重大进展。然而，最常用的用于确定目前暴露在地球表面的岩石已经变形的温度的分析技术，众所周知也对组成岩石的矿物颗粒在变形过程中可能发生的化学诱导的减弱的波动很敏感。因此，使用这些温度计计算的变形温度可能是错误的，而利用这些温度数据来模拟地壳中的流动而开发的热力学数值模型可能会得出不切实际的结果。该项目旨在使用一种最新开发的温度计来测试这些温度计的有效性，该温度计将这些化学过程考虑在内。在NSF以前的资助下从喜马拉雅和苏格兰的古代山区收集的岩石样本被选为案例研究，这些样本中的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，其结果可能改变我们对板块力学和热学特征的理解。板块的厚度和强度最初被认为完全是由于温度和地球的地温梯度。我们的研究解决了这一概念，并测试了除了温度外，板材的内部强度是否也会受到水分含量的影响。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成果。除了该项目的科学目标，这项研究还为弗吉尼亚理工大学和德克萨斯农工大学的博士生培训做出了贡献，并为RPI的一名早期职业博士后研究员提供了支持。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.