Metastable transformations of pyroxenes in subducting slabs

地球是一个非常复杂的动态系统，其中火山活动、地震或板块构造等全球性地质现象对人类文明产生重大影响。要正确理解这些全球现象，就需要建立涉及岩石、矿物和流体的化学和物理过程的微观模型，并了解在高压和高温下地球形成材料的物理性质的变化。物质结构的变化（相变）和地球内部主要稳定矿物成分之间的化学反应已经令人信服地与观测到的地震速度变化（不连续）联系在一起。然而，与此同时，地球物理学家收集到越来越多令人信服的证据，证明地球在组成、温度和密度上是不均匀的。俯冲带等动态地质环境也可能产生与正常地幔相距甚远的条件。所有这些事实都为在高压实验中探索低温亚稳态状态提供了新的动力，以寻找以前未知的亚稳态转变，这可能会影响我们对地震学观测结果的解释。辉石矿物是地壳和上地幔的重要组成部分。辉石稳定相图的总体布局已经很好地建立，然而，直到最近，实验技术的局限性阻碍了在相应深度高于300 km的条件下对亚稳效应的系统研究。配备了一种新的实验方法，同步加速器单晶微衍射，研究人员最近发现了一些有趣的，以前未知的辉石家族的相变，发生在亚稳区。基于这些初步结果，他们假设辉石的亚稳相图比以前假设的要复杂得多，并且以前未知的多态转变可能会影响俯冲板的浮力，并且可以通过地震检测到。在这个项目中，他们建议结合先进的实验和计算方法，包括同步加速器单晶微衍射，拉曼光谱，第一性原理量子力学计算和热力学建模，对辉石系统进行系统的调查，覆盖从环境压力到60 GPa，从环境温度到1200°C的范围，代表了俯冲带内的估计条件。晶体学研究，如同步加速器单晶微衍射实验，提供了有关结构参数的基本信息，如密度、原子配位几何、键长等，这些参数控制着矿物的物理和化学性质，对于建立可靠的地球物理和地球化学模型是必不可少的。另一方面，计算量子力学模型允许利用这些晶体学结果来计算矿物的热力学性质，这些性质很难或不可能在高压和高温下进行实验测量。热力学建模将使科学家能够把晶体学和热力学信息放在地球物理背景下，并确定新发现的转变的含义。该项目预计将有助于改进和优化国家同步加速器用户设施中同步加速器单晶微衍射实验的实验方法，为夏威夷大学的研究生和本科生在这些设施中参与最先进的实验创造独特的研究和教育机会。并显著改善夏威夷的室内实验基础设施，用于学生培训和研究。

Earth is a very complex and dynamic system in which global geologic phenomena such as volcanism, earthquakes or plate tectonics have major impacts on human civilization. Proper understanding of these global phenomena requires microscopic models of chemical and physical processes in which rocks, minerals and fluids are involved, as well as understanding of changes in the physical properties of the Earth forming materials at high pressure and temperature. Changes in the structure of materials (phase transitions) and chemical reactions between the major stable mineral components of the Earth's interior have been convincingly linked with observed seismic velocity changes (discontinuities). At the same time, however, geophysicists gather more and more convincing evidence that the Earth is heterogeneous in composition, temperature and density throughout the Earth. Dynamic geologic environments, such as subduction zones can also produce conditions that are quite far from the normal mantle. All these facts fuel new motivation for exploring the lower-temperature metastable regime in high pressure experiments in search for previously unknown metastable transformations, which may impact our interpretation of observations from seismology. Pyroxene minerals constitute a significant portion of the Earth's crust and upper mantle. The general layout of the pyroxene stable phase diagram has been well established, however, until recently, the limitations of experimental techniques prevented systematic investigations of the metastability effects at conditions corresponding to depths higher than 300 km. Equipped with a novel experimental method, synchrotron single-crystal micro-diffraction, the investigators recently discovered a number of intriguing, previously unknown phase transformations in the pyroxene family, occurring in the metastable regime. Based on these preliminary results they hypothesize that metastable phase diagrams of pyroxenes are far more complex than previously assumed, and that the previously unknown polymorphic transformations may affect buoyancy of the subducting slab and could be seismically detectable. In this project they propose to use a combination of advanced experimental and computational methods, including synchrotron single-crystal micro-diffraction, Raman spectroscopy, first principles quantum mechanics calculations and thermokinetic modeling to conduct a systematic investigation of the pyroxene system and covering range of pressures from ambient to 60 GPa and temperature from ambient to 1200°C, representing estimated conditions within a subduction zone. Crystallographic investigations, such as synchrotron single-crystal micro-diffraction experiments provide fundamental information about the structural parameters such as density, atom coordination geometry, bond lengths, etc., which govern the physical and chemical properties of minerals and are indispensible for building reliable geophysical and geochemical models. Computational quantum mechanics models, on the other hand, allow utilization of these crystallographic results to calculate thermodynamic properties of minerals, which are difficult or impossible to measure experimentally at high pressure and temperature. Thermokinetic modeling will enable scientists to put the crystallographic and thermodynamic information in the geophysical context and determine the implications of the newly discovered transformations. The project is expected to contribute to improvement and optimization of experimental methodology for synchrotron single-crystal micro-diffraction experiments at the national synchrotron user facilities, create unique research and education opportunities for graduate and undergraduate students from the University of Hawaii to participate in state-of-the-art experiments at these facilities, and to significantly improve in house experimental infrastructure in Hawaii for student training and research.