Metastable transformations of pyroxenes in subducting slabs

地球是一个非常复杂和动态的系统，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了重大影响。要正确理解这些全球现象，就需要建立岩石、矿物和流体所涉及的化学和物理过程的微观模型，以及了解地球形成物质在高压和高温下的物理性质的变化。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与观测到的地震速度变化(不连续)有令人信服的联系。然而，与此同时，地球物理学家收集到越来越令人信服的证据，表明地球在成分、温度和密度方面在地球各地是不同的。动态的地质环境，如俯冲带，也可能产生与正常地幔相当远的条件。所有这些事实都为在高压实验中探索低温亚稳区域以寻找以前未知的亚稳转变提供了新的动力，这可能会影响我们对地震学观测的解释。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确定，然而，直到最近，由于实验技术的限制，无法系统地研究深度大于300公里的条件下的亚稳效应。配备了一种新的实验方法--同步单晶显微衍射，研究人员最近在辉石家族中发现了一些有趣的、以前未知的相变，这些相变发生在亚稳区。根据这些初步结果，他们假设辉石的亚稳相图比之前假设的要复杂得多，以前未知的多态转变可能会影响俯冲板块的浮力，并可能在地震中被探测到。在这个项目中，他们建议使用先进的实验和计算方法相结合，包括同步加速器单晶显微衍射、拉曼光谱、第一性原理量子力学计算和热动力学建模，以系统地研究辉石系统以及从环境到60 Gpa和从环境到1200°C的压力范围，代表俯冲带内的估计条件。晶体研究，如同步加速器单晶显微衍射实验，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结晶学结果来计算矿物的热力学性质，这些性质在高压和高温下很难或不可能通过实验测量。热动力学模型将使科学家能够将结晶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的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创造独特的研究和教育机会，参与这些设施中最先进的实验，并显著改善夏威夷用于学生培训和研究的内部实验基础设施。

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.