Metastable transformations of pyroxenes in subducting slabs

俯冲板片中辉石的亚稳态转变

• 批准号: 1344942
• 负责人: Przemyslaw Dera
• 金额: $ 51.5万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1344942&HistoricalAwards=false
• 关键词: Metastable; transformations; pyroxenes; subducting; slabs

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.

地球是一个非常复杂和动态的系统，火山、地震或板块构造等全球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，需要涉及岩石、矿物和流体的化学和物理过程的微观模型，以及了解地球形成材料在高压和高温下物理特性的变化。材料结构的变化（相变）以及地球内部主要稳定矿物成分之间的化学反应已令人信服地与观测到的地震速度变化（不连续性）相关。然而，与此同时，地球物理学家收集了越来越多令人信服的证据，表明地球在整个地球的成分、温度和密度方面是不均匀的。动态地质环境，例如俯冲带，也可能产生远离正常地幔的条件。所有这些事实都激发了在高压实验中探索低温亚稳态状态的新动力，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。辉石矿物构成了地壳和上地幔的重要部分。辉石稳定相图的总体布局已经很好地建立，然而，直到最近，实验技术的限制阻碍了对300公里以上深度条件下的亚稳定性效应的系统研究。借助同步加速器单晶微衍射这一新颖的实验方法，研究人员最近在辉石族中发现了许多有趣的、以前未知的相变，这些相变发生在亚稳态状态。基于这些初步结果，他们假设辉石的亚稳态相图比之前假设的要复杂得多，并且之前未知的多晶型转变可能会影响俯冲板片的浮力，并且可以通过地震检测到。在该项目中，他们建议结合使用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、第一原理量子力学计算和热动力学建模，对辉石系统进行系统研究，涵盖从环境到60 GPa的压力范围和从环境到1200°C的温度范围，代表俯冲带内的估计条件。同步加速器单晶微衍射实验等晶体学研究提供了有关密度、原子配位几何、键长等结构参数的基本信息，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晶体学和热力学信息置于地球物理背景下，并确定新发现的转变的影响。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，让他们参与这些设施中最先进的实验，并显着改善夏威夷用于学生培训和研究的内部实验基础设施。