The role of grain-scale non-equilibrium thermodynamics in the production and evolution of oceanic crust and lithosphere

颗粒尺度非平衡热力学在洋壳和岩石圈产生和演化中的作用

基本信息

批准号：
1826310
负责人：
Paul Asimow
金额：
$ 30.62万
依托单位：
California Institute of Technology
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2018
资助国家：
美国
起止时间：
2018-08-01 至 2020-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1826310&HistoricalAwards=false
关键词：
role grain scale non equilibrium

项目摘要

One of the most powerful approaches that scientists have for understanding chemical systems, both natural and synthetic, is equilibrium thermodynamics. This approach allows us to predict, in detail, the state that systems will settle into if given enough time to react completely. In studying the melting and crystallization of the magmas that erupt at mid-ocean ridges and form new ocean floor, equilibrium has been the basic assumption and tool that scientists have long relied on, on the basis of the reasonable assumption that temperatures are quite high in magmatic systems and melt production and migration is happening reasonably slowly. Much has been learned from work based on this idea. However, it has limitations. Some reactions are very slow. Exposed samples of the mantle may have a texture like "marble cake" and, if the regions of different composition are big enough, they cannot react with each other completely. Diffusion of elements through large crystals limits the rate at which they can reach equilibrium. This reasoning leads to the conclusion that a framework for thinking about melting, melt migration, and crystallization that addresses the approach to equilibrium (rather than just the end state) is necessary to test these assumptions, address harder problems, and gain a full understanding of the origin of the seafloor and the information about Earth's deep interior that can be gained by picking up rocks there. This is a challenging endeavor because it requires new categories of numerical models built on entirely different equations, and able to follow systems through time. This work will borrow numerical approaches from materials engineering (fields like metallurgy) that explicitly include a description of a parcel of Earth's mantle at the scale of individual mineral grains, and tracks how those grains grow or shrink, react with one another, and contribute atoms to the liquid phase as melting proceeds. This tool will be applicable to numerous Earth science problems and will enable solid Earth scientists to think about volcanism, mid-ocean ridges, and subduction zones in entirely new ways.This grant will support a multi-scale computational study of melting processes in Earth's mantle, specifically focusing on the role of non-equilibrium thermodynamics in determining the chemical and textural evolution of the melting source and the composition of melts. The numerical framework for grain-scale non-equilibrium thermodynamics under development can self-consistently simulate processes such as coarsening, phase transformation, major and trace element diffusion, reactions, and melting of minerals or rocks. The work will begin by constraining model parameters and validating the model's explanatory power against kinetic laboratory experiments. That will set the stage for applying the model to the decompression melting of oceanic mantle beneath spreading centers. Developments to be studied include the use of phase-field techniques to describe interfacial dynamics, adding grain-boundary diffusion, developing an algorithm for melt extraction, and adopting a thermodynamic database for sub-solidus and magmatic phase relations. The first task will proceed via application of the model to coarsening of solid and melt-bearing assemblages, characterizing the roles of chemical and interfacial mobilities, volumetric free energies, bulk composition, and grain boundary diffusion on coarsening and phase transformation rates. Experimental validation will examine reaction rates and textures between periclase, quartz, enstatite, and forsterite. The main application will then be to investigate the microstructural and chemical evolution of mantle peridotite during decompression melting beneath mid-ocean ridge spreading centers. This phase will study the topology of melt during production and migration, textural evolution due to melting and phase transformations (e.g., garnet to spinel), and the effect of grain size and decompression rates on composition. Investigations will consider two scenarios: a semi-closed-system, near-fractional melting model and an open-system, reactive flow melting model. The project supports training a postdoctoral researcher, developing grain-scale non-equilibrium thermodynamic modeling in the Earth sciences, and expanding that development to numerous problems in material physics and engineering related to coarsening, diffusion, and phase transformation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

科学家对理解天然和合成的化学系统最有力的方法之一是平衡热力学。这种方法使我们能够详细预测如果有足够的时间完全反应，则系统将解决。在研究岩浆中山脊上爆发并形成新的海底的岩浆的融化和结晶时，平衡一直是科学家长期以来依赖的基本假设和工具，因为合理的假设是，温度在岩浆系统中的温度相当高，并且融化的产生和迁移正合理地缓慢地发生。根据这个想法，从工作中学到了很多东西。但是，它有局限性。一些反应非常慢。地幔的裸露样品可能具有“大理石蛋糕”之类的质地，如果不同构图的区域足够大，它们将无法完全反应。元素通过大晶体的扩散限制了它们达到平衡的速率。这种推理得出的结论是，要考虑融化，融化迁移和结晶的框架，该框架解决了平衡方法（而不是仅仅是终端状态），对于测试这些假设，更严重的问题，并充分了解海底的起源以及可以通过在那里岩石来获得地球深层的信息，这是必要的。这是一项具有挑战性的努力，因为它需要建立在完全不同方程式上的数值模型的新类别，并且能够随着时间的推移遵循系统。这项工作将从材料工程（像冶金等田地）中借用数字方法，这些方法明确地包括对单个矿物谷物的规模描述地球套的描述，并跟踪这些谷物如何生长或收缩，彼此反应，并在融化过程中对液相造成原子的作用。该工具将适用于许多地球科学问题，并使固体地球科学家能够以全新的方式思考火山症，中海脊和俯冲带。该赠款将支持对地球地幔中熔融过程的多规模计算研究，特别是针对非平衡热力学的作用，以确定化学物质和融合层次的构造和融化融合的作用。正在开发中的晶尺度非平衡热力学的数值框架可以自一模拟过程，例如矿物或岩石的变形，相变，主要和痕量元素扩散，反应以及熔化的过程。这项工作将首先限制模型参数并验证模型的解释能力针对动力学实验室实验。这将为将模型应用于扩展中心下方的海洋披风的减压熔化奠定了基础。要研究的发展包括使用相田技术来描述界面动力学，增加了晶界扩散，开发用于熔体提取的算法，并采用了用于亚果属和岩浆相关的热力学数据库。第一个任务将通过将模型应用于固体和融化的组合来进行，以表征化学和界面迁移率的作用，体积的自由能，批量组成以及晶界扩散在块状和相变速率上。实验验证将检查周围，石英，enstatite和脚石之间的反应速率和质地。然后，主要的应用将是研究地幔橄榄岩在减压层中中部山脊扩散中心下方的微结构和化学演化。该阶段将研究生产和迁移期间熔体的拓扑，由于熔化和相变（例如石榴石到尖晶石）而引起的质地演变以及晶粒尺寸和减压速率对组成的影响。调查将考虑两种情况：半锁定系统，近乎分流的熔化模型和一个开放式，反应性流熔化模型。该项目支持培训一名博士后研究人员，在地球科学中开发了谷物规模的非平衡热力学建模，并将这些发展扩展到与材料物理和工程有关的许多问题，与重新分布，扩散和相变相关。该奖项反映了NSF的法定任务，并通过评估范围来反映了众所周知的支持者的知识范围。