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

平衡热力学是科学家们用来理解化学系统的最有力的方法之一，无论是天然的还是合成的。这种方法使我们能够详细地预测系统在有足够的时间完全反应的情况下将进入的状态。在研究大洋中脊喷发并形成新洋底的岩浆的熔化和结晶过程时，平衡一直是科学家长期以来依赖的基本假设和工具，其依据是一个合理的假设，即岩浆系统的温度相当高，熔体的产生和迁移相对缓慢。从基于这一思想的工作中已经学到了很多东西。然而，它有局限性。有些反应非常缓慢。暴露的地幔样品可能具有类似于“大理石蛋糕”的质地，如果不同成分的区域足够大，它们就不能完全相互反应。元素在大晶体中的扩散限制了它们达到平衡的速度。这种推理得出的结论是，一个框架来思考融化，熔体迁移和结晶，解决平衡的方法（而不仅仅是最终状态）是必要的，以测试这些假设，解决更困难的问题，并获得对海底起源的全面了解，以及通过在那里拾取岩石可以获得的有关地球内部深处的信息。这是一项具有挑战性的奋进，因为它需要建立在完全不同的方程上的新类别的数值模型，并且能够通过时间跟踪系统。这项工作将借用材料工程（冶金等领域）的数值方法，其中明确包括在单个矿物颗粒的尺度上描述地球地幔的一个包裹，并跟踪这些颗粒如何生长或收缩，相互反应，并在熔化过程中将原子贡献给液相。这一工具将适用于许多地球科学问题，并将使固体地球科学家以全新的方式思考火山活动、大洋中脊和俯冲带。该基金将支持对地幔熔融过程的多尺度计算研究，特别关注非平衡热力学在确定熔融源的化学和结构演化以及熔融物成分方面的作用。发展中的颗粒尺度非平衡热力学数值框架可以自洽地模拟矿物或岩石的粗化、相变、主元素和微量元素扩散、反应和熔融等过程。这项工作将开始约束模型参数和验证模型的解释能力对动力学实验室实验。这将为将该模型应用于扩张中心下的海洋地幔减压熔融奠定基础。要研究的发展，包括使用相场技术来描述界面动力学，添加晶界扩散，制定一个算法熔体提取，并通过一个热力学数据库的亚固相线和岩浆相关系。第一个任务将通过应用该模型粗化的固体和熔体轴承组合，表征化学和界面迁移率，体积自由能，散装组合物和晶界扩散的粗化和相变速率的作用。实验验证将检查方镁石，石英，顽火辉石和镁橄榄石之间的反应速率和纹理。其主要应用将是研究洋中脊扩张中心减压熔融过程中地幔橄榄岩的显微结构和化学演化。这一阶段将研究生产和迁移过程中熔体的拓扑结构，由于熔化和相变引起的质地演变（例如，石榴石到尖晶石），以及粒度和减压速率对组成的影响。调查将考虑两种情况：一个半封闭系统，近部分熔化模型和一个开放系统，反应流熔化模型。该项目支持培养博士后研究人员，开发地球科学中的颗粒尺度非平衡热力学模型，并将其发展扩展到与粗化，扩散和相变相关的材料物理和工程中的许多问题。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估而被认为值得支持。

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.