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

无论是天然的还是合成的化学体系，科学家们拥有的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地预测系统将进入的状态，如果给予足够的时间来完全反应的话。在研究在大洋中脊喷发并形成新海底的岩浆的熔融和结晶过程中，平衡一直是科学家们长期依赖的基本假设和工具，其基础是合理的假设，即岩浆系统的温度相当高，熔体的产生和迁移发生得相当缓慢。基于这一想法，人们从工作中学到了很多东西。然而，它也有局限性。有些反应非常慢。暴露在地幔中的样品可能具有类似大理石蛋糕的质地，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。这一推理得出的结论是，有必要建立一个关于熔融、熔体迁移和结晶的框架，解决接近平衡的方法(而不仅仅是终态)，以检验这些假设，解决更困难的问题，并充分了解海底的起源以及通过在那里捡到岩石可以获得的关于地球深处的信息。这是一项具有挑战性的工作，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，并能够随着时间的推移跟踪系统。这项工作将借鉴材料工程(如冶金等领域)的数值方法，明确包括在单个矿物颗粒尺度上描述地幔的一小块，并跟踪这些颗粒如何生长或收缩，彼此之间的反应，以及在熔融进行时如何向液态贡献原子。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能够以全新的方式思考火山作用、大洋中脊和俯冲带。这笔赠款将支持一项关于地幔融化过程的多尺度计算研究，特别是侧重于非平衡热力学在确定熔融源的化学和结构演化以及熔体成分方面的作用。正在开发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。这项工作将从约束模型参数开始，并根据动力学实验室实验验证模型的解释能力。这将为将该模型应用于扩张中心下方大洋地幔的减压熔融奠定基础。有待研究的进展包括使用相场技术来描述界面动力学，增加晶界扩散，开发熔体提取算法，以及采用热力学数据库来研究亚固相线和岩浆相关系。第一个任务将通过将该模型应用于固体和熔体组合的粗化，来表征化学和界面迁移率、体积自由能、主体成分和晶界扩散对粗化和相变率的作用。实验验证将检查方镁石、石英、顽辉石和镁橄榄石之间的反应速度和结构。然后，主要应用于研究减压熔融过程中大洋中脊扩张中心之下地幔橄榄岩的微结构和化学演化。这一阶段将研究生产和迁移过程中熔体的拓扑结构，熔融和相变(例如，石榴石到尖晶石)的结构演变，以及粒度和减压速率对成分的影响。研究将考虑两种情况：半封闭系统、近分相熔化模型和开放系统、反应流熔化模型。该项目支持培养博士后研究员，在地球科学中开发颗粒尺度的非平衡热力学模型，并将这一发展扩展到与粗化、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智力优势和更广泛的影响审查标准进行评估，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.