Grain and Phase Boundaries in Mantle Assemblages: Atom Probe and Electron Microscopy/Diffraction Approaches

在此资助下进行的研究旨在加强对结晶固体中晶界的化学和物理学的理解，特别强调代表地球上地幔的硅酸盐材料中的边界。 典型的结晶固体由无数的晶体（晶粒）组成，通常具有随机的空间取向，跨越带状二维边界结合在一起。 这些边界具有特定的结构和化学性质，在基本方式上与两侧的晶粒不同;边界通常控制固体整体的物理性质，例如，在地球和行星科学中，社区的兴趣反映了这些特性的广度，尽管特别是机械和电化学特性：同一矿物颗粒之间的边界，不同矿物颗粒间的边界（相边界）和矿物与熔体之间的边界直接影响着以下现象：（i）地幔变形速率（ii）地震波能量的衰减，以及（iii）岩浆的化学成分（特别是微量元素，这有助于理解地球中的熔融和岩浆迁移过程）。在这项具体研究中，突出的是一种新的和令人兴奋的分析技术应用于晶粒和相界结构和化学的研究：原子探针层析成像（APT）。在样品制备和数据分析中给予适当的关注，APT可以在原子尺度上表征边界化学，从而可以提出和仔细检查有关材料动力学的精确问题。这种方法的一个优点是，人们可以学习从亚纳米到超纳米尺寸的机械和化学反应的缩放物理学。这项研究对经济和劳动力发展都有影响。离子和共价键合固体（如矿物）中的晶粒和相边界结构和化学是电池/燃料电池、光伏和结构应用的高级陶瓷开发中的主要关注点。 采用APT在一个有效的方式进行化学设计的晶粒/相界是一个直接延伸的研究支持在这里。 “有效方法”的告诫与APT方法的进步（标本制备、成像条件、数据分析）有关：这些必要的进步将构成计划参与者积累和传播的教育的不小部分。 该研究特别侧重于（a）上地幔化学/矿物学（橄榄石加辉石）的变形岩石和（B）橄榄石晶界与寄主岩浆的界面中的晶粒和相边界结构和化学。前一个重点检查地幔聚合物变形（实验）扩散蠕变晶粒/相边界滑动。 描述偏应力的空间方向对边界结构和化学的影响，可以阐明地幔塑性不稳定性的物理学方面，这是板块构造产生和维持的一个关键问题。 后一个重点解决了玄武岩岩浆结晶的问题和晶界可能在（i）储存不相容的微量元素以及（ii）影响/影响部分结晶的大型岩浆体的动员方面发挥的作用。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

The research pursued under this grant seeks to enhance understanding of the chemistry and physics of grain boundaries in crystalline solids, with particular emphasis on boundaries in silicate materials representative of the upper mantle of the Earth. Typical crystalline solids consist of myriad crystals (grains), frequently having random spatial orientation, bonded together across ribbon-like, two-dimensional boundaries. These boundaries have specific structure and chemistry, different in fundamental ways from the grains on either side; the boundaries frequently control the physical properties of the solid overall, e.g., strength, viscosity, electrical and ionic conductivity, optical transmittance, etc. In Earth and planetary science, the community’s interests reflect the breadth of these properties, although particularly the mechanical and electrochemical ones: boundaries between grains of the same mineral, boundaries between grains of different minerals (phase boundaries) and boundaries between minerals and melts affect directly phenomena such as (i) the rate of mantle deformation (which effects plate tectonics), (ii) the attenuation of seismic wave energy, and (iii) the chemical composition of magmas (particularly of trace elements, which facilitates understanding of melting and magma-migration processes in the Earth). Prominent in this specific research is the application of a new and exciting analytical technique to the study of grain- and phase-boundary structure and chemistry: atom probe tomography (APT). With appropriate care in specimen preparation and data analysis, APT can characterize boundary chemistry at the very atomic scale, allowing precise questions concerning materials dynamics to be posed and scrutinized. A beauty of the approach is that one can so learn the physics of scaling of mechanical and chemical responses from the sub-nanometer to the kilometer-plus dimensions. The research has implications both economic and in workforce development. Grain- and phase-boundary structure and chemistry in ionic and covalent-bonded solids (as are minerals) is a primary concern in the development of advanced ceramics for battery/fuel cell, photovoltaic, and structural applications. Employing APT in an effective way for the chemical design of grain/phase boundaries is a direct extension of the research supported here. The “effective way” caveat has everything to do with advances in APT approach (specimen preparation, imaging conditions, data analysis): these necessary advances will constitute no small part of the education accumulated—and promulgated—by the program participants. The research specifically focuses on grain and phase boundary structure and chemistry in (a) deformed rock of upper-mantle chemistry/mineralogy (olivine plus pyroxenes) and (b) olivine grain boundary interface(s) with a host magma. The former focus examines mantle aggregates deformed (experimentally) in diffusion creep–grain/phase boundary sliding. Characterizing the impact of the spatial orientation of deviatoric stress on boundary structure and chemistry can elucidate aspects of the physics of plastic instability in the mantle—a crucial issue in creating and sustaining plate tectonics. The latter focus addresses issues in the crystallization of basaltic magma and the role grain boundaries might play in (i) storing incompatible trace elements as well as (ii) affecting/effecting the mobilization of large magma bodies that are partially crystallized.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.