Mechanisms and Kinetics of Porphyroblast Nucleation

变质岩对压力、温度及其化学环境的变化作出反应，将现有的矿物转化为新的矿物。这些矿物组合的变化创造了岩石历史的记录，地质学家可以用来重建古代的造山和侵蚀事件，并了解这些事件的原因和它们是如何发生的。启动一种矿物组合向另一种矿物组合转化所需的关键第一步是产生新矿物的亚微观晶体(核)，因此，为了使岩石学家准确阅读保存在变质岩中的地球历史记录，对这一成核过程的详细了解是必不可少的。特别是，如果新矿物的成核和生长速度明显慢于压力、温度和化学环境的变化，矿物的变化就不能跟上这些物理条件的变化。因此，岩石学记录可能包含空白或可能被误读。该项目的目标是了解变质矿物成核所涉及的原子尺度机制，并量化自然界中成核的速率。这些知识将提高地质学家重建地球历史事件的能力，这些事件塑造了今天山脉和沉积盆地的分布，这些信息对于定位、开发和保护各种自然资源至关重要，包括水、土壤、工业矿物和多种能源。在这个项目中，经典成核理论的预测将与不同岩石套件中的石榴石斑岩的观测和测量进行比较。这项研究将包括最近出现的技术和模拟技术在概念上的三个新应用。首先，将结合高分辨率X射线计算机断层扫描和电子背散射衍射测量来确定石榴石晶体和可能控制其成核的衬底之间的外延取向关系的普遍性和共同的结晶学特征。其次，在X射线计算机层析成像定量微结构数据的约束下，成核和生长过程的新的数值模型将被用来确定不同岩石组中成核速率的时间变化，并估计相关的反应亲和力、界面能效应和成核速率的稳态极限。第三，将通过分子静力学计算来计算各种石榴石-石榴石晶界的界面能，作为与石榴石成核直接相关的多相界面能类似计算的第一步。

Metamorphic rocks respond to changes in pressure, temperature, and their chemical environment by transforming existing minerals into new minerals. These changes in mineral assemblage create a record of the rocks' history that geologists can use to reconstruct ancient episodes of mountain building and erosion, and to understand the causes of these events and how they occur. The vital first step required to initiate the transformation of one mineral assemblage to another is creation of submicroscopic crystals (nuclei) of the new minerals, so detailed knowledge of this process of nucleation is essential in order for petrologists to read accurately the record of Earth history preserved in metamorphic rocks. In particular, if new minerals nucleate and grow significantly more slowly than changes occur in pressure, temperature, and chemical environment, the mineral transformations cannot keep pace with those changes in physical conditions. The petrologic record may then contain gaps or may be misread. The goal of this project is to understand the atomic-scale mechanisms involved in nucleation of metamorphic minerals, and to quantify the rates at which nucleation takes place in nature. This knowledge will improve the ability of geologists to reconstruct events in Earth history that have shaped the present-day distribution of mountain ranges and sedimentary basins, information that is vital to locating, exploiting, and conserving a variety of natural resources, including water, soil, industrial minerals, and multiple energy sources.In this project, the predictions of classical theories of nucleation will be compared to observations and measurements made of garnet porphyroblasts in diverse suites of rocks. The research will encompass three conceptually new applications of recently emergent technologies and simulation techniques. First, high-resolution X-ray computed tomography and electron backscattered diffraction measurements will be combined to determine the prevalence and common crystallographic characteristics of epitaxial orientational relationships between garnet crystals and substrates that may have controlled their nucleation. Second, new numerical models of nucleation and growth processes, constrained by quantitative microstructural data from X-ray computed tomography, will be used to determine the time variation of nucleation rates in a diverse suite of rocks, and to estimate relevant reaction affinities, interfacial energy effects, and steady-state limits on nucleation rates. Third, interfacial energies for a variety of garnet-garnet grain boundaries will be computed by means of molecular-statics calculations, as a first step toward similar calculation of polyphase interfacial energies directly relevant to garnet nucleation.