Rates and Mechanisms of Intracrystalline and Intergranular Diffusion

发生在地壳深处或上地幔的变质过程无法直接观察到，它们跨越的时间尺度比人类的寿命长得多，因此对它们的了解必须来自对它们产生的岩石的研究。为了重建地质历史-例如，为了了解一个山脉带形成需要多长时间，它有多古老，它产生或被侵蚀的速度有多快-我们必须能够阅读这些过程的记录，这些过程被编码在矿物中，这些矿物形成于很深的地方，并通过构造过程被带到地表。这项研究的中心目标是更好地了解如何阅读石榴石中的此类记录，石榴石是一种具有将其历史细节编码到其化学成分中的非凡能力的矿物。石榴石在生长过程中，通常会从晶体的核心到边缘产生组成元素浓度的差异，并且这些变化会通过扩散进行修改，即原子在固体晶体结构中的运动。扩散的修改发生在不同程度上，反映了不同的时间长度的晶体在不同的温度和压力在其生长后的历史。将这些扩散变化转化为地质过程的详细信息的关键是对不同元素在石榴石结构中扩散的速率和机制的定量了解。先前的工作已经对石榴石中丰富的元素的扩散速率进行了可靠的测量，但是为了收集高温过程的信息，需要对丰度非常低的微量元素进行这样的数据。因此，本研究的第一个目标是确定微量元素的扩散速率，通过分析和建模的浓度分布的演变，从核心到边缘的石榴石晶体具有特殊的，良好的约束历史。第二个目标是发展新的理论来解释，在一个基本的方式，原子尺度的扩散机制，通过复杂的计算机模拟（分子动力学模拟）的原子在扩散运输过程中的运动。第三个目标是量化不同类型扩散的速率，即晶间扩散--意味着沿沿着晶体边界而不是在晶体本身内扩散。铝的晶间扩散速率是矿物生长过程中变质反应和化学交换速率的主要控制因素。在特殊情况下，它可以被量化，在这种情况下，晶体与周围环境发生反应，产生新的矿物质，石榴石中微量元素扩散的新数据应该增加严格的应用，如高温加热/冷却的历史和高温的时间尺度，温度热事件和交代事件，Sm-Nd和Lu-Hf石榴石年代学，石榴石与副矿物之间的变质平衡的理解，以及地幔矿物和熔体的稀土模式的解释。分子动力学模拟应该回答为什么石榴石和其他矿物不服从扩散的弹性应变理论这一令人困惑的问题，从而应该揭示迄今尚未认识到的控制固态扩散动力学的因素。铝的晶间扩散率的新估计应扩展到更高的温度以前的结果，显着提高我们的能力，定量处理的长度尺度和时间尺度的变质反应，变质过程中的化学平衡的范围和程度。

Metamorphic processes that take place deep in the earth's crust or in its upper mantle cannot be observed directly, and they span time scales much longer than human lifetimes, so knowledge of them must come from study of the rocks they produce. To reconstruct geologic histories -- to learn, for example, how long it takes for a mountain belt to form, how ancient it is, how rapidly it arose or was eroded -- we must be able to read the record of those processes that is encoded in minerals that formed at great depth and have been brought to the surface by tectonic processes. The central goal of this research is to learn better how to read such records in garnet, a mineral with a remarkable ability to encode details of its history in its chemical composition. Garnet, during growth, commonly develops differences in the concentrations of constituent elements from the cores of crystals out to their rims, and these variations are modified by diffusion, the movement of atoms through the solid crystal structure. Diffusional modifications occur to varying degrees, reflecting the various lengths of time that the crystals spent at different temperatures and pressures during their post-growth histories. The key to transforming these diffusional modifications into detailed information on geologic processes is quantitative knowledge of the rates and mechanisms of diffusion of different elements through the garnet structure.Prior work has produced robust measurements of diffusion rates for elements that are abundant in garnet, but to glean information on high-temperature processes, such data are needed for trace elements, those with very low abundance. Thus the first objective of this research is to determine diffusion rates of trace elements by analyzing and modeling the evolution of concentration profiles from core to rim in garnet crystals with special, well-constrained histories. A second objective is to develop new theories to explain, in a fundamental way, the atomic-scale mechanisms of diffusion, by sophisticated computer modeling (molecular-dynamics simulation) of the motions of atoms during diffusional transport. A third objective is to quantify rates of a different type of diffusion, namely intergranular diffusion -- meaning diffusion along crystal boundaries rather than within the crystals themselves. The rate of intergranular diffusion of aluminum is a principal control on rates of metamorphic reactions and chemical exchange among minerals as they grow. It can be quantified in special circumstances, in which a crystal is 'caught in the act' of reacting with its surroundings to produce new minerals, by analysis and modeling of the compositional changes that build up at crystal's outer periphery as it reacts.New data on trace-element diffusion in garnet should add rigor to applications as diverse as high-temperature heating/cooling histories and timescales of high-temperature thermal and metasomatic events, Sm-Nd and Lu-Hf garnet geochronology, understanding of metamorphic equilibration between garnet and accessory minerals, and interpretation of rare-earth patterns of mantle minerals and melts. Molecular-dynamics simulation should answer the puzzling question of why garnet and other minerals fail to obey elastic-strain theories for diffusion, and should thereby reveal so-far-unrecognized factors governing the kinetics of diffusion in the solid state. New estimates for intergranular diffusivity of Al should extend previous results to higher temperatures, significantly improving our ability to treat quantitatively the length scales and time scales of metamorphic reactions, and the range and degree of chemical equilibration during metamorphism.