Rates and Mechanisms of Intracrystalline and Intergranular Diffusion

发生在地壳深处或上地幔中的变质过程无法直接观察到，而且它们跨越的时间尺度比人类的寿命长得多，因此对它们的知识必须来自对它们产生的岩石的研究。要重建地质历史--例如，要了解一条山脉带的形成需要多长时间，它有多古老，它形成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，这些过程编码在深层次形成的矿物中，并被构造过程带到地表。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，石榴石是一种矿物，具有在化学成分中编码其历史细节的非凡能力。在生长过程中，石榴石通常会在从晶体核心到边缘的组成元素浓度上产生差异，这些差异会被扩散和原子在固体晶体结构中的运动所改变。扩散修正在不同程度上发生，反映了晶体在其生长后历史中在不同温度和压力下花费的不同时间长度。将这些扩散作用转化为有关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的扩散速度和机制的定量了解。以前的工作对石榴石中丰富的元素的扩散速度进行了可靠的测量，但为了收集高温过程的信息，对于丰度很低的痕量元素来说，需要这样的数据。因此，这项研究的第一个目标是通过分析和模拟具有特殊约束历史的石榴石晶体中从核心到边缘的浓度分布的演化来确定微量元素的扩散速率。第二个目标是开发新的理论，通过对原子在扩散传输过程中的运动进行复杂的计算机模拟(分子动力学模拟)，从根本上解释原子尺度的扩散机制。第三个目标是量化不同类型扩散的速率，即晶间扩散--即沿着晶体边界扩散，而不是在晶体内部扩散。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。在晶体与周围环境反应生成新矿物的特殊情况下，可以通过分析和模拟晶体在反应过程中外围形成的成分变化来对其进行量化。有关石榴石中微量元素扩散的新数据应该会为各种应用增加严谨性，如高温热事件和交代事件的高温加热/冷却历史和时间，Sm-ND和Lu-Hf石榴石年代学，对石榴石和副矿物之间变质平衡的理解，以及对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石榴石和其他矿物为什么不符合弹性应变扩散理论这一令人困惑的问题，从而揭示迄今为止未被认识到的控制固体扩散动力学的因素。对Al晶间扩散系数的新估计应该会将先前的结果扩展到更高的温度，显著提高我们定量处理变质反应的长度尺度和时间尺度以及变质过程中化学平衡的范围和程度的能力。

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.