Diffusion Kinetics of Selected Trivalent Ions and Hf in Garnet: Experimental Studies and Applications

扩散是一个基本的原子过程，它是固体在相对高温下各种行为的基础，因此是科学和工程中许多领域的主要兴趣主题。尽管研究时间较晚，但由于对成岩矿物扩散控制性质的模拟对造山带演化的时间尺度、构造板块俯冲速率、地球内部动力学过程等提供了重要的约束，因此对成岩矿物扩散性质的研究已引起人们的极大兴趣（例如熔融关系和岩浆生成、对流），喷发前岩浆房中熔体的停留时间尺度，具有预测火山活动等的潜力。拟议的研究重点是（a）实验测定了Sm ~（3+）、Lu ~（3+）、Y ~（3+）和Hf ~（4+）在天然石榴石中的扩散动力学性质，这些动力学性质是影响扩散过程的重要物理和化学因素的函数;（B）将这些数据应用于大范围地质作用时间尺度的定量模拟。整合这些数据与早期研究的结果，该小组对石榴石中二价（2+）阳离子的扩散特性将构成一个重要的一步，发展预测模型的扩散固体，是具有根本重要性的一般领域的固态扩散。根据样品和其他细节，实验诱导的扩散分布将通过离子探针中的西姆斯（二次离子质谱）深度分析，nanoSIMS中的步进扫描或我们最近获得的电子探针中的步进/束扫描进行分析，这特别适合于痕量元素分析。地球科学界长期以来一直需要石榴石中3+和4+阳离子对压力依赖性的实验数据，因为涉及这些阳离子的地质过程在高压下发生。即将获得的实验数据将填补这一重大空白。由于Y3+的扩散率似乎比主要的二价阳离子的扩散率明显慢（铁、镁、锰、钙），目前已有可靠数据（除Ca外），同时模拟石榴石中Y3+和二价阳离子的浓度分布，使用在本研究过程中开发的计算机程序，将对各种变质和岩浆过程的时间尺度施加严格的限制。变质岩石中石榴石的Sm-Nd和Lu-Hf定年方法已被广泛采用，但同一岩石的结果往往不一致。与Sm-Nd系统不同的是，母体核素Lu 3+和子体核素Hf 4+的扩散率之间似乎存在很大差异。这个属性和相对慢得多的Hf 4+的扩散率，我们的初步实验数据似乎表明，有有趣的影响石榴石全岩Lu-Hf等时线的发展，需要仔细考虑，并将在拟议的研究中解决，年龄和176 Lu/177 Hf初始比率，这是来自天然样品中的等时线的正确解释。此外，根据石榴石的扩散动力学性质，分析变质岩石中石榴石的Sm-Nd和Lu-Hf年龄的差异，将有助于限制天然样品中石榴石的生长速率，这是构造变质过程模拟的一个非常重要的参数。

Diffusion is a fundamental atomic process that underlies a variety of behavior of solids at relatively high temperatures, and thus is a subject of major interest in a number of fields in science and engineering. Although late in coming, there is now a major interest in the study of diffusion properties of rock-forming minerals as the modeling of diffusion-controlled properties have provided important constraints on the time scales of evolution of mountain belts, rate of subduction of tectonic plates, dynamical processes within the Earth's interior (e.g. melting relations and magma generation, convection), residence time scales of melts in magma chambers prior to eruptions that hold the potential of predicting volcanic activities etc. The proposed research is focused (a) on the experimental determination of diffusion kinetic properties of selected cations, namely Sm3+, Lu3+, Y3+ and Hf4+, in natural garnets as function of the important physical and chemical factors that influence diffusion process, and (b) applications of the data to the quantitative modeling of the time scales of a large spectrum of geological processes. Integration of these data with the results of earlier studies by the team on the diffusion properties of divalent (2+) cations in garnet would constitute a major step towards developing predictive models of diffusion in solids that is of fundamental importance to the general field of solid state diffusion. Depending on the sample and other details, the experimentally induced diffusion profiles would be analyzed by SIMS (secondary ion mass spectrometry) depth profiling in an Ion-probe, step scanning in a nanoSIMS or step/beam-scanning in our recently acquired electron microprobe, which is especially suited for trace element analysis. There has been a long standing need in the Earth sciences community for experimental data on the pressure dependence of 3+ and 4+ cations in garnets as the geological processes of interest involving these cations take place at high pressures. The experimental data to be obtained would fill this major gap. As the diffusivity of Y3+ seems to be significantly slower than those of the major divalent cations (Fe, Mg, Mn, Ca), for which reliable data are now available (except for Ca), simultaneous modeling of the concentration profiles of Y3+ and the divalent cations in garnet, using computer programs to be developed in the course of this study, would impose tight constraints on the time scales of a wide range of metamorphic and magmatic processes. Garnets in metamorphic rocks have been extensively dated by Sm-Nd and Lu-Hf geochronological systems, usually with discrepant results for the same rock. Unlike the Sm-Nd system, there seems to be a large difference between the diffusivities of parent nuclide, Lu3+, and the daughter nuclide, Hf4+. This property and the relatively much slower diffusivity of Hf4+, as our preliminary experimental data seem to suggest, have interesting implications on the development of garnet-whole rock Lu-Hf isochrons that need to be considered carefully, and would be addressed in the proposed research, for the proper interpretation of ages and 176Lu/177Hf initial ratios, which are derived from the isochrons in natural samples. Additionally, analysis of the discrepant Sm-Nd and Lu-Hf ages of garnets in metamorphic rocks in terms of the diffusion kinetic properties of the species would help constrain the growth rates of garnets in natural samples, which is a very important parameter for modeling tectono-metamorphic processes.