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

扩散是一种基本的原子过程，它是固体在相对较高温度下各种行为的基础，因此是科学和工程中许多领域主要感兴趣的主题。由于扩散控制性质的模拟对山脉带演化的时间尺度、构造板块的俯冲速率、地球内部的动力学过程(例如熔融关系和岩浆生成、对流)、喷发前熔体在岩浆室中的停留时间尺度具有预测火山活动的潜力等，目前对造岩矿物扩散性质的研究现在引起了极大的兴趣。这些数据包括：(A)由于影响扩散过程的重要物理和化学因素的作用，在天然石榴石中的分布；(B)数据在对大范围地质过程的时间尺度进行定量建模方面的应用。将这些数据与该团队早期对石榴石中二价阳离子扩散性质的研究结果相结合，将构成朝着开发对固体扩散的一般领域至关重要的固体扩散预测模型迈出的重要一步。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SIMS(二次离子质谱仪)深度剖析、纳米SIMS中的阶跃扫描或我们最近获得的电子探针中的阶跃/束流扫描来分析，这特别适合于痕量元素分析。长期以来，地球科学界一直需要关于石榴石中3和4个阳离子的压力依赖性的实验数据，因为涉及这些阳离子的地质过程是在高压下发生的。将要获得的实验数据将填补这一重大空白。由于Y3的扩散速度似乎明显慢于主要的二价阳离子(Fe、Mg、Mn、Ca)，目前已有可靠的数据(除Ca外)，因此利用在本研究过程中开发的计算机程序同时模拟Y3和石榴石中的二价阳离子的浓度分布，将对广泛的变质和岩浆作用的时间尺度施加严格的限制。变质岩中的石榴石在Sm-ND和Lu-Hf年代学系统中得到了广泛的测年，但对同一岩石通常结果不同。与Sm-ND系统不同，母体核素Lu3和子体核素Hf4的扩散系数似乎有很大的差异。我们的初步实验数据似乎表明，这一性质和Hf4相对较慢的扩散系数对石榴石-全岩Lu-Hf等时线的发展具有有趣的影响，需要仔细考虑，并将在拟议的研究中讨论，以正确解释年龄和176Lu/177Hf初始比值，这些比值是从自然样品中的等时线得出的。此外，对变质岩中石榴石的Sm-ND和Lu-Hf年龄差异的分析将有助于限制天然样品中石榴石的生长速度，这是模拟构造变质过程的一个重要参数。

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.