Fe-Mg Interdiffusion in Olivine and Magnesiowüstite Under Hydrous Conditions: Implications for Mantle Processes

我们的实验研究旨在量化Fe-Mg在橄榄石和镁武铁矿中相互扩散动力学对水/羟基浓度的依赖性。对于许多名义上的无水矿物，少量的“水”可以显著提高许多（如果不是全部的话）与扩散相关的特性，包括蠕变速率、位错恢复动力学、电导率和离子扩散。然而，在大多数情况下，这些观察结果只是定性的。水的存在是否增强了扩散率或蠕变率？是还是不是？我们最近刚刚确定了橄榄石的扩散率和水/羟基含量（水逸度）之间的定量关系；没有镁武铁矿。为了利用实验室实验的观测结果来建立地幔的地球化学演化和地球动力学行为模型，量化动力学性质对含水量的依赖是至关重要的。因此，我们提出了一项实验室研究，以扩大我们对橄榄石中Fe-Mg互扩散率与水浓度的关系的研究，并包括镁武铁矿。在0.05至15 GPa的压力范围内，对单晶进行了高温高压扩散实验。采用卢瑟福后向散射光谱（RBS）和电子探针微量分析（EPMA）测量得到的扩散谱。采用傅里叶变换红外（FTIR）分析方法测定羟基浓度。此外，透射电子显微镜（TEM）和x射线衍射分析被用来检查是否存在类似humite的层。对于橄榄石，需要实验来确定扩散系数与温度和压力的关系，并验证我们最初测量的扩散系数与水逸度的关系。对于菱镁矿，所有这些实验都是必需的。我们实验的一个重要目标是在明确的热化学条件下确定两种重要地幔矿物的阳离子（Fe-Mg）扩散率和水/羟基浓度之间的定量关系。拟议的研究旨在利用这些结果在点缺陷热力学的背景下，获得对水影响离子扩散和电导率等输运性质的机制的基本理解。由此产生的理论框架对于将实验室结果可靠地外推到地幔环境以及利用实验观测建立地球内部动态行为模型至关重要。

KohlstedtEAR-0106981Support is requested to complete our experimental investigation designed to quantify the dependence of the kinetics of Fe-Mg interdiffusion in olivine and magnesiowustite on water/hydroxyl concentration is proposed. For many nominally anhydrous minerals, a small amount of 'water' significantly enhances many, if not all, diffusion-dependent properties including creep rate, dislocation recovery kinetics, electrical conductivity and ionic diffusion. In most cases, however, these observations are only qualitative. Does the presence of water enhance diffusivity or rate of creep ... yes or no? We have just recently determined a quantitative relationship between diffusivity and water/hydroxyl content (water fugacity) for olivine; none exists for magnesiowustite. To utilize observations determined from laboratory experiments to develop models of the geochemical evolution and geodynamical behavior of the mantle, it is critical to quantify the dependence of kinetic properties on water content. Therefore, a laboratory study is proposed to extend our investigation of the dependence of Fe-Mg interdiffusivity on water concentration in olivine and to include magnesiowustite. High-temperature, high-pressure diffusion experiments are being carried out on single crystals at controlled water fugacities at pressures in the range 0.05 to 15 GPa. Rutherford backscattering spectroscopy (RBS) and electron probe microanalysis (EPMA) are employed to measure the resulting diffusion profiles. Fourier transform infrared (FTIR) analyses are used to determine the hydroxyl concentration. In addition, transmission electron microscopy (TEM) and x-ray diffraction analyses are employed to check for the presence of humite-like layers. For olivine, experiments are needed to determine the dependence of diffusivity on temperature and pressure as well as to verify our initial measurement of the dependence of diffusivity on water fugacity. For magnesiowustite, all of these experiments are needed. An important goal of our experiments is to determine quantitative relationships between cation (Fe-Mg) diffusivity and water/hydroxyl concentration for two important mantle minerals under well-defined thermochemical conditions. The proposed research is designed to use these results in the context of point defect thermodynamics to gain a fundamental understanding of the mechanism(s) by which water influences transport properties such as ionic diffusion and electrical conductivity. The resulting theoretical framework is essential for reliably extrapolating laboratory results to mantle environments and for utilizing experimental observations to develop models of the dynamic behavior of the Earth's interior.