Nb-Ta Diffusion in Rutile: Kinetic Fractionation of Geochemical Twins

在太阳系的行星和卫星中，地球是独一无二的，它有一个高度分化的大陆地壳。这种地壳的形成需要一个复杂的过程，包括岩石熔融和岩浆结晶的几个阶段。地球分异的模型是由地壳和地幔岩石中的元素丰度来指导的。一般的假设是，矿物和熔体在所有这些熔化和结晶过程中的元素浓度是完全平衡的。矿物和熔体之间的平衡浓度比已经在实验室中通过实验确定，并且通常用于模拟壳幔分异。铌（Nb）和钽（Ta）元素对被认为是区分地壳分异中不同熔融状态的关键，但已发表的平衡模型在很大程度上未能再现所观察到的Nb-Ta分布模式。在为这一提议做准备的一系列实验中，Marschall博士和他的合作者研究了Nb和Ta在金红石（二氧化钛）中的扩散。金红石是许多岩石中Nb和Ta的主要矿物载体。他们的实验明确地表明，Nb在金红石中的扩散速度明显快于Ta，并且他们表明Nb和Ta可能在地壳熔融过程中通过扩散分离。以天然岩石中金红石的粒度范围和地壳岩石熔融所涉及的温度和时间尺度为例，论证了Nb-Ta平衡是不可能的。研究结果表明，低Nb/Ta比记录的大陆地壳是部分？而不是完整的金红石与熔体的平衡，以及Nb-Ta元素对在地壳分异过程中的平衡熔融假设必须放弃，本文的目的是详细确定Nb和Ta在金红石中的扩散。这些数据将适用于地球化学建模和确定地壳深部地质过程的时间尺度。该研究阐述了表征Nb和Ta在金红石中的扩散作为各种参数的函数，如温度和金红石的微量元素组成。这将允许识别的机制，纳入和扩散的Nb和Ta在金红石，它将量化的参数空间的平衡与不平衡熔化在自然系统中。后者将用于发展和改进地壳分异的地球化学模型，除了地质界对金红石矿物日益感兴趣之外，材料科学家对金红石中微量元素掺入和扩散机制的定量数据的需求也越来越大。金红石的技术应用包括水的光催化水解装置和光伏装置，这些都是基于金红石的半导体特性。然而，它在这些电器中的性能仍然受到纯金红石的低电导率和再现所需性能的困难的阻碍。用Nb掺杂金红石可以将其变成半导体甚至准金属导体。然而，对于Nb掺杂的金红石半导体的工业应用，必须确定Nb在金红石中的扩散率作为温度、氧压和杂质浓度的函数。这些数据是技术加工所必需的，以便生产具有明确组成和所需性能的均匀材料。本研究旨在提供这些数据。

The Earth is unique among the planets and moons of the solar system in having a highly differentiated continental crust. The formation of this crust required a complex process including several stages of melting of rocks and crystallization of magmas. Models for the differentiation of the Earth are guided by the abundances of elements in crustal and mantle rocks. The general assumption is that minerals and melts are in complete equilibrium with respect to their element concentrations in all of these melting and crystallization processes. Equilibrium concentration ratios between minerals and melts have been determined experimentally in the laboratory and are conventionally used to model crust-mantle differentiation. The element pair niobium (Nb) and tantalum (Ta) has been identified as critical for the distinction of different melting regimes involved in crustal differentiation, but published equilibrium models have largely failed to reproduce the observed Nb-Ta distribution patterns.In a series of experiments in preparation for this proposal, Dr. Marschall and his collaborators have investigated the diffusion of Nb and Ta in rutile (titanium dioxide). Rutile is the dominant mineral host of Nb and Ta in many rocks. Their experiments unequivocally demonstrate that Nb diffuses significantly faster in rutile than Ta, and they demonstrate that Nb and Ta may be separated by diffusion during crustal melting processes. Taking the range of grain sizes of rutile in natural rocks and the temperatures and time scales involved in the melting of crustal rocks, it is demonstrated that Nb-Ta equilibrium cannot be expected. The findings suggest that the low Nb/Ta ratio documented for the continental crust is the results of partial?as opposed to complete?equilibration of rutile and melt, and that the assumption of equilibrium melting has to be abandoned for the element pair Nb-Ta in the processes of crustal differentiation.The goal of this study is a detailed determination of the diffusion of Nb and Ta in rutile. This data will be applicable to geochemical modeling and for the determination of time scales of geological processes operating deep in the Earth crust. The study sets out to characterize Nb and Ta diffusion in rutile as a function of various parameters, such as temperature, and the minor element composition of rutile. This will allow the identification of the mechanisms of incorporation and of diffusion of Nb and Ta in rutile, and it will quantify the parameter space for equilibrium vs. disequilibrium melting in natural systems. The latter will be employed to develop and refine the geochemical models for crustal differentiation.Apart from the growing interest of the geological community in the mineral rutile, there is an even greater demand by material scientists for quantitative data on trace-element incorporation and diffusion mechanisms in rutile. Technical applications of rutile include devices for the photocatalytical hydrolysis of water and photovoltaic installations, which are based on the semiconducting properties of rutile. However, its performance in these appliances is still hindered by the low electrical conductivity of pure rutile, and the difficulties of reproducing the desired properties. Doping of rutile with Nb can turn it into a semiconductor or even a quasi-metallic conductor. Nevertheless, for an industrial application of Nb-doped rutile semiconductors, it is essential to determine the diffusivities of Nb in rutile as a function of temperature, oxygen pressure and concentration of impurities. These data are needed for the technical processing in order to produce homogeneous materials with a well-defined composition and the desired properties. This study sets out to provide these data.