Diffusion-diffusive phase transformations in alkali feldspar

碱长石是一种介于钠长石（NaAlSi3O8）和钾长石（KAlSi3O8）之间的二元固溶体，是地壳中最丰富的矿物群。在压力和温度的变化过程中，它经历了几次相变，产生了带有遗传信息的微观结构，如孪晶、编织结构或脱溶层。碱长石在高温下表现出连续的混相。在约600℃以下，一个混相间隙打开。在从超溶剂温度冷却的过程中，中间碱长石溶解形成富钠和富钾片层的共生体，称为perhite。随着时间的推移，薄片经历粗化和增加化学分离，这是时间和温度相关的，并已校准应用于地质速度测定。到目前为止，从perthites重建热历史的准确性是有限的，因为校准主要是基于exsolution实验，它整合了所有潜在的过程，包括Na-K扩散，新相边界的形成和相干应力的演化。为了改进析出液的定量，这些过程中的每一个都必须独立校准，并且必须理解它们的耦合。我们将利用一种新的示踪剂/互扩散技术，基于Na-K和K示踪剂扩散系数的测定，包括温度、成分、方向和应变依赖，开发Na-K-互扩散模型。这将由分子动力学和蒙特卡罗模拟中使用的缺陷和迁移能的从头计算来补充。最后进行了脱溶实验，并利用原子探针层析成像技术对纳米尺度的脱溶薄片进行了分析。这将使我们第一次能够直接测定碱长石中的相干溶剂和相互扩散。广泛的理论、实验和分析方法的结合使该项目独一无二，并将允许基于碱长石中扩散和扩散相变的大地测速应用的实质性改进。参与的研究人员联盟结合了所有相关领域的专业知识，包括矿物学的见解和热力学和扩散相变的背景（R. Abart），原子建模（C. delago），扩散理论，实验和蒙特卡罗模拟（S. Divinski）。

Alkali feldspar, a binary solid-solution between albite (NaAlSi3O8) and K-feldspar (KAlSi3O8), pertains to the most abundant mineral group in the Earth’s crust. During changes in pressure and temperature it undergoes several phase transformations producing microstructures such as twinning, braid texture, or exsolution lamellas, which bear genetic information. At high temperatures alkali feldspar shows continuous miscibility. Below about 600°C a miscibility gap opens. During cooling from super-solvus temperatures, intermediate alkali feldspar exsolves forming an intergrowth of Na-rich and K-rich lamellas, known as perthite. With time, the lamellas undergo coarsening and increasing chemical separation, which are both time- and temperature dependent and have been calibrated for application in geo-speedometry.So far, the accuracy at which thermal histories can be reconstructed from perthites is limited, because calibration is largely based on exsolution experiments, which integrate over all underlying processes including Na-K diffusion, formation of new phase boundaries, and evolution of coherency stress. For an improved quantification of exsolution each of these processes must be calibrated independently, and their coupling must be understood. We will develop a model for Na-K-interdiffusion based on the determination of Na- and K tracer diffusion coefficients including temperature-, composition-, direction-, and strain-dependence using a novel tracer/interdiffusion technique. This will be complemented by ab-initio calculations of defect- and migration energies to be used in molecular dynamics and Monte Carlo simulations. Finally, exsolution experiments will be done and the nm scale exsolution lamellas will be analyzed using atom probe tomography. This will allow, for the first time, direct determination of the coherent solvus and of interdiffusion in strained alkali feldspar.The combination of the broad spectrum of theoretical, experimental and analytical methods make this project unique and will allow for substantial improvement of geo-speedometry applications based on diffusion and diffusive phase transformations in alkali feldspar. The consortium of researchers involved combines expertise in all relevant fields including the mineralogical insight and background in thermodynamics and diffusive phase transformations (R. Abart), in atomistic modeling (C. Dellago), diffusion theory, experimentation, and Monte Carlo simulation (S. Divinski).