Diffusive Isotopic Fractionation and the Structure of Silicate Liquids

这个项目是对岩浆分子结构的研究，也被称为“硅酸盐液体”。岩浆是由地球内部的岩石熔化产生的，在其他行星内部也会形成。这项研究的目的是使用一种新的方法来确定硅酸盐液体在化学上是否与有机聚合物相似，或者更类似于液态金属。这些信息对于了解火山是如何形成的以及行星在数十亿年里是如何变化的都是必要的。该结果在陶瓷和材料工程中也有应用，并将对地质和高温材料科学做出重大贡献。该方法是利用硅酸盐液体中主要化学元素（Ca, Mg, Fe， K）的扩散同位素分馏来探测熔体结构和扩散机制。本文认为，同位素之间的区别程度直接关系到元素与铝硅酸盐熔体结构的结合，并且随着熔体结构的变化，阳离子的分馏行为会随着液体成分的变化而变化。以前的工作已经证明了同位素区分的存在，并且在涉及特定玄武岩和流纹岩成分的扩散实验中，它会导致容易测量的影响。尚未完成的是系统地改变液体成分，以阐明同位素对组成的依赖性，并评估液体组分之间的扩散耦合。这既可以用天然的液体组成来完成，也可以用更简单的系统来完成，在这些系统中，同位素效应更容易与液体结构和热力学性质的特殊性联系起来。为了解释岩浆系统中的同位素变化和更好地理解硅酸盐液体在分子水平上的动态行为，需要了解扩散诱导的分馏效应。实验方法的第二个补充方面将是使用热扩散效应来探测简单硅酸盐液体中的物种形成，并将这些结果与基于扩散实验的模型进行比较。

This project is an investigation of the molecular structure of magma, also referred to as "silicate liquid". Magma is produced by melting of rocks in the Earth's interior, and also forms inside other planets. The objective of the study is to use a new approach to determine whether silicate liquids are chemically similar to organic polymers, or instead are more similar to liquid metals. This information is needed to understand how volcanoes form and how planets change over billions of years. The results also have applications in ceramics and materials engineering, and will be a significant contribution to both geology and high temperature materials science.The approach is to use diffusive isotopic fractionation of major chemical elements (Ca, Mg, Fe, K) in silicate liquids to probe melt structure and the mechanisms of diffusion. The thesis is that the degree of discrimination between isotopes relates directly to the bonding of the elements to the alumino-silicate melt structure, and that the fractionation behavior of the cations will vary with liquid composition as melt structure changes. Previous work has demonstrated that isotopic discrimination exists, and that it causes easily measurable effects in diffusion experiments involving specific basalt and rhyolite compositions. What has not been done is to systematically vary the liquid compositions to elucidate the dependence of isotopic effects on composition, and to evaluate diffusive coupling between components of the liquids. This can be done both with natural liquid compositions, as well as on simpler systems where it will be easier to relate the isotopic effects to specifics of the liquid structure and thermodynamic properties. Diffusion-induced fractionation effects need to be understood in order to interpret isotopic variations in magmatic systems and to better understand the dynamic behavior of silicate liquids at the molecular level. A second complementary aspect of the experimental approach will be to use thermal diffusion effects to probe speciation in simple silicate liquids, and to compare these results to models based on diffusion experiments.