Multicomponent Diffusion in Natural Silicate Melts

扩散是一个普遍存在的基本过程，它是由原子、离子和分子在一个相中的随机运动引起的。如果存在不均匀，扩散的随机运动导致从高浓度到低浓度的净传质，以消除不均匀。虽然扩散是一个微观过程，但它在许多宏观现象中发挥着关键作用，从人体内的营养物质转移、污染物的扩散，到各种火成岩过程，如火山爆发和火成岩形成。火山爆发始于气泡的增长，气泡的速度受气体分子从熔体扩散到气泡中的控制。火成岩的形成是岩浆中晶体生长的集体效应，其中扩散起着重要作用。更广义的扩散概念也被用来理解大陆的平均海拔。这项研究的目标是了解天然硅酸盐熔体中的扩散。天然硅酸盐熔体通常含有许多主要的氧化物成分。多组分(指三个或更多组分)体系中的扩散是复杂的，因为不同的氧化物组分可能以协调的方式移动。因此，一种成分的扩散不仅受其自身的影响，还受其他成分的影响。因此，即使扩散的最终效果是均匀，但复杂的扩散可能会导致硅酸盐熔体中的某些氧化物从低浓度扩散到高浓度，称为上坡扩散。在文献中，只对天然硅酸盐熔体中的扩散进行了部分处理：如果一种组分表现出“正常”的扩散行为，即通量从高到低，则将其量化以获得有效的二元扩散系数。否则，研究人员只是简单地注意到这种向上扩散的行为，然后在没有量化的情况下回避它。这种上行扩散在实验和自然系统中经常遇到。在之前获得NSF拨款的情况下，密歇根大学张友学的研究小组研究了多组分扩散，并在过去三年中取得了显著进展。新的拨款将使继续研究和开发成为可能。将研究具有天然玄武岩和英安岩成分的熔体中的扩散。有8种主要氧化物组分，扩散用7x7扩散矩阵(49未知数)描述。具有适当组成的玻璃将被合成以形成扩散偶。将开展高温高压扩散耦合实验。淬火玻璃的成分分布将使用电子探针进行测量。利用Levenberg-MarQuardt算法对数据进行拟合，提取扩散矩阵。一种可行的假设是，天然硅酸盐熔体的扩散特征向量与温度和熔体组成无关，这一点将得到评估。获得的扩散矩阵将用于计算矿物溶解实验中的扩散剖面，并预测各种过程中在天然岩浆中的扩散，包括晶体生长和溶解、岩浆混合以及熔体包裹体与宿主矿物之间的捕获后相互作用。结果将在科学会议和教学中公布和传播。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Diffusion is a ubiquitous and fundamental process arising from the random motion of atoms, ions and molecules in a phase. If there is inhomogeneity, the random motion of diffusion results in net mass transfer from high concentration to low concentration to eliminate the inhomogeneity. Although it is a microscopic process, diffusion plays a crucial role in numerous macroscopic phenomena ranging from nutrient transfer in the human body, dispersion of pollutants, to a variety of igneous processes, such as explosive volcanic eruptions and igneous rock formation. Explosive volcanic eruptions begin with bubble growth, whose rate is controlled by the diffusion of gas molecules from the melt into the bubbles. Igneous rock formation is the collective effect of crystal growth in magma, in which diffusion plays an important role. The more generalized concept of diffusion has also been applied to understand the mean elevation of continents. The goal of this research is to understand diffusion in natural silicate melts. A natural silicate melt typically contains many major oxide components. Diffusion in a multicomponent (meaning three or more components) system is complicated because different oxide components may move in a coordinated fashion. Hence, the diffusion of one component is affected not only by itself, but also by other components. Therefore, even though the ultimate effect of diffusion is to homogenize, the complicated diffusion may lead some oxides in silicate melts to diffuse from low to high concentration, termed uphill diffusion. In the literature, diffusion in natural silicate melts was treated only partially: If a component shows "normal" diffusion behavior, meaning the flux is from high to low concentration, it is quantified to obtain the effective binary diffusivity. Otherwise, researchers simply note the uphill diffusion behavior, and then shy away from it without quantification. Such uphill diffusion is frequently encountered in experiments as well as in natural systems. With a prior NSF grant, Youxue Zhang's research group at the University of Michigan investigated multicomponent diffusion and made remarkable progress in the last three years. The new grant will enable continued research and development. Diffusion in melts with natural basalt and dacite compositions will be studied. There are 8 major oxide components and the diffusion is described by a 7 by 7 diffusion matrix (49 unknowns). Glasses with appropriate compositions will be synthesized to form diffusion couples. High-temperature and high-pressure diffusion couple experiments will be carried out. The compositional profiles in the quenched glass will be measured using an electron microprobe. The data will be fit using the Levenberg-Marquardt algorithm to extract the diffusion matrix. One working hypothesis is that the diffusion eigenvectors for natural silicate melts are independent of temperature and melt composition, which will be evaluated. The obtained diffusion matrix will be applied to calculate diffusion profiles in mineral dissolution experiments, and to predict diffusion in natural magmas during a variety of processes, including crystal growth and dissolution, magma mixing, and post-entrapment interaction between a melt inclusion and the host mineral. The results will be published and disseminated in scientific meetings and in teaching.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.