Diffusion is a ubiquitous process originating from random motion of particles in a liquid, solid or gas. In simple systems, such random motion leads to a net mass transfer from a region of high concentration to one of low concentration. Although it is a microscopic process, diffusion plays a critical role in numerous macroscopic phenomena ranging from nutrient transfer in the human body, dispersal of pollutants, to a variety of igneous processes, such as explosive volcanic eruptions and igneous rock formation. For example, explosive volcanic eruptions begin with bubble growth, whose rate is controlled by the diffusion of gas molecules from the melt into the bubbles. Incidentally, bubble growth in melts is similar to the daily encountered bubble growth in beer, champagne and soft drinks. The more generalized diffusion concept has also been applied to understand the mean height of continents. Igneous rock formation is the collective effect of crystal growth in magma, in which diffusion plays an important role. A natural silicate melt typically contains many major oxide components, and hence the diffusion is multicomponent diffusion, more complex than diffusion in simple systems. The goal of this research is to understand multicomponent diffusion in silicate melts, which would advance the ability to interpret and quantify rates of such processes.
One manifestation of the complications due to multicomponent diffusion is that an oxide component in silicate melts often diffuses from low concentration to high concentration, termed uphill diffusion. In the literature, diffusion in natural silicate melts is treated partially: If a component shows 'normal' diffusion behavior, meaning the flux is from high concentration to low concentration, it is quantified using the effective binary diffusion treatment. On the other hand, if a component displays uphill diffusion, researchers simply note the behavior, and then shy away from it without quantification. Such uphill diffusion is frequently encountered in experiments using silicate melts as well as in natural systems. As science advances rapidly and becomes increasingly more quantitative and predictive, it is time to confront the challenge of multi-component diffusion in natural silicate melts. The base melt composition chosen for this investigation will be similar to a mid-ocean ridge basalt, the most abundant terrestrial rock with a narrow compositional range. There are 8 major oxide components and the diffusion is described by a 7 by 7 diffusivity matrix. 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 diffusivity matrix. For verification, the obtained matrix will be used to calculate diffusion profiles in previous mineral dissolution experiments, and the calculated results will be compared with measured profiles. After verification, the diffusivity matrix will be applied to predict real 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.
