Multicomponent Diffusion in Natural Silicate Melts

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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.