Self-Diffusion of Silicon and Oxygen in Silicate Melts: An Experimental Study

The rate of diffusive transport of Si and O in silicate melts is important not only in understanding kinetics of magmatic processes but also in studying the structure of silicate melts. The coefficients of self-diffusion of Si and O were determined experimentally in two simple silicate melts of geologic interest: jadeite composition; NaAlSi₂O₆, and diopside composition, CaMgSi_2O. These melts were selected because they represent: (1) contrasting degrees of polymerization, i.e., jadeite melt is highly polymerized, whereas diopside melt is de-polymerized [1], (2) extremes in the spectrum of viscosities, i.e., jadeite melt at the high viscosity end and diopside melt at the low viscosity end [2], and (3) contrasting behavior in terms of their pressure-induced variations in viscosities, i.e., jadeite melt shows a decrease of viscosity with increasing pressure [3], whereas diopside melt shows an increase of viscosity with increasing pressure [2].