Diffusion and Redox Reactions of Sulfur in Silicate Melts

Abstract

Although large-scale transport in magmas is usually controlled by convection, diffusion of sulfur in silicate melts, nevertheless, plays a crucial role in the kinetics of various magmatic processes. The most important process is probably the degassing of magmas and melts initiated by an oversaturation of the melt with respect to dissolved volatiles. Such oversaturation can be caused in nature, i.e., by decompression when a magma ascends to the Earth surface. But the degassing of melts is also of major interest for technical applications, e.g., during fining of melts in industrial glass production. The kinetics of volcanic eruptions and the fining of glass melts are basically controlled by the dynamics of bubble nucleation and growth, and these properties are strongly affected by the diffusion of volatiles from the melt into bubbles (Nemec 1980a,b; Sparks et al. 1994; Muller-Simon 2011, this volume). Hence, understanding the diffusion of volatiles in melts provides a necessary tool for modeling bubble nucleation and growth. However, the diffusivities of volatiles such as H2O, CO2, SO2 and H2S in the melt do not only affect the degassing rate, they may also cause a fractionation of volatiles between gas phase and melt as well, due to different diffusivities. For instance, in hydrous magmas the diffusion of H2O is usually much faster than CO2 and sulfur diffusion (Baker et al. 2005; Zhang and Ni 2010), so rapid, disequilibrium removal of bubbles from the melt may cause an artificial enrichment of some components in the bubbles. Other processes which may be governed by sulfur diffusion in the melts are the dissolution and precipitation of minerals. In glass manufacturing, crucial problems are the kinetics of dissolution of the raw materials in the melt batch and of …