Can spinodal decomposition occur during decompression-induced vesiculation of magma?

Abstract

Volcanic eruptions are driven by decompression-induced vesiculation of supersaturated volatile components in magma. The initial phase of this phenomenon has long been described as nucleation and growth. Recently, it was proposed that spinodal decomposition (an energetically spontaneous phase separation that does not require the formation of a distinct interface) may occur during decompression-induced magma vesiculation. This suggestion has attracted attention, but is currently based only on textural observations of decompression experiment products (e.g., independence of bubble number density on decompression rate and homogeneous spatial distribution of bubbles). In this study, I used a simple thermodynamic approach to investigate whether spinodal decomposition can occur during decompression-induced vesiculation of magma. I plotted binodal and spinodal curves on the chemical composition-pressure plane by approximating hydrous magmas under several temperature and compositional conditions as two-component symmetric regular solutions of silicate and water, using experimentally determined water solubility values. The spinodal curve was consistently much lower than the binodal curve at pressures sufficiently below the second critical endpoint. In addition, the final pressure of all decompression experiments performed to date fell between these two curves. This suggests that spinodal decomposition is unlikely to occur in the pressure range of magmatic processes in the continental crust, and that decompression-induced vesiculation results from nucleation and subsequent growth, as previously considered. Furthermore, by substituting the determined spinodal pressure into the formula of non-classical nucleation theory, the surface tension between silicate melt and bubble nucleus can be estimated.