Mizuki Nishiwaki 

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 a phase of 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 considerable attention and is currently only based on qualitative textural observations of decompression experiment products (the independence of bubble number density on decompression rate; the homogeneous spatial distribution of the bubbles). In this study, I used a simple thermodynamic approach to quantitatively investigate whether spinodal decomposition can occur during the decompression-induced vesiculation of magma. I drew the binodal and spinodal curves for each magma on the chemical composition-pressure plane by approaching hydrous magmas at several conditions of temperature and chemical composition as two-component symmetric regular solutions of silicate and water, and using experimentally determined values of water solubility in these magmas. The spinodal curve was much lower than the binodal curve for all the magmas at pressures sufficiently below the second critical endpoint (approximately less than 1000 MPa). In addition, the final pressure of all the 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 or at realistic decompression rates, and that decompression-induced magma vesiculation results from nucleation, as previously suggested. To test this thermodynamic approach, I estimated the ‘microscopic’ surface tension between the melt and the bubble nucleus in previous decompression experiments by substituting the pressure of the spinodal curve into the equation for the dependence of surface tension on the degree of supersaturation. The resulting values were significantly more scattered than those obtained by the conventional method of inversion of the experimental bubble number density using the classical nucleation theory formula. Therefore, it is difficult to judge whether the equation of surface tension as a factor of supersaturation (based on the nonclassical nucleation theory) is appropriate for magma systems. Further careful consideration should be given to both experimental and theoretical aspects of magma vesiculation.