  Volcanic eruptions are driven by decompression-induced vesiculation of
volatiles in magma. Its initial phase has long been described as nucleation.
Recently, it was proposed that spinodal decomposition (SD; an energetically
spontaneous phase separation without forming a distinct interface) may occur
during magma vesiculation. This suggestion is currently based only on
qualitative textural observations of the products of decompression experiments.
In this study, I used a simple thermodynamic approach to quantitatively
investigate whether SD can occur during magma vesiculation. Using the previous
water solubility data, I plotted the binodal and spinodal curves on the
chemical composition-pressure plane for several hydrous magmas, treating them
as two-component symmetric regular solutions of silicate and water. The
spinodal curves were much lower than the binodal curves at pressures
sufficiently below the second critical endpoints. In addition, the final
pressure of all the decompression experiments performed to date fell between
these two curves. This suggests that SD is unlikely to occur in the pressure
range of magmatic processes in the continental crust or at realistic
decompression rates i.e. magma vesiculation results from nucleation, as
previously suggested. To test this thermodynamic approach, I estimated the
microscopic surface tension between the melt and bubble nucleus in previous
decompression experiments by substituting the spinodal pressure into the
nonclassical nucleation theory (non-CNT) equation for the dependence of the
surface tension on the degree of supersaturation. The resulting values were
significantly more scattered than those obtained by the conventional method:
inversion of the experimental bubble number density using the classical
nucleation theory (CNT) formula. Thus, it is difficult to judge whether the
non-CNT equation is appropriate for magma systems.
