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

Mizuki Nishiwaki 

Volcanic eruptions are driven by decompression-induced vesiculation of supersaturated volatiles in magma. The initial phase has long been described as a process of nucleation and growth. Recently, it was proposed that spinodal decomposition―an energetically spontaneous phase separation that does not require a distinct interface―may occur during decompression. This idea has attracted attention, but support for it is currently limited to textural observations of experimental products, such as bubble number density independent of decompression rate and bubbles that are uniform in size and homogeneously distributed. In this study, hydrous magma was modeled as a two-component symmetric regular solution of silicate and water, allowing the spinodal curve to be determined from water solubility data coinciding with the binodal curve. In the low-pressure region from the magma chamber to the surface (≲ 300 MPa), the spinodal curve lies at very low pressures (≲ 10 MPa), and all previous laboratory decompression experiments were conducted within the nucleation region. Decompression paths, governed by the competition between decompression and water diffusion in the melt, indicate that spinodal decomposition without nucleation would require continuously very high decompression rates (tens of MPa/s), which are unlikely in natural systems where rates increase nonlinearly from deep to shallow regions. Although this simple model cannot fully capture the complexity of natural systems, the energetic framework provided by equilibrium thermodynamics remains a valid and insightful perspective for understanding magma vesiculation.