The eruptive behavior of volcanic systems is governed by the mechanism of H2O fluid phase separation from a supersaturated hydrous melt during magma ascent. The vesicle number density (VND mm-3) serves as a standard parameter for quantifying the efficiency of fluid-melt separation. Nucleation driven vesicle formation increases the VND significantly with decompression rate, making it a potential parameter for estimating magma ascent velocity. However, recent studies have suggested the occurrence of spinodal decomposition in hydrous alkali-rich melts, characterized by a decompression rate-independence of VND.

In this study, sodium-rich phonolite melts were hydrated and decompressed in an internally heated pressure vessel under superliquidus temperatures and 200 MPa. The experiments were conducted under H2O saturation and slight undersaturation conditions prior to decompression. The hydrous melts were continuously decompressed at rates of 0.064–1.7 MPa/s until final pressures ranging from 80–30 MPa were achieved.

The results reveal consistently high logVNDs of ~5.5 for the initially formed vesicle population, regardless of decompression rate, with uniform vesicle sizes. These findings align with the principles of spinodal decomposition. However, following the onset of coalescence, the samples exhibit larger vesicle diameters up to ~500 µm and significantly reduced VNDs, depending on decompression rate and final pressure. The lowest logVNDs of 0.5–0.8 occur at the slowest decompression rate, while the highest logVNDs of 3.1–3.7 are observed at the fastest decompression rate.

This coalescence-driven VND trend mimics the decompression rate-dependence expected for vesicle nucleation, suggesting a transition in vesiculation dynamics. The rapid shift from a high VND that is decompression rate-independent to a decompression rate-dependent coalescence-dominated stage with markedly lower VNDs have significant implications for eruption dynamics. This transition may reflect a shift from a closed system with explosive eruption potential to an open system capable of efficient outgassing, possibly resulting in an effusive eruptive style.