Bubble Genesis: Experimental Investigation of Vesicle Formation and Evolution in Hydrous Phonolitic Melts

The eruption behavior of volcanic systems is governed by the separation of an H₂O fluid phase from a supersaturated hydrous melt during magma ascent. Vesicle number density (VND, mm^-3) is a key parameter used to quantify the efficiency of this fluid-melt separation. While VND typically increases with decompression rate due to nucleation-driven vesicle formation—making it a potential proxy for magma ascent velocity—recent studies suggest that spinodal decomposition, which produces decompression rate-independent VND, may dominate in hydrous alkali-rich melts.

We conducted super liquidus decompression experiments on phonolitic melts under H₂O-saturated and undersaturated conditions, across decompression rates of 0.064–1.7 MPa/s. The samples show consistently high initial logVNDs of ~5.5, regardless of decompression rate, with uniform vesicle diameters of 2–8 µm, supporting spinodal decomposition as the vesicle formation mechanism. As vesicle coalescence begins and progresses, vesicle sizes increase up to ~500 µm and VND decreases significantly, becoming dependent on decompression rate and final pressure. The lowest logVNDs of 0.5–0.8 occur at the slowest rates, while the highest coalesced VNDs with 3.1–3.7 are observed at the fastest rates.

This transition—from a high-VND, decompression rate-independent regime to a coalescence-driven, rate-dependent regime—marks a critical shift in vesiculation dynamics. It reflects a development from a closed, potentially explosive system to a more open, outgassing system favoring effusive eruption. These findings offer vital insights into how degassing processes may influence eruption style and contribute to improved volcanic hazard assessment.