The effect of initial H2O concentration on decompression-induced phase separation and degassing of hydrous phonolitic melt.

Supersaturation of H₂O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H₂O concentration (c_H2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts with c_H2Oini ranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s⁻¹ and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt% c_H2Oini with a logVND of ~ 5 (in mm⁻³). The logVND increases strongly towards lower and higher c_H2Oini by one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt% c_H2Oini. At lower c_H2Oini, the initial VND is preserved during further decompression up to melt porosities of 30–50%. At higher c_H2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion, c_H2Oini determines the VND of initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions. [ABSTRACT FROM AUTHOR]