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Supersaturation of H 2 O 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 H 2 O 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 log VND  of ~ 5 (in mm −3 ). The log VND  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.

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