Experimental investigation of homogeneous vesicle formation and growth in hydrous silicate melts during decompression

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Volcanic activity is essentially driven by dynamic degassing processes during ascent of volatile bearing magma. These processes often lead to explosive eruptions. The fundamental physico-chemical parameters that control the homogeneous formation of vesicles and their growth in a silicate melt are the initial concentration of dissolved volatiles, the supersaturation pressure (difference between the vapor pressure in the melt and the ambient pressure), the interfacial tension between melt and fluid (surface tension), the diffusivity of volatile components and the viscosity of the melt. The separation of volatiles from a homogeneous melt is associated with a complex interaction between initial formation, diffusive growth, expansion, coalescence and segregation of fluid vesicles. This phase separation is accompanied by a decrease in density and a change in the viscosity of the magma. Reliable formation and growth rates of H2O vesicles are important to improve phase separation and growth models, and to better understand dynamic degassing processes of volcanic eruptions. The aim of the project extension is the final systematic investigation of the formation and growth of H2O vesicles during continuous decompression in the simplified system SiO2-Al2O3-Na2O-K2O-H2O, starting from 200 MPa at 1123 K in hydrothermal autoclaves and from 200 MPa at 1323 K in the internally heated gas pressure vessel. Both devices are equipped with piezoceramic controlled valves for continuous decompression and allow fast cooling of the samples. The simplified SiO2-Al2O3-Na2O-K2O-H2O system is well suited for this project since H2O solubility and diffusion data as well as viscosity data, depending on the H2O content of the melts, are available over a wide pressure and temperature range. Considering experimental limitations, the expected fundamental results of this project prolongation will contribute to significantly improve future experimental studies on vesicle formation and degassing of hydrous melts with natural compositions. In addition, the expected experimental results together with computer-aided modelling will contribute substantially to a deeper understanding of magma degassing, which ultimately drives volcanic eruptions.