EXPERIMENTAL VESICULATION AND OUTGASSING OF CRYSTAL-BEARING DACITE

Dacitic magmas have the capacity to erupt effusively or explosively. We seek a better understanding of how gas exsolution and crystallinity control effusive versus explosive eruptive behavior. This is controlled heavily by the magma’s viscosity, which is a function of temperature, crystallinity, and volatile content. Dissolved volatiles, especially water, lower the viscosity of the melt. Conversely, degassing increases melt viscosity, making it harder for exsolved bubbles to move the melt. We conducted a series of experiments to gain insight into the relationship between bubble nucleation and growth, and how these processes are affected by crystallinity at a given water content and temperature. Hydrous synthetic dacite glass samples containing 4.2 wt.% dissolved H₂O and zero, 50, 60, 70, or 80 vol % quartz crystals, were synthesized in a Hot Isostatic Press (HIP) and quenched at a final pressure at ~63 MPa, comparable to a shallow magma reservoir. Cylindrical cores, ~5 mm diameter and ~10 mm length, were heated and held at 700, 750, or 800°C, at ambient pressure (~1 atm) in a uniaxial parallel-plate viscometer, until bubble-driven expansion ceased. The matrix glass transition is ~483°C, above which temperature the samples initially expand (liquid thermal expansion), then shrink (viscous deformation), then expand again at a much faster rate (bubble nucleation and growth). Amounts of viscous deformation decreased respectively with increasing crystallinity. We are using X-ray micro-computed tomography to gather 2D and 3D data to identify nucleation sites, preexisting bubble growth, and methods of outgassing. At 750°C, crystal-free samples (F0) grew ~34%, while F50 grew ~19.3%, F60 grew ~15%, F70 grew 4.7%, and F80 grew ~3.5%. At 800°C, F0 grew 43.4%. Crystallinities, such as 50 or 60 vol %, allow for rapid expansion of the melt and may lead to brittle failure and fragmentation of the melt. However, highly crystalline magmas (i.e. 70 or 80 vol %) hinder bubble growth and may promote fracturing. These fractures can then operate as conduits for outgassing, stalling the system and reduce its explosive potential.