Experimental study of water degassing from phonolite melts: implications for volatile oversaturation during magmatic ascent

We have experimentally studied degassing of phonolite melts between 50 and 200 MPa and 825 and 850 °C. Our results indicate that decompressed phonolite melts contain equilibrium water contents regardless of decompression rate or final pressure. In addition, the total porosities of our experiments agree with predicted equilibrium values, regardless of final pressure or decompression rate. Combined observations of mean bubble diameters, porosities, and arrangement of bubbles in our experiments indicate that degassing occurs preferentially in the large bubble fraction of the population. Because of this, retardation of growth in the small bubble population occurs. This may lead to bimodal bubble size distributions in completely degassed phonolites. Our results indicate that water saturated phonolites at temperatures >800 °C can degas in equilibrium, while rhyolites follow a disequilibrium trend, at decompression rates of ∼0.5–1.0 MPa/s. Thus, hotter, water-saturated phonolitic magmas are likely to maintain equilibrium magma water contents during ascent at rates up to 20 m/s, which are typical of highly explosive eruptions. These results may apply to eruptions such as the 79 AD eruption of Vesuvius, which is the type location for Plinian-style, explosive eruptions. However, caution must be used when applying these results to lower temperature, more differentiated, and water-undersaturated phonolite magmas. If fragmentation can occur at depths of ∼2 km or greater in the conduit, our results indicate that highly explosive eruptions of phonolitic magma may occur in the absence of water oversaturation in the magma during ascent. Fast ascent rates and high shear stresses in the conduit may provide a better explanation for fragmentation in this case.