Alan Whittington, University of Missouri, Columbia MO 65201 (whittingtona@missouri.edu)
Magmas undergo significant changes in crystallinity and dissolved volatile content even during closed system fractionation. Key changes that must be quantitatively understood in order to model plutonic and volcanic processes include the effects of degassing (bubble formation) and crystallization on viscosity, density, and thermal budget. Recent advances in experimental technique have allowed precise viscosity and volume measurements to be made on hydrous liquids at atmospheric pressure and low temperatures (near the glass transition), where the kinetics of water exsolution are slow.
At constant temperature, liquid viscosity decreases non-linearly with increasing water content. This decrease is most marked for the first wt.% of added water, and becomes less important at higher total water contents. Combined with the non-linear decrease of water solubility with increasing pressure, this results in a dramatic viscosity increase for water-saturated magmas of any composition as they approach the EarthÕs surface, particularly during the final 3 km or so of ascent (often resulting in fragmentation). Viscosity increases on degassing are most dramatic for highly silicic liquids such as granites and rhyolites, but are also significant (³4 orders of magnitude) for intermediate liquids such as phonolites and andesites.
Dissolution calorimetry experiments on three series of hydrous glasses indicate that mixing between water and silicate liquids is near-ideal, so that exsolution of water from the melt into the vapor phase (bubbles) will result in a negligible temperature change of the magma for silicic and intermediate compositions. Crystallization during magma degassing will drive the melt towards increasingly silicic, alkalic and water-rich compositions, usually resulting in little change or even a decrease in the viscosity of the liquid . However, the presence of crystals will typically cause a slight increase in the bulk viscosity and density of the magma, while bubbles will decrease both viscosity and density. In addition, both crystals and bubbles can introduce a significant component of non-Newtonian behavior into bulk magma rheology.