THE VISCOSITY OF HYDROUS DACITIC LIQUIDS: IMPLICATIONS FOR THE RHEOLOGY OF EVOLVING SILICIC MAGMAS

The viscosity of a series of six synthetic dacitic liquids, containing up to 5.04 wt.% dissolved water, was measured by parallel-plate viscometry. The temperature of the 10¹¹ Pa.s isokom decreases from 792˚C for the anhydrous liquid, to 591˚C and 407˚C for water contents of 0.97 and 5.04 wt.% H₂O. Including additional measurements at high temperatures by concentric-cylinder and falling-sphere viscometry, the viscosity (η) can be expressed as a function of temperature and water content w according to:

log₁₀ η = -4.43 + (7618.3 – 17.25 log₁₀ (w + 0.26)) / (T – (406.1 – 292.6 log₁₀ (w + 0.26)))

where η is in Pa.s, T is temperature in K, and w is in weight percent. This parameterization reproduces the 76 viscosity data with a root-mean square deviation (RMSD) of 0.16 log units in viscosity, or 7.8 K in temperature.

The measurements show that water decreases the viscosity of the dacitic liquids more than for andesitic liquids, but less than for rhyolites. At low temperatures and high water contents, andesitic liquids are more viscous than the dacitic liquids, which are in turn more viscous than rhyolitic liquids, reversing the trend seen for high temperatures and low water contents. This suggests that the relative viscosity of different melts depends on temperature and water content as much as on bulk melt composition and structure. At magmatic temperatures, rhyolites are orders of magnitude more viscous than dacites, which are slightly more viscous than andesites. During degassing, all three liquids undergo a rapid viscosity increase at low water contents, and both dacitic and andesitic liquids will degas much more efficiently than rhyolitic liquids.

During cooling and differentiation, changing melt chemistry, decreasing temperature and increasing crystal content all lead to increases in the viscosity of magma (melt plus crystals). Under closed system conditions, where melt water content can increase during crystallization, viscosity increases may be small. Conversely, viscosity increases are very abrupt during ascent and degassing-induced crystallization. Crystallized magmas of similar bulk composition may have undergone very different rheological histories during their formation, depending on their temperature-crystallinity-water content path, and on the evolution of the residual liquid composition.