Viscosity of crystal-bearing melts and its implication for magma ascent

Abstract

New experiments on the viscosity of partially crystallized andesite were performed high temperature using the falling sphere methods. Because experiments with partly crystallized samples are difficult to carry out (i.e. due to high sensitivity of phase equilibria to P,T and water content), we set up a new technique to control precisely the volume fraction and the size of crystals. We simply add zircon to the melt because: a) previous study suggested that the saturation of zircon in melts of andesitic composition as a function of both temperature and composition is low, and b) easy to crush in fixed size range. Zircon-bearing magmas were synthesized at 1473 K and 300 MPa using an internally heated gas pressure vessel. All the experimental samples were then analyzed using microprobe technique. Results gave an average value of ZrO2 dissolved in the melt of about 1.6 wt %. The solubility of Zr in andesitic melt is up to two-three times higher than predicted by literature model (Watson and Harrison, 1983). Falling sphere experiments were performed using as starting material composed of dry andesitic glass, zircon crystals (15, 30 and 40 vol%) and water. The water content of the andesitic melt after experiments ranged between 0.5 and 4.08 wt%. Image analyses show that the viscosity measurements are not affected by differences in crystals shape among the samples. Falling spheres results show a viscosity 10 times higher than that of andesitic melts for samples containing 15 vol% crystals and large discrepancies from previous literature models is found in the hydrous samples. At higher vol% of crystals we did not observe any movement of the sphere. This implies that such magmas show strongly Non-Newtonian viscosity, i.e. a threshold of accelerating force needs to be passed before the sphere could move.