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

Experiments were performed at high temperature and pressure to determine the effective viscosity of a crystal-bearing andesite using the falling sphere method. Because viscosity experiments with partly crystallized samples are difficult to realize (i.e., due to high sensitivity of phase equilibria to P, T and water content), we have added zircon crystals to adjust precisely the volume fraction and the size of crystals in a magma. Using this approach, the anhydrous melt composition does not vary significantly with temperature and water content of the melt and, hence, the effects of crystals on effective viscosity can be worked out accurately. The investigated systems (magma analogue) were composed of an andesitic melt containing 0.5 wt% to 4.1 wt% H2O and 15 vol% to 40 vol% of zircon crystals with grain size