PHASE-EQUILIBRIUM CONSTRAINTS ON THE VISCOSITY OF SILICIC MAGMAS - 1 - VOLCANIC-PLUTONIC COMPARISON

By using recently determined experimental phase equilibria we show tha t the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at similar to 10(4.5) Pa s, irrespective of th eir temperature and level of emplacement. Magmas crystallizing as gran itic plutons are not water-poor and thus not more viscous than their e xtrusive equivalents. Instead, comparison between pre-eruption magma v iscosities of extrusive silicic-intermediate and intrusive granitic ma gmas shows that the former are on average slightly more viscous. Given the typical strain rates in silicic magma chambers, magma rheological behavior is expected to be dominantly Newtonian, bubbles having a min or rheological influence at depth although exceptions can exist. Thus whether a silicic-intermediate magma is erupted or frozen at depth dep ends primarily on the rheological properties of surrounding terranes o r on external tectonic factors, but not on the rheology of the magma i tself. However, preeruptive viscosities of extrusive magmas rarely exc eed 10(6) Pa.s, which suggests that crystal-melt mushes with higher vi scosities cannot leave the magma storage regions beneath volcanoes. Th e narrow range of viscosities displayed by silicic-intermediate magmas results from both the strong control that pressure exerts on volatile solubilities in silicate melts and thermal limitations required to pr oduce acid magmas. Considerations of the relationships between magma c rystallinities, bulk SiO2, and preeruptive melt H2O contents show that the higher the melt H2O content is the higher the maximum crystallini ty that a given magma will be while still being potentially erupted. A n empirical correlation is proposed that enables us to estimate preeru ptive melt H2O contents of erupted magmas by knowing their crystallini ty and bulk SiO2.