Phase equilibrium constraints on the viscosity of silicic magmas: 1. Volcanic‐plutonic comparison

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Abstract
By using recently determined experimental phase equilibria we show that the viscosity of granitic magmas emplaced at upper crustal levels is approximately constant at ∼104.5 Pa s, irrespective of their temperature and level of emplacement. Magmas crystallizing as granitic plutons are not water‐poor and thus not more viscous than their extrusive equivalents. Instead, comparison between pre‐eruption magma viscosities of extrusive silicic‐intermediate and intrusive granitic magmas 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 minor rheological influence at depth although exceptions can exist. Thus whether a silicic‐intermediate magma is erupted or frozen at depth depends primarily on the rheological properties of surrounding terranes or on external tectonic factors, but not on the rheology of the magma itself. However, preemptive viscosities of extrusive magmas rarely exceed 106 Pa.s, which suggests that crystal‐melt mushes with higher viscosities cannot leave the magma storage regions beneath volcanoes. The 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 produce acid magmas. Considerations of the relationships between magma crystallinities, bulk SiO2, and preemptive melt H2O contents show that the higher the melt H2O content is the higher the maximum crystallinity that a given magma will be while still being potentially erupted. An empirical correlation is proposed that enables us to estimate preemptive melt H2O contents of erupted magmas by knowing their crystallinity and bulk SiO2.