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Authors: Misiti, V.* 
Freda, C.* 
Taddeucci, J.* 
Romano, C.* 
Scarlato, P.* 
Longo, A.* 
Papale, P.* 
Poe, B.* 
Title: The effect of H2O on the viscosity of K-trachytic melts at magmatic temperatures
Issue Date: 2006
Series/Report no.: /235 (2006)
DOI: 10.1016/j.chemgeo.2006.06.007
Keywords: Viscosity
Falling sphere method
Vogel–Fulcher–Tamman equation
Subject Classification04. Solid Earth::04.08. Volcanology::04.08.02. Experimental volcanism 
04. Solid Earth::04.08. Volcanology::04.08.03. Magmas 
Abstract: Viscosity of hydrous trachytes from the Agnano Monte Spina eruption (Phlegrean Fields, Italy) has been determined at 1.0 GPa and temperatures between 1200 and 1400 °C using the falling sphere method in a piston cylinder apparatus. The H2O content in the melts ranged from 0.18 to 5.81 wt.%. These high-temperature hydrous viscosities, along with previous ones determined at low-temperature (anhydrous and hydrous) and at high-temperature (anhydrous), at 1 atm on the same melt composition, represent the only complete viscosity data set available for K-trachyticmelts, frommagmatic to volcanic conditions.Viscosity decreases with increasing temperature andwater content in the melt.At constant temperature, viscosity appears to significantly decreasewhen the first wt.% ofH2Ois added.At H2O content higher than 3 wt.% the effect of temperature on viscosity is slight. Moreover, the deviation from Arrhenian behaviour towards greater “fragility” occurs with increasing water content. We combined low- and high-temperature viscosities (also from literature) and parameterized themby the use of a modified Vogel–Fulcher–Tamman equation, which accommodates the non-Arrhenian temperature dependence ofmelt viscosity.Moreover, in order to explore the extent to which the improved knowledge of Agnano Monte Spina trachyte viscosity may affect simulation of volcanic eruption at Phlegrean Fields, we included our viscosity models in numerical simulations of magma flow and fragmentation along volcanic conduits. These simulations show that the new parameterizations (and hence the new equations) give stronger predictions in the temperature interval relevant for magmatic and eruptive processes.
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