Viscosity of andesite melts and its implication for magma mixing prior to Unzen 1991-1995 eruption

摘要

The viscosity of an iron-bearing melt with composition similar to Unzen andesite wasdetermined experimentally in the high (109-1010.5 Pa·s) and low (5-1000 Pa·s) viscosity rangeusing a parallel plate viscometer and the falling sphere method, respectively. Falling sphereexperiments were carried out in an internally heated argon pressure vessel and in a pistoncylinder apparatus at 1323 to 1573 K and 200 to 2000 MPa. Creep experiments wereperformed in the temperature range of 747 - 845 K at 300 MPa. The water content of the meltvaries from nominally dry to 6.2 wt% H2O. The Fe2+/Fetot ratio was determined for eachsample in the quenched glass using a colorimetric method. Pressure has minor influence onthe viscosity compared with the effect of temperature, water content (main compositionalparameter controlling the viscosity) or with the Fe2+/Fetot ratio (especially important at lowwater content of the melt). Based on our new viscosity data and literature data with measuredFe2+/Fetot ratio we propose a new empirical equation to estimate the viscosity η (in Pa·s) ofandesitic melts as a function of temperature T (in K), water content w (in wt%) and Fe2+/Fetotratio. The derived relationship reproduces the experimental data (87 in total) in the viscosityrange from 100.5 to 1013 Pa·s with a 1σ standard deviation of 0.17 log units. However,application of this calculation model is limited to Fe2+/Fetot>0.3 and to temperatures above Tg.Moreover, in the high viscosity range the variation of viscosity with water content isconstrained only by few experimental data and needs verification by additionalmeasurements.The viscosity data are used to interpret mixing processes in the Unzen magma chamber priorto 1991-1995 eruption. We demonstrate that the viscosities of the rhyolite and andesite meltsfrom the two end-member magmas are nearly identical prior and during mixing, enablingefficient magma mixing.