D.R. Neuville1, A.-M. Lejeune2, Y. Linard1, R.S.J. Sparks2
1Laboratoire de Physique des Gˇomatˇriaux, CNRS-URA 734, Institut de Physique du Globe, 4 Place Jussieu, 7252 Paris Cˇdex 05, France
2Department of Geology, University of Bristol, Wills Memorial Building, QueenÕs Road, Bristol BS8 1RJ, UK
Modelling of processes involving transport of magmas is made difficult by the particularly strong variations of the viscosity of silicate melts with temperature, chemical composition and crystallinity. In order to assess the current eruption at the Soufriere Hills Volcano, Montserrat, and to understand how rheological properties change during degassing and groundmass crystallization, we have started a rheological study of the lavas recently erupted in Montserrat. These lavas consist of a very high viscosity andesitic magma (1013.2 Pa.s at 1165 K under a 7.4 N m-2 stress), containing typically less than 15% melt of rhyolitic composition.
As a first step, we have investigated the rheology of the whole rock andesite melt and the estimated rhyolitic melt phase near the glass transition range. The experiments have been performed with a creep apparatus in the interval 109 - 1014 Pa.s. Measurements show strong increases of the viscosity with time, by nearly three orders of magnitude in 900 min at 1080 K. Such a time dependence of the viscosity is only observed for iron-bearing melts, lavas recently erupted in Montserrat containing 7% FeO.
To define those viscosity variations, we have measured the viscosity of different synthetic iron-bearing melts (iron-anorthite, - diopside and -andesite) as a function of time and temperature. The results show that the magnitude of the viscosity increase varies with the iron content of the melt. At constant temperature, these variations are complex and can reach up to five orders of magnitude. Because redox changes were insignificant during the experiments, the viscosity variations are essentially due to presence of iron. The chemical effects resulting from the changing composition of the liquid phase predominate over the physical effects originating from the presence of solid inclusions. There is a great rheological similarity between synthetic and Montserrat melts which will be discussed.