HR: 11:50h
AN: V51L-07    [PDF]
TI: Experimental Temperature-X(H$_{2}$O)-Viscosity Relationship for Peraluminous Leucogranites, and Comparison With Synthetic Silicic Liquids
AU: * Whittington, A
EM: whittingtona@missouri.edu
AF: University of Missouri-Columbia, Department of Geological Sciences, Columbia, MO 65211-1380 United States
AU: Richet, P
EM: richet@ipgp.jussieu.fr
AF: Institut de Physique du Globe, Laboratoire de Physique des Geomat\'{e}riaux, 4 place Jussieu, Paris, 75252 France
AU: Behrens, H
EM: h.behrens@mbox.mineralogie.uni-hannover.de
AF: Institut fuer Mineralogie, Universitaet Hannover, Welfengarten 1, Hannover, D-30167 Germany
AU: Holtz, F
EM: f.holtz@mbox.mineralogie.uni-hannover.de
AF: Institut fuer Mineralogie, Universitaet Hannover, Welfengarten 1, Hannover, D-30167 Germany
AU: Scaillet, B
EM: bscaille@cnrs-orleans.fr
AF: Institut des Sciences de la Terre d'Orl\'{e}ans, CRSCM-CNRS, 1A rue de la F\'{e}rollerie, Orl\'{e}ans, 45071 France
AB: Viscosities of liquid albite (NaAlSi$_{3}$O$_{8}$) and a peraluminous Himalayan leucogranite have been measured between 10$^{9.0}$ and 10$^{13.8}$ Pa.s for water contents between 0 and 3.4 wt.%. This leucogranite is the first naturally occurring liquid composition to be investigated over the full range of T-X(H$_{2}$O) conditions that may be encountered in both plutonic and volcanic settings (rhyolites of near-identical composition are known from Peru).\\ At typical magmatic temperatures of 750 $\deg$C, the viscosity of the leucogranite is 10$^{11.0}$ Pa.s for the anhydrous liquid, dropping to 10$^{6.5}$ Pa.s for a water content of 3 wt.% H$_{2}$O. For the same temperature, the viscosity of liquid NaAlSi$_{3}$O$_{8}$ is reduced from 10$^{12.2}$ to 10$^{6.3}$ Pa.s by the addition of 1.9 wt.% H$_{2}$O. Combined with published high-temperature viscosity data, these results indicate that water reduces the viscosity of NaAlSi$_{3}$O$_{8}$ liquids to a much greater degree than that of natural granitic liquids. Furthermore, the viscosity of NaAlSi$_{3}$O$_{8}$ liquid becomes substantially non-Arrhenian at water contents as low as 1 wt.% H$_{2}$O, while that of the leucogranite appears to be near-Arrhenian to at least 3 wt.% H$_{2}$O. Therefore the viscosity of hydrous NaAlSi$_{3}$O$_{8}$ liquid does not provide a good model for natural granitic or rhyolitic liquids, especially at lower temperatures and water contents.\\ Qualitatively, the differences can be explained in terms of configurational entropy theory because addition of water should lead to higher entropies of mixing in simple model compositions than in complex natural compositions. This hypothesis also explains why the water reduces magma viscosity to a larger degree at low temperatures, and is consistent with published viscosity data for hydrous liquid compositions ranging from NaAlSi$_{3}$O$_{8}$ and synthetic haplogranites to natural samples. Therefore predictive models of magma viscosity need to account for compositional variations in more detail than via simple approximations of the degree of polymerization of the melt structure.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting