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The viscosity of silicic magmas: a review from the viewpoint of experimental petrology.

Abstract : From the rheological standpoint any magma can be usefully considered as a three phases system made of a mixture of melt, crystal and bubbles of exsolved volatiles in varying proportions. The rheological behaviour of such a polyphased system will be the interplay of the dynamical response of each of these phases with respect to any applied stress. Decisive progress in the determination of the viscosity of hydrous silicate melts has been made in recent years and several empirical equations are now available. These may be valid over a rather large compositional range (e.g., Hess and Dingwell, 1996; Holtz et al., 1999) or apply to a more specific (yet widespread) group of granitic magmas (Scaillet et al., 1996 for peraluminous granites). These empirical formulations supersede the classical model of Shaw (1972), especially in the low melt water content range. In particular, these new data have shown that hydrous silicic melts are distinctly non-Arrhenian over large temperature ranges (i.e. > 1000°C) although notable exceptions do exist (ie hydrous peraluminous silicic magmas). However, granitic magmas sensu lato display fairly smaller melting/crystallization temperature intervals, typically 100-200°C, over which the temperature dependence of the melt viscosity can be considered as Arrhenian. The melt viscosity is also strongly dependent on the strain rate: at low strain rates, silicic melts display a newtonian behaviour, while at high strain rates, non-newtonian behaviour are observed, shear viscosities decreasing with increasing strain rates (Webb and Dingwell, 1990). The low strain rates prevailing in plutonic environments imply that the melt in magmas at depths obeys predominantly a Newtonian behaviour. However, magmas transported through dykes, be it from the source region or from the plumbing system of volcanoes, may well experience high strain rates at which the onset of non-newtonian behaviour is observed. Applications of the above empirical models to such cases will likely give a maximum value for the melt viscosity. The role of crystals on magma viscosity is still a subject of active research. Experiments carried out on model systems have shown that the Eintein-Roscoe equation satisfactorily reproduces the data for crystal contents < 30% in volume (e.g., Lejeune and Richet, 1995). Beyond, magma viscosities are generally observed to increase rapidly up to reaching solid behaviour at around 60% vol crystals. Phase equilibria carried out on several granitic rocks have conclusively shown that granitic magmas arrive in a nearly molten state at their emplacement level, their load of crystals barely exceeding 10% (e.g. Clemens and Wall, 1981; Scaillet et al., 1995). This indicates that during extraction and ascent, the rheology of granitic magmas is mostly controlled by the melt properties. Crystallization paths calculated from those phase equilibria have shown that the crystal content is lower than 30% during 60-80% of the crystallization interval, a result of the eutectic-like crystallization of silicic magmas. Consequently, the viscosity increase arising from the increase in crystallinity hardly exceeds 1 order of magnitude during cooling, except at near solidus conditions. Melt viscosities of silicic magmas tightly cluster at 104.5 Pa s, irrespective of the temperature, melt water contents and the plutonic or volcanic nature (pre-eruptive conditions) of the rock. This shows that the fate of silicic magmas, whether volcanic or plutonic, is not due to fundamental differences in rheological properties. The fact that there are virtually no volcanic rocks with crystal contents higher than 50%, corresponding to magma viscosities of 106 Pa s, indicates that this is a fundamental rheological barrier, beyond which silicic magmas hardly move upward. Volcanic rocks having higher crystal content may have had their viscosity lowered by the presence of bubbles, at least in the low strain rate regime. The restored fluid contents of most volcanic rocks are, however, in the range 1-5 wt.% (eg, Wallace et al., 1995; Scaillet et al., 1998), which indicates that the rheological role of bubbles at depth is probably minor. Thus, although the relative rheological role of the melt, crystals and bubble can be expected to vary significantly among magmas, a general subdivision for crustally derived magmas can be proposed as follows: melt properties will be critical in determining whether a magma is extracted from its source; the crystal content will be instrumental in controlling the fluid dynamics of a magma chamber and the eruptability of the magma; bubbles will play a fundamental role in volcanic environments, that is only at very shallow levels. Still, aside from volcanic environments, a viscosity of 104.5 Pa s can be safely used for most intents and purposes, such as in numerical simulations of the fluid dynamics at work in silicic magma chambers. Clemens JC and VJ Wall, Can Mineral, 19, 111-131, 1981. Hess KU and, DB Dingwell, Am. Mineral., 81, 1297-1300, 1996. Holtz, F, J Roux, S Ohlhorst, H Beherens, and F Schulze, Am. Mineral, 84, 27-36, 1999. Lejeune AM, and P Richet, J. Geophys. Res., 100, 4215-4229, 1995. Scaillet, B., M Pichavant, and J. Roux, J. Petrol., 36, 664-706, 1995. Scaillet, B, F Holtz, and M Pichavant, J. Geophys. Res., 101, 27691-27699, 1996. Scaillet , B, B Clemente, BW Evans, and M Pichavant, J. Geophys. Res., 95, 15695-15701, 1998. Shaw, HR., Am. J. Sci., 272, 870-893, 1972. Wallace, P, AT Anderson, and AM Davis, Nature, 377, 612-616, 1995. Webb SL and DB Dingwell, J. Geophys. Res., 95, 15695-15701, 1990.
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Submitted on : Friday, September 29, 2006 - 10:14:42 AM
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  • HAL Id : hal-00102118, version 1



Bruno Scaillet, Alan Whittington, Michel Pichavant. The viscosity of silicic magmas: a review from the viewpoint of experimental petrology.. Hutton symposium, 1999, Clermont-Ferrand, France. ⟨hal-00102118⟩



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