A rheological investigation of vesicular rhyolite

Abstract

Res., 50: 307-322.The rheology of vesiculating rhyolitic systems exerts a strong control on the transport of silicic magmas in the subvolcanic to volcanic environments. We present here an investigation of vesiculating and vesiculated rhyolites using dilatometric methods. This study examines the effect of vesicle content on the viscosity of a natural supercooled rhyolitic liquid with 0-70% vesicles.The experimental samples of rhyolitic glass are derived from fusion of a natural obsidian from Little Glass Butte, Oregon. Crystal-free rhyolite glasses of varying porosity were prepared by fusing obsidian powder in a Pt crucible. Differing porosities were obtained by varying the temperature (1300-1650°C) and duration (0.5-6 h) of the fusions. Cylindrical samples of the resulting vesiculated rhyolites were cored from the crucible using diamond tools and their ends were ground flat and parallel for dilatometry. The porosity of each sample was determined from Archimedean buoyancy density determinations and comparison with bubble-free rhyolite (2.331 g/cm 3, porosity= 1 -P/Po). The density of foamed samples was determined using their mass, volume and regular geometry.Viscosities were determined in the parallel plate mode at stresses of 5 × 103 to 105 Pa. The viscosimeter was calibrated using NBS 711 glass. The bubble contents were microscopically investigated using a video-reflected light system and image analysis software. Distribution functions of the size, orientation, aspect ratio and surface porosity were obtained.The viscosity of rhyolite decreases with increasing bubble content. A general relationship of the form: r/(f) = q(0)/ ( 1 + CJ), describes the effect of porosity, f (in volume fraction) on the viscosity, q, where C is a dimensionless constant (=22.4+_2.9) and logmr/(0) = 10.94+0.04 Pa s at 850°C.

Shear viscosity of rhyolite-vapor emulsions at magmatic temperatures by concentric cylinder rheometry

“…For example, in the experiments of Lejeune et al (1999) using the Gent-type apparatus typical strain values are in the range 5^30%. Likewise, in the study of Bagdassarov and Dingwell (1992) total strain was reported as several percent. From the simulations presented in Manga and Loewenberg (2001) it is noted that for Ca in the transitional regime, transient rheological e¡ects (thixotropy) are present until total strain exceeds 100%.…”

“…An inconsistency between the exploratory experimental results of Murase (1962) and the analytical results of Taylor (1932) for the dependence of suspension shear viscosity on bubble fraction (P) was noted in the geological literature by Shaw (1965) over 35 years ago. By drawing on results from experimental (Bagdassarov and Dingwell, 1992;Stein and Spera, 1992;Lejeune et al, 1999Lejeune et al, , 2000Spera and Stein, 2000), simulation (e.g., Loewenberg and Hinch, 1996;Manga et al, 1998;Manga and Loewenberg, 2001) and theoretical studies (Schowalter et al, 1968;Frankel and Acrivos, 1970) a consistent picture can be constructed. This picture enables one to estimate the relative shear viscosity of dilute and concentrated magmatic emulsions across the range of shear rates pertinent to natural systems.…”

“…Despite the common occurrence of volatile-saturated magma, there are surprisingly few laboratory studies of the rheological properties of these emulsions in the range of temperature and shear rate pertinent to magma transport and eruption (Murase, 1962;Stein and Spera, 1992;Bagdassarov and Dingwell, 1992;Lejeune et al, 1999). In this study, we present additional laboratory measurements for the shear viscosity of rhyolite meltvapor emulsions by concentric cylinder rheometry and show that a rhyolite emulsion of porosity PW0.5 o¡ers a resistance to shear £ow more than ¢ve times smaller than melt phase alone.…”

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

“…The presence of crystals and changes in crystallinity can influence viscous flow and the rheological properties of magmas (e.g. Bagdassarov and Dingwell 1992;Smith 1996;Stevenson et al 1996;Manga et al 1998;Griffiths 2000;Pistone et al 2016). However, the Rocche Rosse lava flow is generally a crystal-free glass (with some exceptions; Davì et al 2009Davì et al , 2010, thus crystal content is excluded from calculations.…”

Experimental strategies for the investigation of low temperature properties in granitic and pegmatitic melts

“…Below 1000°C quartz glass dilatometers are used in our lab employing geometries of axial cylinder compression (e.g. Bagdassarov and Dingwell, 1992) fiber elongation (Webb and Dingwell, 1989) and micropenetration of spheres . These low temperature geometries involve volume and shear components of deformation, at least during initial loading, and care must be taken to obtain time invariant, Newtonian data.…”