Viscosity of a Teide phonolite in the welding interval

Abstract

The viscosity of a natural phonolitic composition with variable amounts of H2O has been experimentally determined. The starting materials were crystal-free phonolitic glasses from Montaña Blanca, situated within the Las Cañadas caldera of Teide. Dry phonolitic melt viscosities were determined using concentric cylinder viscometry in the low viscosity range. The glassy quench products of these runs were then hydrated by high pressure synthesis in a piston–cylinder apparatus to generate a suite of samples with water contents ranging from 0.02 to 3.75wt%. Samples thus hydrated were quenched rapidly and prepared (cut and polished) for the determination of water contents by infrared spectroscopy before and after experimental viscometry. The viscosities of the melts (dry and hydrated) were determined at 1bar using a micropenetration technique. Samples were stable under the measurement conditions up to 3.75 wt% H2O. Homogeneity of water content was confirmed by infrared spectroscopy and total water contents were calculated using absorptivity coefficients for compositions extremely close to that investigated here. The variation of viscosity as a function of water content and temperature can be described in the high viscosity interval of relevance to many welding processes by the non-Arrhenian expression:(1)log10η=−5.900−0.286ln(H2O)+(10775.4−394.8(H2O))/(T−148.7+21.65ln(H2O))whereas the high viscosity range alone is adequately described by the Arrhenian expression(2)log10η=−10.622−0.738ln(H2O)+(17114.3−590.4(H2O))×1/Twhere η is the viscosity in Pas, H2O is the water content in wt% and T is the temperature in K.These results are particularly useful for the scaling of conditions extant during the welding of phonolitic products of Montaña Blanca. The welding of glassy phonolitic rocks is enhanced by the lower viscosity of these melts with respect to calcalkaline rhyolites. The ratio of viscosities of phonolitic to calcalkaline rhyolitic melts is a complex function of temperature and water content and reaches up to 104.5 at 0.1wt% H2O and 500°C. Abundant evidence of welding and remobilisation of pyroclastic and spatter products of Teide system volcanism are consistent with these experimental observations.