Obsidian-H₂O viscosities at 1000 and 2000 bars in the temperature range 700° to 900°C

Abstract

Viscosities of synthetically hydrated obsidians have been measured by the falling‐sphere technique at two values of the H2O content. At 4.3 per cent H2O, by weight, the common logarithm of the viscosity in poises ranges from 6.51 at 800°C to 5.88 at 900°C, and at 6.2 per cent H2O from 6.54 at 700°C to 5.25 at 850°C. The effect of pressure on the viscosity is shown experimentally to be slight at these temperatures. The data are consistent with viscosity isotherms determined by Friedman, Long, and Smith on rhyolite glass at H2O contents to 1.25 per cent. Activation energies and pre‐exponential constants determined from this work and that of Friedman et al. show striking similarities to data for binary alkali oxide‐silica systems. This suggests that the discrete ion theory of silicate melts proposed by Bockris and co‐workers is applicable to silicate‐H2O systems. On the basis of this model a generalized graph is presented which permits approximate predictions of viscosities of framework silicate‐H2O systems for H2O contents up to about 10 per cent by weight and temperatures in the range 500° to 1200°C.

Thermodynamic and rheological properties of rhyolite and andesite melts

“…6 translates to temperature differences of less than 40 K. It is well known that the viscosity of molten SiOz and alkali feldspar compositions vary very strongly with minor composition differences (e.g., Leko 1979). In the same way, addition of 1 wt.% H20 has been found to reduce the viscosity of an initially anhydrous obsidian by two orders of magnitude (Shaw 1963). As noted, however, no significant differences in chemical composition could be detected by electron microprobe analyses, and the same very low water content was found for samples A and B.…”

“…Finally, it is well known that water is a component of rhyolite melts which can exert a profound influence on their theological properties at a few weight percent level (Shaw 1963). Its influence on the thermodynamic properties is largely unknown and discussion of this role is beyond the scope of this work since we do not present any new data relevant to this problem.…”

A compositionally zoned ash-flow sheet in southern Nevada

“…The lowest liquidus temperature for the quartz latites, about 860° C, and the greatest equilibrium w.ater content, about 3 percent by weight, are obtained when water-vapor pressure equals lithosta'tic pressure. The experimental determinations of Shaw (1963) indicate that rhyolitic obsidians should have viscosities on the order of 10 7 poises at this temperature and water content. If li'thostatic pressure were greater than water pressure, the liquidus temperature would be higher and the equilibrium water content lower (Kennedy, 1955, fig.…”

Conditions for detection of ground deformation induced by conduit flow and evolution

“…Both mechanisms may induce a strong increase in magma viscosity of several orders of magnitude [Shaw, 1963;Hess and Dingwell, 1996;Llewellin and Manga, 2005]. For example, the viscosity of an andesitic magma at 900°C containing 5 wt% is around 1 10 4 Pa.s, but can reach values of 10 11 Pa.s if the magma is fully degassed [Sparks, 1997].…”