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3.7 e. The effect of water on the viscosity of natural silicic magmas (N.S. Bagdassarov, R.J. Stevenson, D.B. Dingwell and C. Romano)

Water, as the major volatile in volcanic systems, has a significant effect on the rheology of melts. Viscosity was determined on six microlite-free rhyolitic melts using both natural (water-bearing) and remelted samples fused at 1650 °C. Both parallel-plate and micropenetration methods were employed, with the aim of documenting the effect of water on the temperature-dependence of viscosity. This is important as the Shaw method to calculate the viscosity of melts containing water tends to underestimate the effect of water on viscosity at low water contents (see 3.7b).

Four of the samples were pristine obsidians which originated from calc-alkaline lava flows and contained < 0.5 wt% water. In addition, experiments were performed on a peraluminous macusanite pebble (0.4 wt% water) and a rare natural glass with 1.8 wt% water associated with pyroclastic deposits. Glass compositions (ICP-AES and electron microprobe) and water contents (FTIR) were analysed before and after viscometry experiments, and were unchanged within error. Furthermore, the normalised chemical compositons of remelted and natural glasses, especially for the calc-alkaline samples, were unchanged within error, indicating that the presence or absence of water was the only variable influencing any changes in viscosity or activation energy of viscous flow with reference to the remelted samples.

Results suggest that activation energies of viscous flow, and viscosities, decrease on the addition of small amounts of water (e.g. as little as 0.1 wt%) and more so than Shaw's widely used method would predict. In addition, for equivalent water contents, there is a greater change in temperature (ΔT at constant viscosity) and/or change in viscosity (Δη at constant temperature) for calc-alkaline versus peralkaline rhyolitic melts.

These data suggest that the more peralkaline, more depolymerised melt structure of peralkaline rhyolites results in water having a smaller influence on apparent viscosity. The volcanological implication of this result is that water can diffuse and degas, and vesiculation can occur more readily in peralkaline melts, thus contributing to less-violent eruptive styles than for calc-alkaline magmas.

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