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3.9 d. Vesiculation processes in a water-rich calc-alkaline obsidian (R.J. Stevenson, N.S. Bagdassarov and C. Romano)

Bubble growth is an important process that contributes to the explosivity of volcanic eruptions. The timescale for bubbles to grow in viscous silica-rich melts may range from as little as a few seconds for explosive pyroclastic eruptions, to many days or weeks for the surfaces of silicic lava flows. The effect of temperature and melt viscosity on the kinetics of bubble growth (volume expansion) was, for the first time, measured on a natural water-rich, calc-alkaline rhyolite with ~ 1.8 wt% water from Red Hills, New Mexico. Such water-rich obsidians with minor bubble contents are rare in the geological record. This glass, associated with pyroclastic deposits in the field, is interpreted to represent quenched conduit-wall material.

Shear viscosity was measured using the micropenetration method over the temperature range 450° - 590 °C, and water contents were monitored before and after viscometry by FTIR. The time-dependence of the volume increase as a result of vesiculation was determined in a vertical push-rod dilatometer during annealing times up to 300 hours. Unlike water-poor rhyolitic magmas from lava flows, there is no lag-time for the onset of vesiculation. Instead, at temperatures close to the glass transition temperature (Tg), growing bubbles appear to cause crack nucleation and fracturing of the surrounding melt, resulting in an effective mechanical weakening of the cooling magma. In magmas undergoing decompression within the upper parts of volcanic conduits, this process is therefore likely to cause melt fragmentation. At volcanologically-relevant temperatures, (e.g. 750° - 850 °C) the viscosity of the melt-phase is lower than that calculated using Shaw's method. By implication, vesiculation rates are likely to be faster than predicted - especially near fragmentation horizons. According to our experiments, fracturing can occur in the magma if the melt temperature is greater than hydrous Tg in the bulk melt and lower than anhydrous Tg around bubble walls. In addition, this process could occur in the centre of pyroclasts at lower temperatures (< 700 °C) before and after clast deposition.

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