In situ experimental study of rhyolitic magma degassing

It is now well established that degassing of magma en route to the surface is one of the major process driving volcanic erutions. We have experimentally simulated magma ascent from pressure (P) and temperature (T) conditions corresponding to natural magma chambers, in order to characterize volatile components (X) vesiculation during decompression. Therefore, we have developped, uded and validated two experimental tools allowing the observation of water exsolution from the melt in real time. The starting material was a synthetic hydrous glass of rhyolitic composition. We performed experiments in hydrothermal diamond anvil cells. Water vesiculation was observed from 8-12 kbar to room pressure, at 700-900°C and for 7 to 18 wt.% initially dissolved H2O in the melt. To allow temperature measurements as close as possible to the sample we have prepared and calibrated temperature sensors implanted into diamond anvils. A second set of experiments was performed in an internally heated pressure vessel equipped with transparent sapphire windows. Water degassing was characterized in situ in the sample chamber, for P-T conditions of 1-3 kbar, 700-1000°C respectively and with 4 to 7 wt.% H2O initially dissolved in the melt. Water bubbles nucleation, growth and coalescence were characterized for the studied experimental conditions. Results are presented and discussed; they are compared to the natural case.