HR: 12:05h
AN: V42A-08    [Abstracts]
TI: Experimental Study of the Impact of Constant Versus Accelerating Decompression on Isothermal Bubble Growth in Rhyolitic and Phonolitic Melts
AU: * Malcolm, J C
EM: ftjcm1@uaf.edu
AF: University of Alaska Fairbanks, Geophysical Institute Dept. of Geology and Geophysics, Fairbanks, AK 99775
AU: Larsen, J F
EM: faust@gi.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute Dept. of Geology and Geophysics, Fairbanks, AK 99775
AB: For a constant mass influx and conduit of cross-sectional area, the rate of magma ascent must increase towards the surface as water is exsolved. In addition the confining pressure (due to the overlying magma column) felt by a "parcel" of magma does not change linearly with depth since the solubility of water in melt exsolution as a function of pressure is not linear and the exsolved gas obeys the perfect gas law. This non linear confining pressure profile will also tend to accelerate the parcel of magma towards the surface. Combined, these two effects result in the parcel of magma experiencing an accelerating decompression rate as it ascends towards the surface. This study compares the porosity and bubble size distribution resulting from a constant decompression rate with that deriving from an equivalent accelerating rate. For example, using CONFLOW, a water-saturated rhyolitic magma with an initial pressure of 150MPa, final pressure before fragmentation of 14MPa and initial ascent rate of 4.5 m/s experiences an average decompression rate of 1MPa/s. The equivalent accelerating rate is initially 0.25MPa/s and accelerates to >25MPa/s. A series of controlled decompression experiments were performed to investigate the impact of the different rate regimes on bubble-melt equilibrium for two different melts. High silica rhyolite melts saturated with water at 200MPa and 825°C were decompressed to lower pressures for constant rates of 0.1MPa/s, 0.25MPa/s,0.5MPa/s and 1MPa/s and for equivalent accelerating rates and then rapidly quenched isobarically. The experiments were repeated for phonolitic melts saturated at 200MPa and 880°C. In order to quantify the degree of disequilibrium, the developed sample porosity was compared with that calculated due to equilibrium exsolution. Further quantitative assessment of the effect of rate regime and magma composition was determined through comparison of bubble size distribution and bubble number density.
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005