HR: 0800h
AN: V41A-1359    [Abstracts]
TI: Bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure
AU: * Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712 United States
AB: Vulcanian eruptions and dome collapses involve disruption of cooled, degassed, and thus highly viscous, magma. Theoretical models (e.g., Toramaru, 1995) argue that in highly viscous melts, in which bubble growth is negligible, bubble nucleation should result in exponentially increasing number densities of bubbles as viscosity ($\eta$) increases. To investigate how bubbles nucleate in highly viscous melts, we performed experiments in which water-saturated, high-silica rhyolitic melts were decompressed suddenly from high pressure, held at lower pressure for various amounts of time, and then quenched rapidly. The decompressions were carried out at 500 to $700\deg$ C to achieve viscosities up to $>$10$^{8}$ Pa s. Most bubbles nucleated on faces of Fe-Ti oxide crystals, and so nucleation is heterogeneous. We found that when $\eta$ $<$ 10$^{6}$ Pa s, nucleation occurs in less than 10 seconds, producing up to $>$10$^{8}$ cm$^{-3}$ of bubbles. At similar super-saturations, nucleation takes between 10 and 60 seconds to occur when $\eta$ increases to $\sim$10$^{7}$ Pa s, but produces similar number densities of bubbles as seen at lower viscosity. The delay in nucleation increases dramatically with higher viscosity, with nucleation not occurring after 360 seconds when $\eta$ = 10$^{7.5}$ Pa s. Our preliminary results thus show that viscosity suppresses the rate of nucleation, but so far we have not found that number densities increase significantly. Continued experiments will explore how long nucleation can be suppressed at relatively high viscosities, and whether number densities exponentially rise when nucleation finally occurs. The long delay in nucleation may suggest that most disruption that occurs in eruptions of cooled or degassed magma results from expansion of pre-existing bubbles. Toramaru A., JGR, v 100, p 1913-1931, 1995.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting