HR: 0800h
AN: V21E-0671    [Abstracts]
TI: Magma ascent rates using melt pockets in phenocrysts
AU: Anderson, A T
EM: canderso@uchicago.edu
AF: Department of Geophyical Sciences, University of Chicago, Chicago, IL 60637 United States
AU: * Liu, Y
EM: yangl@utk.edu
AF: Department of Geophyical Sciences, University of Chicago, Chicago, IL 60637 United States
AU: Wilson, C J
EM: cjn.wilson@auckland.ac.nz
AF: Geology Department, Auckland University, Auckland, PB 92019 New Zealand
AB: Assessing magma ascent rates in eruptive conduits is important for eruption dynamics. Existing approaches include amphibole breakdown (limited to ascent rates of < 2 mm/s, Rutherford and Hill 1993), mass discharge rate (Scandone and Malone 1985), and groundmass texture and microlites (Hammer and Rutherford 2002, Martel and Schmidt 2003). These approaches focus on relatively small eruptive masses of andesitic and dacitic magmas. For large rhyolitic eruptions, especially those associated with caldera collapse, the magma ascent rate is probably much larger. We present a new approach based on volatile diffusion in phenocryst-hosted melt pocketes (reentrant inclusions). Melt pocketes connect surrounding bubbly glass with phenocryst-hosted bubble-free glass through a neck that may be narrow (hourglass shape) or wide (Anderson 1991). With eruptive decompression the host melt vesiculates leading to a lowering of the concentration of volatiles in the melt (now glass). Consequently, the volatile concentration at the transition between bubble-free melt in the melt pocket and surrounding bubbly melt decreases with time. The melt at the other (remote) end of the pocket may remain constant if ascent is fast. If ascent is slow, then the melt at the remote end of the pocket will also decrease in volatile content as volatiles diffuse through the melt. To simplify the problem, we assume a cylindrical melt pocket that connects to bubbly melt at the edge of the phenocryst. We assume that volatiles at the open end of the melt pocket remain in equilibrium with gas during closed-system ascent whereby gas is retained in the foam and equilibrates through thin bubble walls. A preliminary result for the 26.5 ka Oruanui eruption at Taupo, New Zealand suggests ascent rates of 0.01 MPa/s (40 cm/s) to 0.001 MPa/s (4 cm/s) for samples from the same clast. This large range of variation partly reflects different shapes of reentrants or complex plumbing arrangements associated with multiple vents. Modeling of the variations in shapes of glassy melt pockets offers potential to more fully constrain variations in ascent rate (an inverse problem).
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005