HR: 0800h
AN: V31B-0487    [Abstracts]
TI: Preeruptive gas in rhyolitic magma: melt inclusion evidence
AU: * Anderson, A T
EM: canderso@uchicago.edu
AF: University of Chicago, Department of Geophysical Sciences 5734 S.Ellis Ave., Chicago, Il 60637, United States
AB: Melt inclusions in Bishop quartz phenocrysts are one of three kinds: 1) wholly enclosed melt inclusions; 2) hourglass inclusions; and 3) melt pockets. Virtually all wholly enclosed melt inclusions lack bubbles, if they cooled rapidly during eruption. Hourglass inclusions are characterized by a narrow neck of glass connecting a body of glass in the interior of a phenocryst with glass at the crystal rim. For example a 17 gram relatively crystal- poor and vesicle-rich, early-erupted pumice clast (BC97 – 16F7 -12) yielded about 9000 quartz crystals and fragments in the 200 to 500 micron diameter size range (about half of the total crystal mass). Of these, 175 are whole (more than about 50 % covered with adhering glass). A random selection of 40 crystals out of the 175 whole quartz crystals contains 14 melt inclusions larger than 50 microns diameter, 33 of which are bubble free. The remaining 7 inclusions include 5 with a less than 0.1vol. % bubble (possibly of shrinkage origin), one with an 8 micron diameter bubble (2 vol. %) and one with a 45 micron diameter bubble (75 vol. % bubble). The 45 micron bubble is in a cracked inclusion and the 8 micron bubble is poorly resolved, and its host crystal is partly cracked. The point is this: there are virtually no unquestionable bubbles in wholly enclosed melt inclusions in Bishop plinian quartz phenocrysts. Yet melt inclusions 50 microns in diameter are common and big enough to enclose and trap many visible bubbles. In general the number density of bubbles is expected to increase dramatically as the size decreases. If Bishop melt contained 5 wt. % (20 vol. %) gas (Wallace et al., 1995) then a 50 micron diameter melt inclusion might contain hundreds of 5 micron diameter bubbles, for example. This is not the case. If there is 20 vol. % gas in preeruptive crystallizing magma then the gas must be contained in bubbles bigger than 50 microns. These would be difficult to trap in 50 micron diameter melt inclusions. Bubbles do occur in hourglass inclusions, however. Commonly hourglass bubbles are single bubbles some 10's of microns in diameter. The bubbles comprise 1 to 50 vol. % or so of the hourglass. Hourglass inclusions contain plausibly primary preeruptive bubbles. The size distribution of hourglass inclusions is similar to, but larger than, that of wholly enclosed melt inclusions. The volume fraction of gas in hourglass inclusions plausibly results from random partial entrapment and evolution of preeruptive bubbles. If melt inclusions can neck off because they lack an expandable bubble, then the evolved volume of hourglass bubbles divided by the sum of the volume of hourglass plus volume of melt inclusions in a pumice clast is the preeruptive trappable gas volume fraction in the magma. The low abundance of gas in hourglass inclusions contrasts with the great estimated volume fraction of gas in the magma suggesting that most of the preeruptive gas in the magma was in bubbles greater than 50 microns in diameter. The paucity of small bubbles in melt inclusions and hourglass inclusions testifies to the significance of processes such as coalescence, floatation, and heterogeneous nucleation that promote bubble enlargement.
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting