HR: 0830h
AN: V51E-0321    [PDF]
TI: Natural Observations on the Link Between Syn-eruptive Degassing and Microlite Formation in Rhyolitic Obsidians
AU: Mercer, C
EM: celestine_mercer@hotmail.com
AF: University of Oregon, 1272 Geological Sciences University of Oregon, Eugene, OR 97403-1272 United States
AU: * Castro, J M
EM: jon.castro@oberlin.edu
AF: Oberlin College, Department of Geology, Oberlin, OH 44074 United States
AU: Morton, J
EM: jxmort@wm.edu
AF: College of William and Mary, Department of Geology, Williamsburg, VA 23187-8795 United States
AB: Quenched silicic melts often contain microlites interpreted to form syn-eruptively, "in response to loss of a hydrous vapor phase" (Swanson et al., 1989). Based on this link between degassing and crystallization, microlite textures have been widely used to infer the extent magmatic degassing, magma residence times, and magma rise rates. Because the growth of a microlite population depends not only on the imposed undercooling, but also the time available to form and grow nuclei, the possibility exists that two or more parcels of magma that underwent the same degree of degassing could exhibit drastically different groundmass textures, owing to different rise rates and/or residence times. Thus, inferences regarding eruption dynamics based on groundmass textures alone may not be straightforward, and some independent measure of the amount of degassing should accompany textural measurements. In this work, we measure microlite textures and corresponding volatile concentrations in pyroclastic and effusive obsidians from the Inyo volcanic chain, CA in order to determine the extent to which microlites record syn-eruptive degassing. Direct comparisons of clinopyroxene and plagioclase microlite modes, crystal size distributions, and number densities with preserved volatile contents determined by FTIR allow us to test the sensitivity of microlite textures to the degassing history and ascent dynamics of the Inyo magma. Our samples consist of 110 obsidians collected tephra and dome deposits of the 550-650 yr.b.p. eruption of the Inyo Volcanic chain. Obsidians are characterized by low bulk volatile contents (~0.13 to 1.8 wt.% H$_{2}$O) and large variations in mineralogy, mode, number density, and crystal size. Clinopyroxene is the dominant microlite phase in most dome and tephra samples. However, approximately 30% of samples also contain abundant plagioclase in the microlite assemblage. Average pyroxene and plagioclase number densities (N$_{v}$) define two general patterns when plotted against dissolved H$_{2}$O content: 1) a bell-shaped region defined by samples erupted from the Obsidian Dome vent, and 2) a relatively linear, negatively sloping envelope consisting of data from both Obsidian Dome and South Deadman vents. In the bell-shaped distribution, peak N$_{v}$ (~10$^{9}$/cm$^{3}$) occurs at H$_{2}$O contents between 0.6 and 0.7 wt.%. Microlite contents drop off by one order of magnitude on either side of this maximum. This hump-shaped distribution may record two stages of syn-eruptive crystallization: 1) an early nucleation-dominated regime characterized by a sharp increase in the nucleation rate with volatile loss (undercooling) and; 2) a later stage marked by declining nucleation rate and a transition to growth dominated behavior with decreasing dissolved H$_{2}$O. In the second array, average (N$_{v}$)increases with decreasing H$_{2}$O, although number densities are are uniformly low (~10$^{8}$/cm$^{3}$) compared to obsidians in the bell-shaped trend. Apparently, crystal nucleation did not proceed extensively, despite the large undercoolings imposed by degassing. Rapid ascent and therefore a relatively short crystallization interval prior to quenching may explain these microlite-poor obsidians. Our analysis shows that obsidians degassed to the same final values, may develop drastically different final textures and modes, owing to different rates of magma ascent.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting