HR: 1340h
AN: V53D-1598    [Abstracts]
TI: Plagioclase in Dacites of the Current Mt. St. Helens Eruption: Constraints for Magma Origin and Dynamics
AU: Streck, M J
EM: streckm@pdx.edu
AF: Department of Geology, Portland State University, 17 Cramer Hall, Portland, OR 97207
AU: Broderick, C
EM: cindyb@pdx.edu
AF: Department of Geology, Portland State University, 17 Cramer Hall, Portland, OR 97207
AU: * Thornber, C R
EM: cthornber@usgs.gov
AF: Cascades Volcano Observatory, US Geological Survey, 1300 SE Cardinal Ct. Bldg 10 Suite 100, Vancouver, WA 98683
AU: Clynne, M A
EM: mclynne@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd MS910, Menlo Park, CA 94025
AU: Pallister, J S
EM: jpallist@usgs.gov
AF: Cascades Volcano Observatory, US Geological Survey, 1300 SE Cardinal Ct. Bldg 10 Suite 100, Vancouver, WA 98683
AB: The current, now year-long eruption of Mt. St. Helens is remarkable for a variety of reasons. Nearly solid, gas-poor dacite lava is extruding at the surface. Earthquakes are limited to the upper 3 km with most occurring less than 1 km below the surface; no deeper seismicity has been observed. Emissions of SO₂, H₂S, CO₂ are extremely low, suggesting eruption of degassed magma. No direct evidence for mingling of more mafic magma has been observed. We investigated plagioclase crystals in 2004-5 dacite and 1980-86 dacite dome rocks to shed light on the nature and origin of the current magma. Polarized light and Nomarski microscopy in combination with detailed microprobe traverses were used to characterize single plagioclase crystals, focusing on areas near their rims. In addition, we mapped all plagioclase crystals within an area of ~1/3 of each thin section and grouped them according to easily distinguishable mineral features. Although labor-intensive, this new approach was devised to generate data statistically significant enough to compare plagioclase crystal populations of crystal-rich samples. Mappable features within crystals include: i) zones of acicular opx microlitic inclusions (commonly occurring near the rim), ii) resorption surfaces, iii) spatial association of i) and ii). Our key results from the crystal mapping indicate that sequential samples of 2004/5 have remained comparable, but are distinct from 1980-86 dacite dome samples. We interpret this result as strong evidence that the 2004-5 dacite is new magma and not 'left-over' 1980's dacite. The most interesting result from combining compositional profiling with texture is that acicular opx inclusions occur at the lowest observed An content of 40-33 regardless of position within crystal and that overgrowth on resorption surfaces are typically 10-20 mol% higher in An than within opx-rich zones resulting in single to multiple oscillations of An content towards the rim. These features could possibly be explained by fluctuations that occur during ascent when dacite cools due to contact with cooler country rock initiating crystallization to produce lower An and acicular opx, whereas entrainment in hotter, more interior dacite causes resorption and reduction in crystallinity and raises the Ca/Na of the liquid. Overall, our data are compatible with recharge of new dacite magma as driving force for the eruption to date.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005