HR: 0800h
AN: V41B-0603    [Abstracts]
TI: The Origin and Significance of F-rich Amphibole Rims in 2005 Mount St. Helens Dacite and Similar Eruptions
AU: * de Jesus, S
EM: sargon_dejesus@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook St., Providence, RI 02912, United States
AU: Rutherford, M J
EM: malcolm_rutherford@brown.edu
AF: Department of Geological Sciences, Brown University, 324 Brook St., Providence, RI 02912, United States
AB: The ongoing (2004-present) eruption of dacite magma at Mount St. Helens contains amphibole along with plagioclase, low-Ca pyroxene, Ti-magnetite and ilmenite phenocrysts in a variably crystallized groundmass. Amphibole phenocrysts in the later part of the eruption, where the mass eruption rate was relatively low (1 vs. 6 cubic meters/sec), have a rim up to 40 microns thick that increases from low F (900 ppm) to 9000 ppm at the outer margin. The increase in F is accompanied by an increase in Si and Fe, and decreases in Mg and Al. These observations suggest the reaction to form the F-rich amphibole is the result of recrystallization rather than halogen diffusion exchange with pre-existing amphibole. The F- and SiO2-rich character of the overgrowth suggests that it developed as the rising dacitic magma passed beyond the stability of OH-rich amphibole in the slow erupting phase of the MSH eruption. The ability of the interstitial melt to crystallize additional amphibole beyond that in the existing phenocrysts is compositionally limited by the lack of Ca-pyroxene phenocrysts in the magma and the interstitial melt composition. It is postulated that the overgrowths was generated by decreases in pressure and abrasion-induced fining of some amphibole phenocryst material in the solidifying magma during its ascent. This model is being tested by hydrothermal experiments using finely ground with lightly crushed MSH 2005 dacite with and without additions of F.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8425 Effusive volcanism
DE: 8430 Volcanic gases
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting