HR: 1330h
AN: V42B-0343    [PDF]
TI: Analysis of Silicate Melt Inclusions in Plagioclase Phenocrysts in Prehistoric Tephra $\sim$1400 Years B.P. From Augustine Volcano, Alaska.
AU: Tappen, C M
EM: ctappen@amnh.org
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 United States
AU: * Webster, J D
EM: jdw@amnh.org
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 United States
AU: Mandeville, C W
EM: cmandy@amnh.org
AF: Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192 United States
AB: Augustine volcano, located in southern Cook Inlet, Alaska, has been historically active, erupting 6 times in the last 200 years. Eruptions first began prior to 40,000 years B.P. (Beg\'{e}t and Kienle, 1992). There are a minimum of 6 prehistoric tephra layers, G (oldest), I, H, C, M and B (youngest), present on Augustine Island (Waitt et al, 1996). In this study, we analyzed glassy silicate melt inclusions in plagioclase phenocrysts from tephra layer H ($\sim$1400 years B.P.) for major and minor and some trace elements (Cl, F, S, Ba, and Sr) by electron microprobe. We use the data to determine the chemical variation of melt inclusions in specific locations within zoned plagioclase phenocrysts. Plagioclase phenocrysts (0.5 to 4 mm long) exhibit unzoned, oscillatory or patchy zoned regions. Unzoned phenocryst cores lack melt inclusions. Patchy zonation occurs in cores and is sometimes found in intermediate zones between the core and rim. Planar oscillatory zones are distinguished in BSE images by light (An$_{56-90}$) and dark (An$_{46-55}$) bands. In some phenocrysts light and dark layers differ only by 1% An. Most phenocrysts show 2-3 repeated oscillating pairs of light and dark plagioclase compositional layers. Normal and reverse zoning are apparent in phenocrysts. Rims tend to be more calcic than the cores, varying from 1-5% An. Large melt inclusions (60 to 70 $\mu$m long) are located in patchy zoned cores. Small melt inclusions (2 to 10 $\mu$m long) are located at the contact of high calcic and low calcic oscillatory layers. All melt inclusions are trapped along compositional boundaries and occur in the more calcic plagioclase. Petrography suggests that melt inclusions may have been formed by partial dissolution of a less calcic plagioclase layer. The composition of the melt inclusions are rhyolitic (71 to 75% SiO$_{2}$). The chlorine concentrations range from 3020 to 6100 ppm with the more chlorine enriched concentrations occurring in the outer rims of the phenocryst. Sr and Ba vary from 0 to 887 ppm and 0 to 1280 ppm respectively. Dissolved fluorine concentrations range from 0 to 510 ppm and show a positive correlation with Ba. S ranges from 75 to 395 ppm and shows a positive correlation with Cl. Negative correlations of SiO$_{2}$ with Cl for all melt inclusions may suggest mixing with a primitive magma and/or degassing before eruption. The chemistry of some melt inclusions in the same oscillatory zone varies dramatically. SiO$_{2}$, Na$_{2}$O, Al$_{2}$O$_{3}$, K$_{2}$O, Ba and Sr show the widest discrepancy. This may be attributed to post-entrapment diffusive reequilibration between the melt inclusion and phenocryst. \\ (J.E. Beg\'{e}t and J. Kienle, 1992, Nature 356: 701-704) (Waitt, et al.,1996, U.S.G.S. Open File Report 96-516, 44 p.)
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting