HR: 11:35h
AN: V52B-06 INVITED     [Abstracts]
TI: Crystallization of and Conditions in the MSH 2004-05 Dacitic Magma as Indicated by Phenocryst Compositions and Experiments.
AU: * Rutherford, M J
EM: malcolm_rutherford@brown.edu
AF: Dept. of geological Sciences, Brown University, 324 Brook St., Providence, RI 02912 United States
AU: Devine, J D
EM:
AF: Dept. of geological Sciences, Brown University, 324 Brook St., Providence, RI 02912 United States
AB: The Mount St. Helens 2004-5 dacitic magma contains plagioclase, amphibole, low-Ca pyroxene phenocrysts that are cyclically zoned in a largely crystalline groundmass rich in plagioclase, pyroxenes, Fe-Ti oxides and glass. The magma also contains small populations of gabbroic xenoliths and some phenocrysts similar to those in 1980-86 magmas; most of these can be identified by their textures. The rim compositions of the phenocrysts are interpreted to reflect pre-eruption conditions in the magma; the magnetite and ilmenite rim compositions yield temperatures ranging from 850°C early in the eruption, to 870°C and as high as 930 to 950°C early in magma erupted early in 2005. Magma samples erupted later in 2005 yield temperatures of 870° C. The complex cyclic zoning in the 2004-5 MSH magma phenocrysts must reflect conditions and events during the pre-erupton history. The zoning in the amphiboles involves a substitution of Fe+Al+Ti for Mg+Si, suggesting a temperature as well as possibly a pressure and composition control. Plagioclase phenocrysts in the samples show a similar number (<3) cycles from An₇₀ to An₃₅. Interestingly, similar although not identical zonation cycles are present in the MSH magmas erupted in 1986 and earlier, but the zones in the first erupted 1980 magmas are less extreme. The rims of the 1980-86 amphiboles are low in Al₂O₃ (9-11 wt%) at a given Mg# compared to the rims of 2004/5 phenocrysts, consistent with the lower temperature of the latter magmas according to compositions of coexisting Fe-Ti oxides. Experiments on crushed samples of the 2004/5 magma show a similar stability field for amphibole as determined for the 1980 samples even though the new magma contains 65 wt% SiO₂, ~2% higher than the 1980 magma. However, reversal experiments from 100 to 300 MPa produce large euhedral crystals that are lower in Al than any in the cyclically zoned natural phenocrysts. We conclude that to produce the high-Al amphiboles observed, and the an-rich plagioclase zones, the phenocryst growth had to take place at even greater pressures than 300 MPa, or even more likely, an in-mingling of a mafic magma was involved in the phenocryst growth. In either case, the phenocryst-melt assemblage present just prior to eruption of the 2004/5 magma was no stable at the conditions (850-870°C; P(H₂O~140 MPa) existing in the magma storage zone, and amphibole would have tended to recrystallize.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005