HR: 1400h
AN: V23B-03    [Abstracts]
TI: Variations in Argon Isotopes From Plagioclase and Hornblende Phenocrysts at El Chichon Volcano
AU: * Jones, D A
EM: rockjones73@yahoo.com
AF: Geophysical Institute, University of Alaska Fairbanks P.O. Box 757320 , Fairbanks, AK 99775, United States
AU: * Jones, D A
EM: rockjones73@yahoo.com
AF: Department of Geology and geophysics, University of Alaska Fairbanks P.O. Box757500, Fairbanks, AK 99775, United States
AU: Layer, P W
EM: player@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks P.O. Box 757320 , Fairbanks, AK 99775, United States
AU: Layer, P W
EM: player@gi.alaska.edu
AF: Department of Geology and geophysics, University of Alaska Fairbanks P.O. Box757500, Fairbanks, AK 99775, United States
AU: Newberry, R J
EM: ffrn@uaf.edu
AF: Department of Geology and geophysics, University of Alaska Fairbanks P.O. Box757500, Fairbanks, AK 99775, United States
AB: The ratios between the isotopes of argon vary greatly between different reservoirs (atmosphere, crust, mantle). Coupled with the fast diffusion of argon at magmatic temperatures, the argon isotopic system could prove highly valuable for the investigation of magmatic processes such as injection and assimilation. In the past 8,000 years, El Chichon volcano has produced at least 11 eruptive events displaying a relatively constant trachyandesitic composition, attesting to the tapping of a long-lived magma chamber held in steady state between the influx of basaltic magma and differentiation of resident magma. Plagioclase phenocrysts from the 1,500 ybp and older eruptions display variable enrichment of 40Ar (excess argon), while hornblende phenocrysts from the same eruptions show similar, but subdued, enrichment. In contrast, both plagioclase and hornblende phenocrysts from the four post-1,500 ybp eruptions have argon isotopic ratios near atmospheric values. Isochron analysis rules out xenocrystic contamination as the source of excess argon. All eruptive units display a positive correlation between 40Ar and Cl, indicating melt inclusions as a major reservoir for argon within plagioclase. Some units show positive correlation between 40Ar/36Ar and Cl/K, which points to inclusion-hosted excess argon, while others display a negative correlation between 40Ar/36Ar and Cl/K, pointing to lattice-dissolved excess argon. Electron microprobe and x-ray fluorescence data supplement our ability to distinguish these different reservoirs of argon within phenocrysts. The subdued behavior of hornblende is due to slower diffusion and minimal melt inclusions. Variations in the amount and reservoir location of excess argon within a phenocryst are a function of crystal residence and the isotopic signature of the magma chamber.
DE: 1036 Magma chamber processes (3618)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly