The 1982-83 Eruption at Galunggung Volcano, Java (Indonesia): Oxygen Isotope Geochemistry of a Chemically Zoned Magma Chamber |
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Authors: | HARMON RUSSELL S; GERBE MARIE-CHRISTINE |
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Institution: | 1NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre Keyworth NG12 5GG, UK
2Dpartement des Sciences de la Terre, UniversitBlaise Pascal Clermont-Ferrand, F63038, France |
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Abstract: | Mount Galunggung is a historically active volcano in southwesternJava that has erupted four times in the last two centuries.During the most recent event, which occurred during a 9monthinterval in 1982 83, some 305 106 m3 of mediumK,calcalkaline magma was erupted. This eruption was unusualbecause of its duration, the diversity of eruption dynamicsand products, and the range of lava compositions produced. Thecomposition of juvenile material changed gradually during thecourse of the eruption from initial plagioclase (An6075)and twopyrozene bearing andesites with 58% SiO2 to finalplagioclase (An8590), diopside, and olivine (Fo8590)bearing primitive magnesium basalts with 47% SiO2 Mineralogicaland compositional relationships indicate a magmatic evolutioninvolving differentitation of highMg parental melt. Theeruptive volumes of 35 106 m3 andesite, 120 106 m3 maficandesite, and 150 106 m3 basalt are consistent with the ideathat the 1982 83 eruption progressively tapped and draineda magma chamber that had become chemically stratified throughextensive crystal fractionation. Separates of plagioclase and pyroxene have 18O( SMO W) rangesof + 5. 6 to + 6.0 and + 5.3 to + 5.6, respectively, with 18Oplagpxvalues of + 0.4 to + 0.6o, indicating internal Oisotopeequiliburium at temperature of 1100850 C. The magenesianbasalts have magmatic 18O/ 16O ratios similar to those of midoceanridge basalt, and the Oisotope ratios of compositionallyevolved derivative melts show no evidence for contaminationof the galunggung magmas by 18Orich crust during differentiation.Andesites and transitional mafic and sites have a more variableOisotope character, with laves and phenocrysts havingboth higher and lower 18O values than observed in the parentalmagnesium basalts. These features are interpreted to reflectintramagma chamber processes affecting the upper portions ofthe differentiating Galunggung magma body before the 198283eruption. |
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