HR: 1330h
AN: V12A-0561    [PDF]
TI: Crystallinity, textures and chemistry of "old and youthful" lavas from Merapi volcano, Java, Indonesia: understanding evolving eruptive behavior
AU: * Innocenti, S
EM: sinnocen@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Andreastuti, S D
EM: sandreas@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Furman, T
EM: furman@geosc.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Voight, B
EM: voight@ems.psu.edu
AF: Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: Merapi volcano has erupted frequently over the past 40,000 years, displaying a wide range of eruptive behaviors and posing different types of hazards. Because of its high population density and restless activity, Merapi represents an ideal laboratory for investigating magma reservoir processes and their relation to eruptive mechanisms. This presentation focuses on the recent effusive activity and adds insights from older products of Merapi, providing an early contribution to a larger project that includes the detailed study of changes in the eruptive behavior over time. The bulk geochemistry of lavas spanning $\sim$8,000 years requires a diversity of comples magmatic processes. Merapi stratigraphy records a compositional range of 49-57 wt% SiO$_{2}$, with periodic abrupt changes in K$_{2}$O content. Previous studies have demonstrated that Merapi products display a transition from medium-K ({\it Old Merapi}) to high-K series ({\it New Merapi}) at $\sim$2,000 yBP, whereas products of {\it Very Old Merapi} ($>$5,000 yBP) contain both medium and high-K series.The most recent activity of Merapi ($\sim$1888 AD to present) is represented mainly by lava dome growth and pyroclastic flows following dome collapse. Products of this activity display a relative uniformity in geochemical, petrographic, and, to a first approximation, textural parameters. Dome lavas are basaltic andesite in composition and almost all belong to the high-K series; they show high crystallinity values (47-60%) that correlate positively with the K$_{2}$O content. The phenocryst assemblage is chiefly plagioclase (68-90 vol%), clinopyroxene, orthopyroxene and Fe-Ti oxides, in a microcrystalline groundmass made of feldspars and pyroxenes. Plagioclase shows strong evidence of reheated and resorption textures and has cores with An contents as high as 89%. The relative geochemical uniformity of the recent dome lavas (1888AD-present) provides an opportunity for tentative correlations with textural (including Crystal Size Distribution) parameters, and we compare textures with modern lavas with those from {\it Very Old Merapi}. The comparison between geochemistry and quantitative textural data, and in particular CSD analyses, provides insight into chemical and physical parameters intimately related to the mechanisms of magma reservoir storage and replenishment as well as eruptive behavior. These relationships also provide a basis for an interpretation of textural parameters in relation to different types of eruptive activity.
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting