HR: 1340h
AN: V13B-0535    [Abstracts]
TI: Geochemical, Textural and Petrographic Indicators for Eruptive Behavior at Merapi Volcano, Indonesia
AU: * Innocenti, S
EM: sinnocen@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
AU: Andreastuti, S
EM: mvopgm@yogya.wasantara.net.id
AF: Merapi Volcano Observatory, Jl. Cendana 15, Yogyakarta, 55166 Indonesia
AB: Over the 40,000 years if its activity, Merapi volcano has exhibited a wide variety of eruptive style. Recent eruptive activity is dominated by growth and subsequent gravitational collapse of small lava domes; however, the tephrostratigraphic record reveals a history that includes mild effusive activity and is punctuated by highly explosive events (to VEI 4). The long-term geochemical evolution at Merapi is characterized by a gradual increase in silica content from basalt to andesite. The most interesting variation at Merapi is observed in K₂O content, with eruptive products belonging to both medium- and high-K series. An abrupt change from medium- to high-K lavas occurred with the somma-forming eruption of the Tegalsruni tephra ~100 A.D. (Andreastuti 1999). Our work indicates that similar compositional shifts occurred in the earlier history of the Merapi system, although these events are not well-constrained stratigraphically. We have sought to interpret these compositional shifts in terms of changes to the magmatic plumbing system of Merapi as revealed by petrographic and textural analysis. Plagioclase feldspar phenocrysts and microphenocrysts within Merapi lavas display two contrasting styles of crystal size distributions (CSDs) and compositional zoning patterns, which in turn indicate distinct plumbing systems and transport. Briefly, basaltic lavas from throughout Merapi's history show kinked CSD patterns that suggest magma mixing in at least two crustal chambers prior to emplacement. In contrast, recent basaltic andesite dome lavas have smoothly curved CSD patterns that suggest prolonged residence at near-surface conditions under steady-state open system behavior. The high crystallinity of recent Merapi basaltic andesites (~60 vol.%) corresponds to a viscosity approaching the critical rheological locking limit (Marsh, 1981). Current Merapi eruptive behavior oscillates about this rheological limit, with temporary crystalline plugs in the eruptive conduit disrupted by repeated efflux of magma. A slight change in the reservoir-conduit dynamics could interrupt this balance and modify the current eruptive style. We propose that this transition from steady-state to highly explosive activity has happened during Merapi's history, and that such events disrupt the plumbing system to the extent that magmas from different K-affinity are tapped.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005