The geochemical evolution of Tambora volcano, Indonesia

Helene Marie Grall-Johnson, University of Rhode Island

Abstract

A suite of 72 samples from throughout the stratigraphic section of Tambora volcano and nearby Satonda island, Indonesia, was studied. The major element data indicate that the rocks are highly potassic trachybasalts and trachyandesites with K$\sb2$O up to 6 wt%. Major and trace element variations are correlated and indicate four main geochemical episodes that include the major stratigraphic units that have been recognized: (1) the Stratovolcano Lavas, (2) the Caldera Lavas, (3) the Black Sands and Brown Tuff, (4) the 1815 pyroclastics and Post-1815 lavas. The Satonda island sample has the most primitive composition. The geochemical cycles were initiated by fresh pulses of primitive magma into the fractionating magma chamber.^ In terms of major elements, the entire Tambora suite can be generated by fractional crystallization of a Satonda-like melt. The products of the great 1815 Tambora eruption could have been produced by 58% fractionation of pyroxene, plagioclase, olivine, magnetite and biotite, from Satonda magma, in decreasing order of importance.^ Radiogenic isotope data indicate that the source of the Tambora magmas was unchanged during the building stages of the volcano. The source composition, in terms of Pb, Sr and Nd isotopes, is almost identical to the source of Indian Ocean oceanic island basalts and may involve the mixing of three components: depleted mantle, enriched mantle and HIMU.^ The enrichment in incompatible trace elements at Tambora, coupled with a strong depletion in high field strength elements, which is a characteristic of island arc volcanoes, does not seem to be due to a sediment component on the basis of the isotope evidence. The data may reflect a two stage melting process: formation of enriched veins by small degrees of melting of fertile deep mantle; subsequent melting of the veins by adiabatic decompression and dilution by depleted mantle at the asthenosphere-arc lithosphere boundary.^ It is not clear whether the complex tectonic setting of Tambora, at the junction between the Sunda and Banda arcs, where the subducting lithosphere changes from oceanic to continental, has had an impact on the geochemical evolution of the volcano. ^

Subject Area

Geology|Geochemistry

Recommended Citation

Helene Marie Grall-Johnson, "The geochemical evolution of Tambora volcano, Indonesia" (1997). Dissertations and Master's Theses (Campus Access). Paper AAI9831105.
http://digitalcommons.uri.edu/dissertations/AAI9831105

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