Petrological Study of Pinatubo 1991 Eruption: Reconstruction of Magma Plumbing Model

Pinatubo volcano in Philippines caused cataclysmic eruption on June 15, 1991. Before climactic eruption on June 15, there were many Plinian eruptions, and first Plinian eruption occurred on June 12. This first Plinian eruption was produced by andesitic magma that mixed between injected basaltic magma and dacitic magma existing in magma chamber prior to 1991 eruption. There are many previous petrological studies of Pinatubo 1991 eruption, and magma plumbing model was constructed about twenty years ago. This model shows magma mixing prior to eruption and is general at worldwide volcanoes. However, we carried out whole rock and mineral compositions of 1991 products in detail, then gained the new finding of magma plumbing model of 1991 eruption.

Representative samples of andesite are lava from dome released on June 7 and scoria from first Plinian eruption on June 12, although andesitic lava contain mafic (basaltic) enclave. That of dacite is pumice from climactic eruption on June 15. Pinatubo 1991 all products mainly include plagioclase and amphibole phenocrysts, while there is a few olivine, clinopyroxene, biotite, quartz and opaque minerals. In whole rock compositions, all products stand in a straight line, which means the magma mixing trend. However, dacitic samples have compositional variation and line up across to mixing trend. This crossed trend and compositional variation are clearly confirmed in K₂O and Rb contents particularly. Moreover, the compositions of melt inclusions hosted in plagioclase and amphibole show the wide range. Thus, we concluded that two different felsic end-member magmas had existed in magma chamber prior to 1991 eruption. This estimation is consistent with mineral features that cores of plagioclase and amphibole crystals have wide compositional range. We estimated the approximate compositions of two felsic end-member magmas which are divided into less-evolved melt (D1) and more-evolved melt (D2). Both felsic end-member magmas are reproduced by calculating mass balance method which mineral compositions add to glass compositions in melt inclusions. Due to geothermometer calculated by mineral compositions, the temperatures of D1 and D2 are 880˚C and 780˚C, respectively. Dacitic magma chamber constructed zoned magma prior to 1991 eruption and existed during 4 to 10 km depth.