浅間火山2004年9月の噴出物の石基ガラスの含水量測定

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Infrared absorption spectra were taken to determine concentration of water dissolved in rhyolitic glass of pumice, scoria and andesite fragments ejected by the Asama 2004 September eruptions. As the groundmass in the fragments includes a lot of microlites of plagioclase and pyroxene, thickness of the volcanic glass itself is difficult to measure directly. However, an absorption peak, assigned to volcanic glass, is observed at 1,850cm^<-1>, and its peak intensity obtained by a curve-fitting method gives the net thickness of the glass. This net thickness provides the intensities of water per glass thickness without microlite contamination. The relationship between water concentration and two absorption peak-intensity (3,570, 1,630cm^<-1>) is derived from infrared analyses of the aphyric Wada obsidian, whose water content was determined by Karl Fischer titrator. Following the above procedure, the water contents in the glass of the pumice of the September 1st eruption, and scoria and andesite fragments of the September 23rd eruption are estimated to be 0.66, 0.1, 0.1-0.2wt%, respectively. The magma represented by the quenched marginal crust of the pumice preserves considerable amount of water (0.66wt%) for its shallow location beneath the crater floor, prior to the September 1st eruption. The tensile strength of the cap rock presumably kept sufficient pressure in the top of the magma column to dissolve water into the magma, whereas the magma providing the scoria and andesite fragments of the September 23rd eruption lost most of water.
特定非営利活動法人日本火山学会の論文
2006-06-30

浅間火山2004年9月の噴出物の石基ガラスの含水量測定

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