三宅島2000年噴火噴出物のガラス包有物に言已録されたマグマのS濃度とfO_2環境

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Melt inclusions in phenocrysts of ejecta of Miyakejima volcano have been analyzed with electron microprobe focused on sulfur abundance and its speciation. Samples were collected from three different volcanic events to characterize their source magma; (1) submarine eruption on June 27, 2000, (2) summit eruption on August 18, 2000, (3) lateral fissure eruption in 1983. Major element concentrations of melt inclusions derived from submarine eruption and 1983 eruption suggest their common origin. Melt inclusions within ejecta of summit eruption show less evolved feature. Although sulfur concentrations in the melt inclusions vary from 0.32 to 0.05 wt% SO_2 possibly reflecting pre- and post-entrapment degassing, no significant difference in S concentration was observed among three magmas. Measured S^<6+>/S_<total> ratios in melt inclusions with SO_2 higher than 0.1 wt% yield pre-eruptive redox conditions of ΔNNO=+0.2〜+0.5 log unit, and show no correlation with S02 concentration. On the other hand, melt inclusions with SO_2 less than 0.1 wt% indicate redox conditions of ΔNNO=0〜-0.5 log unit, implying rapid degassing of SO_2 during eruption causes decrease of oxygen fugacity in the melts.
2001-08-30