三宅島2000年噴火;噴出物中のガラス包有物組成の特徴とSO2大量放出の要因

概要

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Major-element, Cl, S analyses have been performed with electron microprobe on melt inclusions trapped in phenocryst plagioclase from Miyakejima volcano. 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. Chemical variation observed in melt inclusions from the summit eruption on August 18 can be explained by postentrapment crystallization of host-plagioclase, and no evidence is recognized on the occurrence of magma-mixing. On the other hand, element concentrations of melt inclusions derived from the submarine eruption and the 1983 eruption suggest a common source having a supply signature of less-evolved magma into a differentiated magma. Among the three volcanic events, no significant difference in S concentration was observed. Sulfur concentrations in the melt inclusions vary more than 40%, possibly reflecting pre-entrapment degassing during the ascent of magma from a deeper magma chamber to a shallower one. These observations imply that both insufficient absorption of volcanic gas into groundwater and injection of voluminous volatile-saturated magma into the shallow magma chamber cause the current significant SO2 emission.