メルトインクルージョンに記憶された三宅島深部のマグマ環境

火山 vol.46, 2001からの引用です．

三宅島2000年噴火噴出物のガラス包有物に記録された マグマのS濃度とfO2環境

Sulfur abundance and redox state of melt inclusions from Miyake-jima 2000 eruption products

Atsushi YASUDA*, Setsuya NAKADA* and Toshitsugu FUJII*
* Earthq. Res. Inst., Univ. Tokyo, Japan

Abstract

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% SO2 possibly reflecting pre- and post-entrapment degassing, no significant difference in S concentration was observed among three magmas. Measured S6+/Stotal ratios in melt inclusions with SO2 higher than 0.1 wt% yield pre-eruptive redox conditions of DNNO=+0.2~+0.5 log unit, and show no correlation with SO2 concentration. On the other hand, melt inclusions with SO2 less than 0.1 wt% indicate redox conditions of DNNO=0~-0.5 log unit, implying rapid degassing of SO2 during eruption causes decrease of oxygen fugacity in the melts.

Figure 5. SO2*(wt%) and K2O/MgO (wt%ratio) of melt inclusions corrected for post-entrapment crystallization of host plagioclase.

Figure 7. Data for S6+/Stotal plotted against the corrected sulfur contents in melt inclusions.