抄録
Chemical and isotopic compositions of volcanic gases and rocks are compared between the samples collected immediately before and after the 1986 eruption of Izu-Ohshima volcano. Around the pit crater of Mt. Mihara, the CO2 and H2 contents in fumarolic gases increased before the eruption. The chemical composition of volcanic gases from Mt. Mihara (A crater area) indicates that they are mainly of magmatic origin. The gases from the craters B and C, both formed by fissure eruption, did not come from the magma but were expelled from the hot lava and scoria. δ13C values show that CO2 in these gases were derived from organic matters. This is consistent with the very short duration of fumarolic activity at the B and C crater area. Volcanic sublimates found around the high temperature fumaroles at the A crater area were mainly sulfate minerals. At the B and C crater area, copper chloride (CuCl2 etc.) and copper oxide (CuO etc.) dominated at the B crater area. Most of the sublimates from the C crater area were salammoniac (NH4Cl). Chlorine and fluorine contents of lava and scoria from A crater are found to be relatively uniform. On the other hand, those from B and C crater groups are highly valuable, and strong correlation is found between chlorine and fluorine contents.
