Magma Degassing and Evolution Processes of the 2000 Eruption of Miyakejima Volcano, Japan, Deduced From of Olivine-Hosted Melt Inclusion Analyses

Chemical analyses of melt inclusions in Mg-poor (Mg#68-73) and Mg-rich (Mg#76-84)"@olivines from a bomb and lapilli from the 18 August 2000 eruption of Miyakejima volcano, Japan, were carried out in order to investigate degassing and evolution process "Íf the magma. Analyses of major elements, S and Cl of the melt inclusions were made by EPMA, and H2O and CO2 by FTIR and SIMS (Miyagi et al., 1995; Hauri et al., 2002). Major element composition of Mg-poor olivine-hosted melt inclusions (Mg-poor Ol MIs) is similar to that of groundmass in the bomb, indicating the melt entrapment just before the eruption. The Mg-poor Ol MIs have volatile contents of 0.7-2.5 wt.% H2O, 0.005-0.02 wt.% CO2, 0.05-0.17 wt. % S, and 0.06-0.1 wt. % Cl, that are roughly similar to those of plagioclase-hosted melt inclusions (Saito et al., 2005). Gas saturation pressure of the magma is calculated to be 20-100 MPa on the basis of the H2O and CO2 contents of Mg-poor Ol MIs, corresponds to the depth of 1-4 km. On the other hand, the Mg-rich olivine-hosted melt inclusions (Mg-rich Ol MIs) have SiO2 and K2O-poor but Al2O3-rich composition than the whole rock composition of the bomb and lapilli. They have volatile contents of 1.9-3.5 wt.% H2O, 0.003-0.025 wt.% CO2, 0.06-0.21 wt.% S, and 0.04-0.07 wt.% Cl, that are a little higher H2O and S and lower Cl contents than those of Mg-rich Ol MIs. Gas saturation pressure of the magma is calculated to be 50-150 MPa on the basis of the H2O and CO2 contents of Mg-rich Ol MIs. Ratios of H2O and S contents of both the Mg-poor and Mg-rich Ol MIs are similar to that of volcanic gas emitted from the summit after the 2000 eruption, while their ratios of CO2 and H2O contents are lower than that of volcanic gas. Existence of the Al2O3-rich less-evolved melt with high H2O content is consistent with the petrological and experimental studies that low-MgO high-alumina basalt is derived from primary magma with high H2O content (Uto, 1986; Sisson and Grove, 1993). The volatile contents and major element composition of the Mg-rich Ol MIs suggest degassing of the less-evolved magma occurred together with fractional crystallization at the depth of 2-6 km. The Mg-poor Ol MIs having lower H2O and higher CO2 contents than those of the Mg-rich Ol MIs suggest addition of CO2-rich gas to the degassed and evolved magma. It is necessary to investigate volatile evolution process of the magma at the shallow depth (<4 km) in more detail.