Magma plumbing system of the 2000 eruption of Miyakejima volcano, Japan, deduced from volatile and major component contents of olivine-hosted melt inclusions

Chemical analyses of 56 olivine-hosted melt inclusions and their host olivines from the explosive eruption of Miyakejima volcano, Japan, on 18 August 2000 were carried out by electron probe micro analyzer and secondary ion mass spectrometry in order to investigate the magma plumbing system of the eruption. The bimodal olivine core composition, relatively small rims, and diffusion profiles of the Mg-rich olivines indicate the mixing of evolved magma and less-evolved magma in 2 months before the eruption. The major element composition of the Mg-poor olivine-hosted melt inclusions (Mg-poor OLMIs) is similar to that of the groundmass. The Mg-rich olivine-hosted melt inclusions (Mg-rich OLMIs) have a SiO2- and K2O-poor but Al2O3- and CaO-rich composition, compared to the whole-rock composition of the products. Most of the Mg-rich OLMIs have slightly higher H2O (1.9-3.5 wt %) and S (0.06-0.21 wt %) contents and a lower CI (0.04-0.07 wt %) content than the Mg-poor OLMIs, whereas they have similar CO2 (0.003-0.025 wt %) content. These results suggest that CO2-rich gas was added to the evolved magma in a shallow magma chamber, followed by decompression degassing in a pressure range of 30-80 MPa and the rapid ascent of the less-evolved magma from a deep magma chamber up to a depth of about 2 km. The ratios of the H2O and S contents of the Mg-rich OLMIs are similar to that of the volcanic gas emitted from the summit after the 2000 eruption, which suggests shallow degassing of the less-evolved magma in the conduit since the eruption.