3:30 PM - 5:00 PM
[SCG48-P10] Petrography and estimation of water concentration for Cretaceous granitoids in the Yashiro-jima Island, Yamaguchi Prefecture, Japan.
Water plays an important role in magma genesis, and strongly affects rheology of silicate melts. In this study, we have analyzed compositions of homogenized melt inclusions in zircons separated from the Gamano granodiorite in the Yashiro-jima Island, Yamaguchi Prefecture, in order to develop a technique to estimate water contents of magma which forms plutonic rocks. We have performed homogenization experiments (900 ℃ and 4.5 hours) of melt inclusions in zircons with a piston-cylinder type high-pressure apparatus. The melt inclusion compositions obtained through SEM-EDS analysis are granitic and the water contents are estimated as 4.2-9.5 wt %. The SiO2 contents of the melt inclusions (76.3-79.4 wt %) are higher than the whole-rock composition of the zircon extracted sample (72.4 wt %).
The crystallization sequence of the main mineral constituents suggests that the sample solidified from a magma with water contents more than 3.3 wt %, which is consistent with the water content estimated through SEM-EDS analysis in this study (4.2-9.5 wt %). Pressure estimates using major element compositions and water contents of the melt inclusions yield 403-144 MPa (DERP geobarometer) and 283-122 MPa (Rhyolite-MELTS geobarometer). The water contents estimated in this study provide the minimum pressure estimates of 125-400 MPa based on the water solubility in granitic melts, which is broadly consistent with the pressures estimated by DERP and Rhyolite-MELTS geobarometers. The wide range of estimated pressure probably indicate that the growing zircon crystals have trapped surrounding melts at various pressures, which is consistent with a wide range of zircon occurrences including discreate grains between main mineral constituents and inclusions in main mineral constituents.
The crystallization sequence of the main mineral constituents suggests that the sample solidified from a magma with water contents more than 3.3 wt %, which is consistent with the water content estimated through SEM-EDS analysis in this study (4.2-9.5 wt %). Pressure estimates using major element compositions and water contents of the melt inclusions yield 403-144 MPa (DERP geobarometer) and 283-122 MPa (Rhyolite-MELTS geobarometer). The water contents estimated in this study provide the minimum pressure estimates of 125-400 MPa based on the water solubility in granitic melts, which is broadly consistent with the pressures estimated by DERP and Rhyolite-MELTS geobarometers. The wide range of estimated pressure probably indicate that the growing zircon crystals have trapped surrounding melts at various pressures, which is consistent with a wide range of zircon occurrences including discreate grains between main mineral constituents and inclusions in main mineral constituents.