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[SVC32-P01] Experimental constraints on the H2O-saturated plagioclase liquidus and the storage depth of the Izu-Oshima 1986B basaltic andesite melt
Keywords:Izu-Oshima volcano, pre-eruptive condition, plagioclase liquidus, magma chamber, high P-T experiment
We used the powder of the B lava as the starting material of our experiments. High-temperature equilibrium experiments were carried out for the sample at 1-atm and high-pressure conditions (49, 98 and 196MPa) using a 1-atm fO2-controlled furnace and internally heated pressure vessel at the GSJ, AIST, respectively. For the 1-atm experiments, the fO2 condition was controlled at Ni-NiO buffer using the H2-CO2 mixed gas, and the Pt-wire method was adopted. The samples were heated at the experimental temperatures (1150, 1165, 1180, 1200ºC) for 3 hours and then quenched. For the high-pressure experiments, H2O was saturated in the sample, but the fO2 condition was not controlled. The samples enclosed with H2O in the Ag50Pd50 capsules were heated in the pressure vessel for 3 hours at the experimental temperatures ranging from 1050 ºC to 1130 ºC and then quenched. The quenched samples were polished to thin sections and petrological observations and chemical analyses of glasses were performed using EPMA and FE-EPMA at ERI, University of Tokyo, Japan.
Plagioclase is the liquidus phase at 1 atm, whereas early saturation of Fe-Ti oxide above the plagioclase liquidus occurred in the high-pressure experiments due to the elevated fO2 conditions. The HSPL temperature decreases from 1172 ± 8ºC to 1030 ± 20ºC as the pressure increases from 1atm to 196 MPa. A combination of previously proposed models for the plagioclase liquidus and melt H2O-solubility can predict the experimentally determined HSPL temperatures, even if oxidation-induced magnetite crystallization occurs. On the other hand, the thermodynamic equilibrium calculation program “rhyolite-MELTS” failed to predict the HSPL at high-pressure conditions although it is successfully predicted at 1 atm.
Using the combination of previously proposed models for the plagioclase liquidus and melt H2O-solubility models and the previously reported eruption temperature of ~1100 ± 30ºC, we estimate the pre-eruptive pressure conditions of the B melt to be 42 (+48/-32) MPa, which correspond to depths of 1.9 (+1.9/-1.4) km. The estimated depth is consistent with that of the shallow active dikes previously identified from geophysical studies. The result suggests that the B melt was derived from a small, shallow magma chamber formed in the shallow dike region that is shallower than 3-5 km if the assumed temperature condition is valid.