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We estimated the depth of the magma chamber system beneath Usu volcano, one of the most active volcanoes in Japan, by melting experiments on the eruptive product of the ad 1663 plinian eruption (Us-b pumice). This pumice is a low-K rhyolite with a K2O content of only 1 wt %. The experiments were conducted using internally heated pressure vessels at 98–392 MPa and 700–900°C under nickel–nickel oxide (NNO)-buffered and water-saturated conditions. The pre-eruptive conditions of the 1663 eruption were estimated to be 780 ± 20°C and around 200–250 MPa. The phase equilibria, mineral compositions (plagioclase, orthopyroxene, and Fe–Ti oxides), melt composition, and melt fraction are all in agreement with this conclusion. This pressure corresponds to a depth of 8–10 km, assuming a lithostatic condition with an appropriate crustal density structure. The magma was at near water-saturated conditions. Deviation between the estimated pressure and the true total pressure was estimated to be less than 50 MPa. The Us-b magma was under very uniform conditions with a temperature variation of less than c. 20°C, deduced from the narrow range of the phase equilibrium conditions (crystallizing orthopyroxene without quartz) and the homogeneous composition of plagioclase (c. An 41–45). We also estimated the pre-eruptive conditions of post-1663 eruptions, where it has been proposed that two magmas (high-P and low-P members) mixed prior to each eruption. The pressure of the deep magma was also around 200–250 MPa, whereas that of the shallow magma was estimated to be 100–150 MPa (4–6 km depth), according to the pressure dependence of the plagioclase composition obtained in our experiments. Thus, there appear to be two magma chambers beneath Usu volcano. Our estimate is consistent with geophysical observations made during the 2000 eruption, including identification of an inflation and deflation source at a depth of 10 km, a low-frequency (12 s) tremor source at a depth of 5–6 km, and an earthquake swarm at a depth of less than 4 km. The depth of the rhyolitic magma chamber (8–10 km) is deeper than the level of neutral buoyancy for the magma, and is comparable with that of mafic magma that existed beneath the rhyolitic magma. This suggests that the depth of the 8–10 km deep magma chamber was controlled by the emplacement of the mafic magma and that the rhyolitic magma was produced at that depth. When compared with other silicic magma systems, the Usu magma system has similar characteristics, including a similar pre-eruptive pressure of around 200 MPa and a pressure decrease with time, suggesting upward magma movement towards a neutral buoyancy level. The low potassium content of the Usu magma prohibited crystallization of biotite, K-feldspar or hornblende, which are common phases in magmas with intermediate to high potassium contents. The absence of hydrous minerals at near-liquidus conditions may cause a rapid increase in water content during crystallization of the magma, resulting in a rapid increase of the gas phase and frequent plinian or sub-plinian eruptions (six times since ad 1663) of the volcano. [ABSTRACT FROM PUBLISHER]

To constrain the preeruptive physical and chemical conditions (pressure, temperature, H2O content, and fO2) of the most recent and largest eruption (Aso‐4) of Aso volcano, Japan, we compared phase assemblages and chemical compositions of erupted material with those of experimentally determined run charges of the silicic end‐member magma of Aso‐4 (the KJ5665 pumice and MY5774 welded lens from the Aso‐4 pyroclastic flow deposit; SiO2 = 70.5 wt %). Experimental conditions were as follows: starting material = KJ5665 pumice, pressure = 100–700 MPa, temperature range = 810–950 °C, and injected H2O content = 2.9–6.7 wt %. The starting material comprised mainly rhyolitic glass with plagioclase, orthopyroxene, magnetite, and ilmenite phenocrysts. Hornblende phenocrysts are not found on cross sections of the KJ5665 pumice. Core compositions of these phenocrysts are unimodal. Using the magnetite‐ilmenite thermometer, the preeruptive magmatic temperature and fO2 of KJ5665 were estimated to be ~900 °C and Nickel‐Nickel oxide (NNO) +1.1 log units, respectively. Run products contain plagioclase (An20–48), orthopyroxene (Mg# = 53–72), magnetite, ilmenite, biotite, and K‐feldspar. Hornblende was not observed. Preeruptive conditions of the silicic end‐member magma of the Aso‐4 eruption are estimated to be ~900 °C, ≤ 300 MPa, and ≤ 3 wt % H2O. Key Points: Preeruptive magma conditions of the Aso‐4 silicic end‐member magma were determinedPreeruptive conditions agree with geophysical constraints on magma chamber depth [ABSTRACT FROM AUTHOR]

Abstract: Hydrogen in nominally anhydrous minerals (NAMs) in volcanic rocks can be used as a proxy for dissolved H2O in melt prior to eruption. Plagioclase is a NAM that accommodates hydrogen in concentrations of up to hundreds of wt. ppm H2O. The species of hydrogen in volcanic plagioclase is structural OH. We report the analytical results of OH concentrations in Ca-rich plagioclase from the 1986–1987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in Izu arc. We demonstrate that the island arc low-K tholeiitic basalt magmas erupting from the frontal-arc volcanoes are H2O-saturated and undergo polybaric degassing during the magma ascent. The analyzed OH concentrations in plagioclase range from 20 to 300 wt. ppm H2O, and three levels of OH (20–80 wt. ppm H2O, 100–180 wt. ppm H2O, and 220–300 wt. ppm H2O) are found. These variations in OH indicate that crystallized plagioclase is equilibrated with H2O-saturated melt at three depths beneath the Izu-Oshima volcano prior to eruption: near the surface level (≈1wt.% H2O in melt), at a 4-km-deep magma chamber (≈3wt.% H2O in melt), and at a 8–10-km-deep magma chamber (≈5wt.% H2O in melt). It is proposed that deep-seated island arc low-K tholeiitic basalt magmas erupting from frontal-arc volcanoes are richer in H2O than previously thought, containing approximately 1wt.% H2O based on analyses of “leaked” melt inclusions and phase equilibrium studies at “low-pressure conditions”. [Copyright &y& Elsevier]