Depth and Evolution of a Silicic Magma Chamber: Melting Experiments on a Low-K Rhyolite from Usu Volcano, Japan

Abstract

We estimated the depth of the magma chamber system beneath Usu volcano, one of the most active volcanoes inJapan, 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 1wt %. The experiments were conducted using internally heated pressure vessels at 98^392MPa and 700^9008C under nickel^nickel oxide (NNO)-buffered and water-saturated conditions. The pre-eruptive conditions of the 1663 eruption were estimated to be 780 208C and around 200^250MPa. 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 50MPa. The Us-b magma was under very uniform conditions with a temperature variation of less than c. 208C, 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 200^250MPa, whereas that of the shallow magma was estimated to be 100^150MPa (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 200MPa 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