Studies on diffuse emission of volcanic volatiles in a monogenetic volcano area

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Magmatic volatiles are released not only as plume efflux from summit craters or flank fumaroles but also as diffuse efflux from whole surface of volcanic body. This work tried to monitor the behavior of magma beneath monogenetic volcanoes of the Izu-Tobu volcanoes by means of diffuse degassing of magmatic volatiles. From the bottom of the summit crater of the Teishi Knoll, formed at the 1989 submarine eruption, extremely trace amounts of magmatic-hydrothermal CO2 and CH4 were detected, by means of precise chemical and isotopic analyses of seawater collected just on the crater bottom. No diffuse degassing of magmatic CO2 was observed at the land area of epicenter regions of recent earthquake swarms nor at the fissure eruption region formed 2700 years before. At Oh-muro-yama scoria cone formed 4000 years before, CO2 efflux was slightly higher in the summit crater, the total efflux was (22+2) ton/day.