妙高火山における熱流体の地球化学的研究

概要

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Thermal spring waters from the Myoko volcano comprise near-neutral pH chloride, bicarbonate and sulfate types. Chemical and isotopic evidence suggests that the Na-Cl and Ca-HCO_3 waters are derived from a hydrothermal aquifer formed by mixing of magmatic gases with deep circulating groundwater. The SO_4 water is produced by reaction of surface waters and gaseous phase, which consist for the most part of the CO_2 and H_2S after boiling of the thermal aquifer. The isotopic composition of waters from thermal springs shows mainly their meteoric origin. Hydrochemical and gas geothermometers indicate that high enthalpy chloride type gluids (≦300℃) are present at deep levels beneath the Myoko volcano. The N_2, Ar and He contents in the lowtemperature fumarolic gases (〜100℃), which are essentially fed by steam boiled off from the hydrothermal aquifer, suggest a relatively high contribution of magmatic fluids. On the basis of isotope evidence, it is likely that the thermal fluid in the aquifer contains thermogenic gases derived by thermal decomposition of organic matter in sedimentary basement rocks.
日本地球化学会の論文
1993-11-05