Study of Volcanic Gas by Isotopic Analysis of Noble Gas for Eruption Prediction by Geochemical Techniques

Abstract

Isotopic study of volcanic noble gases is of great significance to make progress in the investigation of eruption prediction by geochemical techniques. Since noble gases are chemically inert and the elemental and isotopic compositions are not affected by chemical reactions between volcanic gases and surrounding rocks of a volcanic vent, a sign of change in magmatic activity is supposed to be reflected directly on the composition of noble gases collected at a fumarole. A long-term variation of ³He/⁴He and ³He/²⁰Ne ratios at Showa-shinzan volcano from 1945 to 1977 is discussed on the basis of noble gas data given by NAGAO et al. (1980). A sudden change in the decreasing rates for both ratios is found around 1965. This change in the noble gas composition correlates with a temporal variation up to 100℃ in fumarolic temperature and with a monthly change in the highest degree of color smoke erupted from this volcano. One of explanations for this event is the temperature decrease of Showa-shinzan magma followed by solidification of the magma surface opening into the vent. In order to check a short-term variation of the noble gas composition, fumarolic gas was collected at the A-fumarole of Zao volcano in short intervals between 10 and 60 minutes, and analysed for the noble gas composition. The ³He/⁴He ratio is constant within experimental errors (2σ). However, the ³He/²⁰Ne ratio shows large variations even among fumarolic gas samples collected within several hours. An abnormally high ³He/²⁰Ne ratio was observed for samples collected in Setember 14, 1980, and it was followed by occurrence of a small scale of mud flow in May, 1981. The ³He/²⁰Ne ratio in September, 1980 is approximately 5 times the usual value of ³He/²⁰Ne ratio at the A-fumarole. This suggests that the noble gas composition such as ³He/²⁰Ne is able to detect even a small change in magmatic activity and is helpful for the eruption prediction by geochemical techniques. The proportion and the elemental and isotopic ratios of the magmatic component of noble gas were estimated according to a mixing model between a primary component released from magma and a secondary component originated from ground water.