Li and Sr isotope systematics of spring water around Ontake volcano in Japan: origin of fluid causes crustal deformation

Since 1978, earthquake swarms have been observed continuously on the east flank of Ontake volcano in central Japan. Earthquakes bigger than M4 have been occurred one or two a year. In September 1984, the M6.8 Western Nagano Prefecture earthquake occurred, and the Otaki village was devastating by collapse of volcanic edifice. In this earthquake swarm region, uplift of 3-6mm has been detected in 2002-2004 (Kimata et al., 2004). Also, a low-resistivity region at a depth 2km beneath the uplift area has been inferred from specific electric conductivity measurements (Kasaya et al., 2002). The resistivity index is lower than 100 ohm/m, suggesting the presence of hydrothermal chamber rather than magma. Our research group has carried out continuous geochemical survey for gas in hot and mineral springs around Ontake volcano since 1981. The results show that an anomalous increase in δ13C of CO2 and 3He/4He ratios was observed at the Shirakawa spring that is near the region of uplift (Takahata et al., 2003). Based on these features, it has been inferred that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarp area (Kimata et al., 2004). However, it has been unknown that the origin of the hydrothermal fluid that causes crustal deformation. The non-traditional lithium (Li) isotopic tracer has a great potential to provide us new knowledge for fluid that causes crustal deformation. Accordingly, we have analyzed 7Li/6Li ratios together with 87Sr/86Sr ratios and chemical compositions of filtrated spring water around Ontake volcano. Analyzed hot and mineral spring water has been sampled biennially since 2000. The results show that the delta 7Li values of spring water from the earthquake swarm region that is located in east flank of Ontake volcano are significantly lower than those of island arc volcanic rocks (δ7Li = ca. +3 ~ +5 per mil). From this, it is inferred that the fluid that causes crustal deformation of studied area contacted with low 7Li rocks at high temperature. Reference: Kasaya et al., 2002. EPS 54, 107-118. Kimata et al., 2004, EPS 56, E45-E48. Takahata et al., 2003. Geochem. J. 37, 299-310.