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Helium Isotope Ratios and Geochemistry of Volcanic Fluids from the Norikura Volcanic Chain, Central Japan: Implications for Crustal Structures and Seismicity

By
Minoru Kusakabe
Minoru Kusakabe
Institute for Study of the Earth's Interior, Okayama University, Misasa, Tottori-ken 682–0193, Japan
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Michiko Ohwada
Michiko Ohwada
Faculty of Sciences, Toyama University, Gofuku, Toyama 930–8555, Japan
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Hiroshi Satake
Hiroshi Satake
Faculty of Sciences, Toyama University, Gofuku, Toyama 930–8555, Japan
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Keisuke Nagao
Keisuke Nagao
Laboratory for Earthquake Chemistry, University of Tokyo, Bunkyo-ku, Tokyo 113–0033, Japan
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Ichiro Kawasaki
Ichiro Kawasaki
Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611–0011, Japan
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Published:
January 01, 2005

Abstract

The crust beneath the Norikura volcanic chain, Hida Highland, central Japan, is tectonically active and characterized by frequent shallow microearthquakes at depths of 2 to 5 km. Recent seismic tomography studies of this area indicate that there are zones of low seismic wave velocities (VP and Vs) beneath the Norikura volcanic chain at depths of 5 to 15 km. Fluid and light gas isotope geochemistry were used to clarify the relationships between hydrothermal activity and seismic activity of this area. The geochemistry of gases from fumaroles, boreholes, and hot springs, found along the Norikura volcanic chain volcanoes, reveals that (1) water vapor in the fumarolic gases is a mixture of arc-type magmatic water and local meteoric water, (2) some of the hot spring waters have fluid-mineral equilibration temperatures up to 200°C, (3) high 3He/4He ratios of 10.3 to 11.2 × 10−6 in fumarolic gases from the Tateyama, Yakedake, and Ontake volcanoes suggest a MORB source for the helium, (4) slightly lower 3He/4He ratios in gases from the boreholes and hot springs indicate a significant contribution of mantle helium, and (5) the ultimate source of carbon is derived mainly from decarbonation of the subducting sediments based on the δ13C-CO2/3He relationship.

The distribution of the high 3He/4He sites coincides well with that of the zones of low seismic wave velocity at a depth of 5 km but not at 10 km. The extensive hydrothermal systems driven by magmatic heat of the Norikura volcanic chain volcanoes rather than emplacement of a felsic magma chamber are responsible for the shallow low-velocity zones, at these depths. The shallow earthquakes (2–5 km) are most likely caused by brittle fracture of rocks above the brittle-ductile transition where the cracks and fractures are filled with hydrothermal fluids.

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Special Publications of the Society of Economic Geologists

Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth

Stuart F. Simmons
Stuart F. Simmons
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Ian Graham
Ian Graham
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Society of Economic Geologists
Volume
10
ISBN electronic:
9781629490342
Publication date:
January 01, 2005

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