Helium Isotope Ratios and Geochemistry of Volcanic Fluids from the Norikura Volcanic Chain, Central Japan: Implications for Crustal Structures and Seismicity
Minoru Kusakabe, Michiko Ohwada, Hiroshi Satake, Keisuke Nagao, Ichiro Kawasaki, 2005. "Helium Isotope Ratios and Geochemistry of Volcanic Fluids from the Norikura Volcanic Chain, Central Japan: Implications for Crustal Structures and Seismicity", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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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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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.