Helium Isotopes in Geothermal and Volcanic Gases of the Western United States, II. Long Valley Caldera
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Journal Article: Helium Isotopes in Geothermal and Volcanic Gases of the Western United States, II. Long Valley Caldera
Abstract
Systematic trends in helium isotope ratios and gas compositions in Long Valley caldera hot-spring and fumarole gases reflect subsurface processes and conditions in the hydrothermal system. High View the MathML source ratios (to 6.5 _ atmospheric) indicate a magmatic helium source diluted to varying degrees by crustal radiogenic helium, as observed in western United States geothermal systems in general. Helium isotope ratios increase eastward across the caldera, which we interpret as an input of magmatic helium to the hydrothermal fluid as it flows eastward over the caldera. Decreasing He/condensible-gas (~ He/CO2) ratios accompanying this trend suggest that CO2 addition and/or preferential helium loss due to boiling are also occurring, although near-surface, pH-related CO2_HCO3 equilibria obscure the nature of this effect.Substantial changes in View the MathML source ratios were observed in Long Valley gases between 1978 and 1985. Helium isotope ratios increased by 15% in the Hot Creek thermal vent, with maximum ratios achieved in 1983. Helium isotope ratios subsequently decreased in this and other hot vents in the caldera. These observations indicate that substantial changes in the mixing ratio of radiogenic and magmatic helium may occur over relatively short periods of time in magmatically active system
- Authors
- J.A. Welhan, R.J. Poreda, W. Rison and H. Craig
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Citation
J.A. Welhan,R.J. Poreda,W. Rison,H. Craig. 1988. Helium Isotopes in Geothermal and Volcanic Gases of the Western United States, II. Long Valley Caldera. Journal of Volcanology and Geothermal Research. 34(3-4):201-209.
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