HR: 10:25h
AN: V52C-01    [Abstracts]
TI: Volatile Flux and Composition at Yellowstone Reflects a Gas-Charged Hydrothermal System Above a Basalt-Fueled Silicic Magma Reservoir
AU: * Lowenstern, J B
EM: jlwnstrn@usgs.gov
AF: U.S. Geological Survey, Mail Stop 910 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Hurwitz, S
EM: shaulh@usgs.gov
AF: U.S. Geological Survey, Mail Stop 439 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Recent papers have documented the immense volatile (CO₂, S, Cl^-, F^-) flux from the Yellowstone Caldera. Carbon dioxide is estimated to escape diffusively through soils at a rate of 45,000 t d^-1 (Werner and Brantley, 2003) compared with 137 t d^-1 of Cl^- released from hot springs into rivers (Hurwitz and others, 2007). These high volatile fluxes and the CO₂/Cl^- ratio of ~300 are inconsistent with simple degassing of a mid- or upper-crustal silicic intrusion. For example, if the estimated CO₂-flux were supplied solely by a 10⁴ km³ silicic-magma reservoir with 500 ppm dissolved CO₂, the reservoir would be exhausted in ~1000 years, less than 0.1% of the longevity of the present Yellowstone volcanic field. Moreover, silicate melt inclusions in phenocrysts from erupted rhyolites contain abundant dissolved Cl^- and F^-, but minimal CO₂ and S, the dominant effluents from the hydrothermal system. The carbon budget is explained best by dominant basalt degassing (~0.3 km³ a^-1 of magma) in the lower and mid- crust, augmented by metamorphic devolatilization of limestone and other sediments, plus what can be sourced from overlying rhyolitic magma. The volatiles then pass into and through the near-surface hydrothermal system. The relative abundances of emitted CO₂ and Cl^- appear to require that the shallow subsurface beneath Yellowstone is gas-saturated down to >2 km. Fournier (1989) concluded that the diverse Yellowstone geothermal waters are ultimately derived from a deep parent fluid with 400 ppm Cl^-. If Cl^- and CO₂ are emitted in proportion to their abundance in the hydrothermal system, then given the CO₂/Cl^- of 300, this parent fluid would contain 12 wt.% CO₂ (5 mol%). Solubility constraints reveal that such fluid would be saturated with CO₂-rich steam within the upper few kilometers. The presence of a compressible and expandable vapor phase has important implications for the origin and interpretation of ground-surface displacements at active calderas such as Yellowstone. Fournier RO (1989) Ann Rev Earth Planet Sci 17, 13-53. Hurwitz S, Lowenstern JB, Heasler H (2007) J Volcanol Geothermal Res 162, 149-171. Werner C, Brantley S, (2003) Geochem Geophys Geosystems 4 (7) 1061, doi:10.1029/2002GC000473.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 8430 Volcanic gases
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting