Thorium-uranium disequilibrium in a geothermal discharge zone at Yellowstone
Whole rock samples of hydrothermally-altered Biscuit Basin rhyolite from Yellowstone drill cores Y-7 and Y-8 were analyzed for /sup 230/Th, /sup 234/U, /sup 238/U, and /sup 232/Th. Extreme disequilibrium was found, with (/sup 230/Th//sup 234/U) ranging from 0.30 to 1.27. Values of (/sup 230/Th//sup 232/Th) and (/sup 234/U//sup 232/Th) define a linear correlation with a slope of 0.16 +/- 0.01, which corresponds to a (/sup 230/Th//sup 234/) age of approximately 19 ka. The (/sup 230/Th//sup 234/U) disequilibrium was apparently caused by U redistribution which occurred mostly at about 19 ka, and is not related simply to the relative degree of hydrothermal alteration and self-sealing of the rhyolite. Mass balance of U requires a large flux of U-bearing ground water through the rhyolite at the time of U redistribution; rough estimates of minimum water/rock ratio range from 10/sup 2/ to 10/sup 4/, for a range of possible ground water U concentrations. Conservative hydraulic calculations indicate that the required ground water flux could have occurred within a period of hundreds of years prior to self-sealing. The disequilibrium data are consistent with a model involving U redistribution during the initial stages of development of a geothermal discharge zone that formed in response to the hydrogeologic effects of glacial melting and unloading during the decline of the Pinedale Glaciation.
- Research Organization:
- Argonne National Lab., IL (USA)
- DOE Contract Number:
- W-31109-ENG-38
- OSTI ID:
- 6996588
- Journal Information:
- Geochim. Cosmochim. Acta; (United States), Journal Name: Geochim. Cosmochim. Acta; (United States) Vol. 51:7; ISSN GCACA
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
580400* -- Geochemistry-- (-1989)
ACTINIDE ISOTOPES
CHEMICAL COMPOSITION
CHEMISTRY
DRILL CORES
EQUILIBRIUM
FEDERAL REGION VIII
FLUIDS
GEOCHEMISTRY
GEOLOGIC MODELS
GEOTHERMAL FLUIDS
GROUND WATER
HYDROGEN COMPOUNDS
HYDROTHERMAL ALTERATION
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
MASS BALANCE
NORTH AMERICA
OXYGEN COMPOUNDS
PUBLIC LANDS
RHYOLITES
ROCK-FLUID INTERACTIONS
ROCKS
THERMODYNAMIC ACTIVITY
THORIUM ISOTOPES
URANIUM ISOTOPES
USA
VOLCANIC ROCKS
WATER
WYOMING
YELLOWSTONE NATIONAL PARK