Thorium-uranium disequilibrium in a geothermal discharge zone at yellowstone

Abstract

Whole rock samples of hydrothermally-altered Biscuit Basin rhyolite from Yellowstone drill cores Y-7 and Y-8 were analyzed for 230 Th, 234 U, 238 U, and 232 Th. Extreme disequilibrium was found, with ( 230 Th/ 234 U) ranging from 0.30 to 1.27. Values of ( 230 Th/ 232 Th) and ( 234 U/ 232 Th) define a linear correlation with a slope of 0.16 ± 0.01, which corresponds to a ( 230 Th/ 234 U) age of approximately 19 ka. The ( 230 Th/ 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 groundwater through the rhyolite at the time of U redistribution; rough estimates of minimum water/rock ratio range from 10 2 to 10 4 , for a range of possible groundwater U concentrations. Conservative hydraulic calculations indicate that the required groundwater 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.