Geochronologic and isotope studies of calcite and silica constraining Quaternary unsaturated- and saturated zone hydrologic flux at Yucca Mountain, Nevada, USA

Abstract

Both unsaturated- and saturated-zone aqueous solutions are capable of precipitating secondary mineral deposits that document the history and origins of past water flux. Calcite and opal occur as thin coatings on open fractures and cavity floors within the thick unsaturated zone at Yucca Mountain. Outermost surfaces of calcite have 14 C ages of between 44,000 and 16,000 radiocarbon years; however, the same surfaces have 230 Th/U ages from 28 ka to more than 500 ka. This discordance, along with negative covariance between conventionally calculated 230 Th/U ages and initial 234 U/ 238 U is best explained by very slow rates of mineral growth where discrete depositional layers are too fine to separate and measure individually. Therefore, isotopic analyses and resulting ages represent mixtures between the deepest and shallowest layers incorporated within a given sub-sample.