Zell E. Peterman Leonid A. Neymark Joseph F. Whelan Brian D. Marshall James B. Paces 1999 <p><span>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&nbsp;</span>¹⁴<span>C ages of between 44,000 and 16,000 radiocarbon years; however, the same surfaces have&nbsp;</span>²³⁰<span>Th/U ages from 28 ka to more than 500 ka. This discordance, along with negative covariance between conventionally calculated&nbsp;</span>²³⁰<span>Th/U ages and initial&nbsp;</span>²³⁴<span>U/</span>²³⁸<span>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.</span></p> application/pdf en Nuclear Energy Agency Geochronologic and isotope studies of calcite and silica constraining Quaternary unsaturated- and saturated zone hydrologic flux at Yucca Mountain, Nevada, USA text