Yuri V. Amelin James B. Paces Leonid A. Neymark 2000 <p>U–Th–Pb isotopic systems have been studied in submillimeter-thick outermost layers of Quaternary opal occurring in calcite–silica fracture and cavity coatings within Tertiary tuffs at Yucca Mountain, Nevada, USA. These coatings preserve a record of paleohydrologic conditions at this site, which is being evaluated as a potential high-level nuclear waste repository. The opal precipitated from groundwater is variably enriched in ²³⁴U (measured ²³⁴U/²³⁸U activity ratio 1.124–6.179) and has high U (30–313 ppm), low Th (0.008–3.7 ppm), and low common Pb concentrations (measured ²⁰⁶Pb/²⁰⁴Pb up to 11,370). It has been demonstrated that the laboratory acid treatment used in this study to clean sample surfaces and to remove adherent calcite, did not disturb U–Th–Pb isotopic systems in opal.</p> <br/> <p>The opal ages calculated from ²⁰⁶Pb∗/²³⁸U and ²⁰⁷Pb∗/²³⁵U ratios display strong reverse discordance because of excess radiogenic ²⁰⁶Pb∗ derived from the elevated initial ²³⁴U. The data are best interpreted using projections of a new four-dimensional concordia diagram defined by ²⁰⁶Pb∗/²³⁸U, ²⁰⁷Pb∗/²³⁵U, ²³⁴U/²³⁸U_activity, and ²³⁰Th/²³⁸U_activity. Ages and initial ²³⁴U/²³⁸U activity ratios have been calculated using different projections of this diagram and tested for concordance. The data are discordant, that is observed ²⁰⁷Pb∗/²³⁵U ages of 170 ± 32 (2σ) to 1772 ± 40 ka are systematically older than ²³⁰Th/U ages of 34.1 ± 0.6 to 452 ± 32 ka.</p> <br/> <p>The age discordance is not a result of migration of uranium and its decay products under the open system conditions, but a consequence of noninstantaneous growth of opal. Combined U–Pb and ²³⁰Th/U ages support the model of slow mineral deposition at the rates of millimeters per million years resulting in layering on a scale too fine for mechanical sampling. In this case, U–Pb ages provide more accurate estimates of the average age for mixed multiage samples than ²³⁰Th/U ages, because ages based on shorter-lived isotopes are nonlinearly biased by younger mineral additions.</p> <br/> <p>Use of the combined U–Th–Pb technique to date Yucca Mountain Quaternary opals significantly extends the age range beyond that of the ²³⁰Th/U dating method and shows that selected fracture pathways in the unsaturated zone felsic tuffs of Yucca Mountain have been active throughout the Quaternary.</p> application/pdf 10.1016/S0016-7037(00)00408-7 en Elsevier 206Pb-230Th-234U-238U and 207Pb-235U geochronology of Quaternary opal, Yucca Mountain, Nevada article