PII S 0016 - 7037 ( 00 ) 00408 - 7 206 Pb – 230 Th – 234 U – 238 U and 207 Pb – 235 U geochronology of Quaternary opal , Yucca Mountain , Nevada

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 U (measured U/U 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 Pb/Pb 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. The opal ages calculated from Pb*/U and Pb*/U ratios display strong reverse discordance because of excess radiogenic Pb* derived from the elevated initial U. The data are best interpreted using projections of a new four-dimensional concordia diagram defined by Pb*/U, Pb*/U, U/ Uactivity, and Th/Uactivity. Ages and initial U/U activity ratios have been calculated using different projections of this diagram and tested for concordance. The data are discordant, that is observed Pb*/U ages of 170 6 32 (2s) to 1772 6 40 ka are systematically older than Th/U ages of 34.1 6 0.6 to 452 6 32 ka. 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 Th/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 Th/U ages, because ages based on shorter-lived isotopes are nonlinearly biased by younger mineral additions. Use of the combined U–Th–Pb technique to date Yucca Mountain Quaternary opals significantly extends the age range beyond that of the Th/U dating method and shows that selected fracture pathways in the unsaturated zone felsic tuffs of Yucca Mountain have been active throughout the Quaternary. Copyright © 2000 Elsevier Science Ltd