Pb-206-Th-230-U-234-U-238 and Pb-207-U-235 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 coatin gs preserve a record of paleohydrologic conditions at this site, which is b eing evaluated as a potential high-level nuclear waste repository. The opal precipitated from groundwater is variably enriched in U-234 (measured U-23 4/U-238 activity ratio 1.124-6.179) and has high U (30-313 ppm), low Tn (0. 008-3.7 ppm), and low common Pb concentrations (measured Pb-206/Pb-204 up t o 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-206*/U-238 and Pb-207*/U-235 ratios displa y strong reverse discordance because of excess radiogenic Pb-206* derived f rom the elevated initial 234U. The data are best interpreted using projecti ons of a new four-dimensional concordia diagram defined by Pb-206*/U-238, P b-207*/U-235, U-238(activity), and Th-230/U-238(activity). Ages and initial U-234/U-238 activity ratios have been calculated using different projectio ns of this diagram and tested for concordance. The data are discordant, tha t is observed Pb-207*/U-235 ages of 170 +/- 32 (2 sigma) to 1772 +/- 40 ka are systematically older than Th-230/U ages of 34.1 +/- 0.6 to 452 +/- 32 k a. The age discordance is not a result of migration of uranium and its decay p roducts under the open system conditions, but a consequence of noninstantan eous growth of opal. Combined U-PI, and Th-230/U ages support the model of slow mineral deposition at the rates of millimeters per million years resul ting 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 mul tiage samples than Th-230/U ages, because ages based on shorter-lived isoto pes are nonlinearly biased by younger mineral additions. Use of the combined U-Th-Pb technique to date Yucca Mountain Quaternary opa ls significantly extends the age range beyond that of the Th-230/U dating m ethod and shows that selected fracture pathways in the unsaturated zone fel sic tuffs of Yucca Mountain have been active throughout the Quaternary. Cop yright (C) 2000 Elsevier Science Ltd.