Geology topics
Yuri V. Amelin
Publications and source records attributed to Yuri V. Amelin.
206Pb-230Th-234U-238U and 207Pb-235U 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 234 U (measured 234 U/ 238 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 206 Pb/ 204 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 206 Pb∗/ 238 U and 207 Pb∗/ 235 U ratios display strong reverse discordance because of excess radiogenic 206 Pb∗ derived from the elevated initial 234 U. The data are best interpreted using projections of a new four-dimensional concordia diagram defined by 206 Pb∗/ 238 U, 207 Pb∗/ 235 U, 234 U/ 238 U activity , and 230 Th/ 238 U activity . Ages and initial 234 U/ 238 U activity ratios have been calculated using different projections of this diagram and tested for concordance. The data are discordant, that is observed 207 Pb∗/ 235 U ages of 170 ± 32 (2σ) to 1772 ± 40 ka are systematically older than 230 Th/U ages of 34.1 ± 0.6 to 452 ± 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 230 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 230 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 230 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.