| Abstract: | 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. 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 (2??) to 1772 ?? 40 ka are systematically older than 230Th/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 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. 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. Copyright (C) 2000 Elsevier Science Ltd. |
| Genre: | Article |
| ProdID: | 70022371 |
| Citation Author: | Neymark, L. A.; Amelin, Y. V.; Paces, J. B. |
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| Citation End Page: | 2928 |
| Citation Issue: | 17 |
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| Citation Language: | English |
| Citation Larger Work Title: | Geochimica et Cosmochimica Acta |
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| Citation Number Of Pages: | 16 |
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| Citation Search Results Text: | 206Pb-230Th-234U-238U and 207Pb-235U geochronology of Quaternary opal, Yucca Mountain, Nevada; 2000; Article; Journal; Geochimica et Cosmochimica Acta; Neymark, L. A.; Amelin, Y. V.; Paces, J. B. |
| Citation Start Page: | 2913 |
| Citation Volume: | 64 |
| Citation Year: | 2000 |
| Type: | citation/reference |
| Text: | 206Pb-230Th-234U-238U and 207Pb-235U geochronology of Quaternary opal, Yucca Mountain, Nevada; 2000; Article; Journal; Geochimica et Cosmochimica Acta; Neymark, L. A.; Amelin, Y. V.; Paces, J. B. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| URL (DIGITAL OBJECT IDENTIFIER): | http://dx.doi.org/10.1016/S0016-7037(00)00408-7 |
| Date Other: | Sat, 1 Jan 2000 00:00 -0600 |
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