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- W2018207410 abstract "Extreme U and Pb isotope variations produced by disequilibrium in decay chains of 238U and 232Th are found in calcite, opal/chalcedony, and Mn-oxides occurring as secondary mineral coatings in the unsaturated zone at Yucca Mountain, Nevada. These very slowly growing minerals (mm my−1) contain excess 206Pb and 208Pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206Pb/238U geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect 207Pb/235U ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages.Opal and calcite from outer (younger) portions of coatings have 230Th/U ages from 94.6 ± 3.7 to 361.3 ± 9.8 ka and initial 234U/238U activity ratios (AR) from 4.351 ± 0.070 to 7.02 ± 0.12, which indicate 234U enrichment from percolating water. Present-day 234U/238U AR is ∼1 in opal/chalcedony from older portions of the coatings. The 207Pb/235U ages of opal/chalcedony samples range from 0.1329 ± 0.0080 to 9.10 ± 0.21 Ma, increase with microstratigraphic depth, and define slow long-term average growth rates of about 1.2–2.0 mm my−1, in good agreement with previous results. Measured 234U/238U AR in Mn-oxides, which pre-date the oldest calcite and opal/chalcedony, range from 0.939 ± 0.006 to 2.091 ± 0.006 and are >1 in most samples. The range of 87Sr/86Sr ratios (0.71156–0.71280) in Mn-oxides overlaps that in the late calcite. These data indicate that Mn-oxides exchange U and Sr with percolating water and cannot be used as a reliable dating tool.In the U-poor calcite samples, measured 206Pb/207Pb ratios have a wide range, do not correlate with Ba concentration as would be expected if excess Ra was present, and reach a value of about 1400, the highest ever reported for natural Pb. Calcite intergrown with opal contains excesses of both 206Pb and 207Pb derived from Rn diffusion and from direct α-recoil from U-rich opal. Calcite from coatings devoid of opal/chalcedony contains 206Pb and 208Pb excesses, but no appreciable 207Pb excesses. Observed Pb isotope anomalies in calcite are explained by Rn-produced excess Pb. The Rn emanation may strongly affect 206Pb–238U ages of slow-growing U-poor calcite, but should be negligible for dating fast-growing U-enriched speleothem calcite." @default.
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- W2018207410 date "2004-11-01" @default.
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- W2018207410 title "Reply to the comment on “Physical and stable-isotope evidence for formation of secondary calcite and silica in the unsaturated zone, Yucca Mountain, Nevada”, by Y.V. Dublyansky, S.E. Smirnov and G.P. Palyanova" @default.
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- W2018207410 doi "https://doi.org/10.1016/j.apgeochem.2004.04.006" @default.
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