Natural radionuclide mobility and its influence on U-Th-Pb dating of secondary minerals from the unsaturated zone at Yucca Mountain, Nevada

Extreme U and Pb isotope variations produced by disequilibrium in decay chains of U-238 and Th-232 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 Pb-206 and Pb-208 formed from excesses of intermediate daughter isotopes and cannot be used as reliable Pb-206/U-238 geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect Pb-207/U-235 ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages. Opal and calcite from outer (younger) portions of coatings have Th-230/U ages from 94.6 +/- 3.7 to 361.3 +/- 9.8 ka and initial U-234/U-238 activity ratios (AR) from 4.351 +/- 0.070 to 7.02 +/- 0.12, which indicate U-234 enrichment from percolating water. Present-day U-234/U-238 AR is similar to 1 in opal/chalcedony from older portions of the coatings. The Pb-207/U-235 ages of opal/chalcedony samples range from 0.1329 +/- 0.0080 to 9.10 +/- 0.21 Ma, increase with micro stratigraphic depth, and define slow longterm average growth rates of about 1.2-2.0 mm my(-1), in good agreement with previous results. Measured U-234/U-238 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 Sr-87/Sr-86 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 Pb-206/Pb-207 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 Pb-206 and Pb-207 derived from Rn diffusion and from direct alpha-recoil from U-rich opal. Calcite from coatings devoid of opal/chalcedony contains Pb-206 and Pb-208 excesses, but no appreciable Pb-207 excesses. Observed Pb isotope anomalies in calcite are explained by Rn-produced excess Pb. The Rn emanation may strongly affect Pb-206-U-238 ages of slow-growing U-poor calcite, but should be negligible for dating fast-growing U-enriched spelcothem calcite. Published by Elsevier Ltd.