2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 8:00 AM-12:00 PM

U-SERIES ISOTOPES, WATER/ROCK INTERACTION, AND EVIDENCE OF UNSATURATED-ZONE FLOW AT YUCCA MOUNTAIN, NEVADA


PACES, James B., U.S. Geol Survey, Box 25046, MS 963, Denver Federal Center, Denver, CO 80225 and NEYMARK, Leonid A., S.M Stoller Corp. c/o U.S. Geol Survey, MS 963, Box 25046, Denver Federal Center, Denver, CO 80225-0046, jbpaces@usgs.gov

234U/238U and -230Th/238U ratios determined by TIMS in both water and volcanic rocks vary throughout the >500-m-thick unsaturated zone (UZ) at Yucca Mountain. Infiltration, with 234U/238U activity ratios (AR) of about 1.5 to 1.8, is modified as it percolates downward by addition of 234U via a-recoil and preferential leaching from pore walls and fracture surfaces. Water in shallower parts of the UZ commonly has 234U/238U AR of 2 to 4, whereas values of 7 to 10 are common at greater depths. Water perched in welded tuffs near the base of the UZ also has large 234U/238U AR (7 to 8), as does ground water in the upper parts of the saturated zone. In contrast to water, 234U is depleted in whole-rock samples throughout the UZ. 234U/238U AR typically ranges from 0.94 to 1.0, although values reach as low as 0.76. Most whole-rock analyses have 230Th/234U AR from 0.92 to 1.02 and plot close to the equiline and infinite-age lines on 234U/238U vs. 230Th/238U AR plots. Systematic differences in the degree of disequilibrium are not observed between subsamples collected from fracture surfaces or fragment interiors. However, samples from the footwall of the Bow Ridge fault, a zone of focused percolation, plot well to the right of the equiline with larger 230Th/234U AR (1.07 to 1.22) than samples away from the fault.

Results imply that U-series isotopes respond differently to water/rock interactions under different UZ hydrologic conditions. Progressive increases in 234U/238U AR with depth in UZ pore and fracture water are consistent with preferential leaching involving small water/rock ratios. Simple steady-state U-removal models can simulate some, but not all, of the observed whole-rock compositions. Long-term leaching leads to 234U-230Th-238U compositions that reflect differences in small amounts of 238U loss and significant 234U/238U fractionation. Compositions plotting to the left of the equiline require Th loss or U gain. In contrast, large 230Th/234U ratios observed at the Bow Ridge fault indicate a more aggressive hydrologic environment with greater 238U leaching and less fractionation of 234U/238U. These studies may help define zones of increased ground-water percolation, constrain water-flux models using mass balances, and quantify the extent of Quaternary water-table fluctuations.