FURTHER EVIDENCE FOR SLOW OPAL GROWTH IN YUCCA MOUNTAIN UNSATURATED ZONE: U-PB ION-MICROPROBE DATA

Secondary calcite and silica in felsic tuffs at Yucca Mountain (YM), Nevada, record a history of fracture water movement through the host rock of the proposed high-level nuclear waste repository. Previously determined variability of ages of YM opal, fine layering, and very slow rates of deposition resulted in attempts to improve spatial resolution of sampling by minimizing sample sizes for U-Pb and U-series dating using conventional thermal ionization mass spectrometry (TIMS) analysis. Sensitive high-resolution ion-microprobe (SHRIMP) methods then were used, and Paces et al. (GSA Annual Meeting Abstr., 2000) presented the first U-series SHRIMP analyses of YM opals.

This present study investigates the potential for U-Pb SHRIMP dating of YM opal at a spatial resolution of about 40 microns. All samples were collected in the Exploratory Studies Facility tunnel constructed in the YM unsaturated zone and analyzed using the SHRIMP-II at Curtin University of Technology, Perth, Western Australia. The analytical procedure was designed to use both U-Pb and U-series systematics, and it was shown that U-Pb ages could be determined with errors less than 10% for U-enriched growth layers in opal.

The new ion-microprobe U-Pb age data support previous conclusions, based on TIMS and SHRIMP U-series and TIMS U-Pb results, that secondary calcite and silica deposition in the YM unsaturated zone began soon after the emplacement of the host tuffs at 12.8 Ma and continued at long-term average rates of £ 5 millimeters per million years throughout the history of YM. Ion-microprobe U-series and U-Pb data simultaneously obtained from single spots change systematically with microstratigraphic depth and do not show appreciable excesses of ²⁰⁶Pb unsupported by ²³⁸U and ²³⁴U decay. This evidence supports the reliability of U-Pb opal age results for YM, which recently were questioned by Dublyansky et al. (J. Geochem. Explor., 2003).