VARIATIONS IN PLEISTOCENE FRACTURE FLOW AT YUCCA MOUNTAIN, NEVADA, INVESTIGATED BY ION MICROPROBE U-SERIES DATING AND CATHODOLUMINESCENCE OF UNSATURATED-ZONE OPAL

Requirements for radioactive waste disposal in the unsaturated zone at Yucca Mountain include isolation of radionuclides for tens to hundreds of thousands of years. Therefore, an increased understanding of Pleistocene hydrologic variability will help predict responses to future environmental changes. Secondary opal and calcite, present as mm- to cm-thick coatings on a small percentage of open cavities, precipitated from water flowing through fractures in the unsaturated zone. Previous studies have shown that these minerals grew slowly (0.5 to 5 mm/m.y.) over the past 10 million years at relatively constant long-term average growth rates. The high U (tens to hundreds of µg/g) and negligible Th concentrations in Yucca Mountain opals allow dating of 30-µm-diameter spots on polished cross sections by ion microprobe (http://shrimprg.stanford.edu) using ²³⁰Th/U ages for opal younger than 200 ka and model ²³⁴U/²³⁸U ages for opal as old as 1.5 Ma. Although precision for isotope ratios is limited to 5 to 25% because of the small ²³⁰Th¹⁶O and ²³⁴U¹⁶O ion-beam intensities, ages increase with depth across 1-mm-thick opal profiles and show good replication of multiple analyses from individual growth bands.

Images of opal cathodoluminescence show light-dark microbanding caused by up to 10-fold variations in U concentrations. Patterns of higher and lower U concentrations are traceable across single opal grains and are replicated in opal from different sites tens of meters apart. U-series ages for opal show that at least some of the cyclic light-dark pairs have an approximate 100-k.y. periodicity. Initial ²³⁴U/²³⁸U calculated for spots with ²³⁰Th/U ages less than about 200 ka remain constant in some opal grains, but vary systematically with age in others. Variations in U concentration and initial ²³⁴U/²³⁸U are most likely related to climate-induced variations in percolation flux and associated bulk-U dissolution. Although estimates of opal growth rates are variable on a 100-k.y. time scale, long-term average growth rates calculated from these data are consistent with rates determined previously. These age and isotopic records for Pleistocene opal can be used to correlate changes in effective moisture at the surface with fracture flow at the depth of the proposed repository.