CONSTRAINING DEPOSITIONAL AGES OF ALLUVIUM BY ION MICROPROBE U-PB DATING OF DETRITAL AND AUTHIGENIC OPAL

Alluvium in Midway Valley in southern Nevada was studied to evaluate the possible location of surface facilities for handling high-level radioactive waste. Milky white to bluish grey opal/chalcedony derived from nearby felsic tuffs is present in the alluvium as pebbles and fracture-coatings within volcanic clasts. This opal originally formed as secondary coatings in subsurface fractures and cavities from meteoric water percolating through the rocks after their eruption at 11-13 Ma. Dating this detrital opal thus provides a maximum depositional age for these otherwise undatable clastic sediments. Authigenic opal is also present as pedogenic cements formed in surface soils or as unsaturated-zone fracture minerals deposited after burial. Dating of this material helps constrain the minimum depositional age of the alluvium. Opal from this environment is a viable U-Th-Pb geochronometer because of its high U concentrations (tens of µg/g), very low common Th and Pb concentrations, and minimal amount of post-depositional water-rock interaction within the unsaturated zone. Detrital and authigenic opals were dated by U-Pb method using the Stanford-USGS Sensitive High-Resolution Ion Microprobe – Reversed Geometry (SHRIMP-RG, http://shrimprg.stanford.edu).

Samples of opal were collected from drill core near the alluvium/bedrock interface 25–52 meters below land surface. Because of their high U/Pb and U/Th ratios, opals have negligible radiogenic ²⁰⁸Pb allowing ²⁰⁶Pb/²⁰⁸Pb-²³⁸U/²⁰⁸Pb isochron dating by SHRIMP-RG. The 18–90 nA primary ¹⁶O^- beam produced pits ~30 µm in diameter and 15–20 µm deep, yielding ~30 nanograms of opal consumed during a 25-minute spot analysis. Measured Pb/U ratios were corrected for instrument bias determined from U-Pb isochron slopes obtained for a 1.915-Ma opal standard analyzed during the same analytical sessions. Ages determined by both SHRIMP-RG and conventional ID-TIMS agree within error. Results of this study constrain the age of basal alluvium within Midway Valley to between 7.29±0.64 Ma (youngest age for detrital opal) and 3.20±0.42 Ma (oldest age for authigenic opal).