U-PB DATING OF OPAL IN DEEPLY BURIED SOILS, YUCCA MOUNTAIN, NEVADA, USING SECONDARY-ION MASS SPECTROMETRY

The uranium-series method commonly is used to date carbonate and silica components of desert soils ranging in age from a few ka to about 350 ka by ²³⁰Th/U or up to 1 Ma using ²³⁴U/²³⁸U model ages assuming initial U isotopic compositions. Methods that allow direct dating of soils older than 1 Ma have not been widely developed or applied, even though many such soils are present in the southwestern U.S.  This work presents initial U-Pb dating results of pedogenic opal older than 1 Ma using secondary-ion mass spectrometry (SIMS).

Opal samples were collected from calcite/silica cements or rinds on rock clasts from core samples of basal alluvium at depths 27 to 37 m below the land surface in Midway Valley, Nevada, adjacent to the proposed high-level nuclear waste repository at Yucca Mountain.  U-series thermal-ionization mass-spectrometry (TIMS) data on samples of these pedogenic materials indicate that ages are older than the dating range of the ²³⁰Th/U method. The high U (tens of µg/g) and low common-Pb and Th concentrations, and negligible radiogenic ²⁰⁸Pb allow ²⁰⁶Pb/²⁰⁸Pb-²³⁸U/²⁰⁸Pb isochron dating of 30-µm-diameter spots on polished cross sections by SIMS using the USGS-Stanford Sensitive High Resolution Ion Microprobe - Reverse Geometry. Isotope analyses of 7 to 17 spots on multiple fragments for each of 5 separate samples yield ²⁰⁶Pb/²³⁸U isochron ages of 1.6 to 2.7 Ma. Measured ²³⁴U/²³⁸U ratios for the same SIMS analytical sessions are within error of secular equilibrium values, in agreement with > 1 Ma opal ages. However, previous TIMS data for younger pedogenic opal in the area indicate the presence of excess initial ²³⁴U, which would have produced excess ²⁰⁶Pb that is unsupported by measured ²³⁸U.  Therefore, corrections for ²⁰⁶Pb excess were applied using an initial ²³⁴U/²³⁸U activity ratio of 1.6, determined by TIMS ²³⁰Th/U dating of younger soils in Midway Valley and assuming that this ratio remained constant. Corrected ²⁰⁶Pb/²³⁸U isochron ages are about 10 percent younger and yield a weighted average age of 1.9±0.2 Ma for the 5 samples of opal from the basal alluvium across much of Midway Valley. Because the erosional surface apparently is not offset across a number of bedrock faults, this U-Pb age for the overlying alluvium can constrain the most recent episode of faulting to be older than 1.9 million years.