Uranium-Series Dating of Speleothems: Current Techniques, Limits, & Applications

Abstract

Carbonate speleothems that contain ppb-ppm levels of uranium can be dated by the 238U-234U-230Th and 235U-231Pa disequilibrium techniques. Accurate ages are possible if the initial concentrations of 230Th and 231Pa are well constrained and if the system has remained closed to post-depositional exchange of uranium, thorium, and protactinium. An estimate of the plausible range of initial 230Th/232xh values combined with the measured 232Th/238U ratio reveal, to first order, whether initial 230Th is significant. If significant, initial 230Th can be constrained by monitoring 232Th and employing isochron techniques to closely estimate the value of the contaminant 230Th/232Th ratio. The sensitivity of the age error to uncertainties in the initial 230Th/232Th ratio decreases with increasing U concentration, increasing age, and decreasing detrital contamination. For many speleothems, particularly those composed of nearly pure calcite, initial 230Th may be trivial. Because Pa is more soluble than Th, 232Th is a poor analog for 231Pa. Therefore, initial 231Pa tends to be more significant than initial 230Th for young samples, although this problem becomes less significant or even insignificant with increasing age. Closed-system behavior can be judged by petrographic considerations, stratigraphic ordering of ages, and 230Th-231Pa concordance. Thermal ionization mass spectrometry (TIMS) and inductively coupled plasma mass spectrometry (ICP-MS) offer significant improvements in counting efficiency and sample throughput compared to traditional decay-counting techniques. For ∼ 100 mg samples containing ∼ 1.0 ppm 238U, bestscenario 230Th age errors, based on the analytical errors (2a) of current TIMS and ICP-MS techniques, are approximately: 500 ± 6 yrs, 10,000 ± 40 yrs, 50,000 ± 180 yrs, 120,000 ± 500 yrs, and 500,000 ± 15,000 yrs. Materials as young as tens of years and older than 600,000 years are potentially dateable by the 238U-234U-230Th method.