Improvements in ²³⁰Th dating, ²³⁰Th and ²³⁴U half-life values, and U-Th isotopic measurements by multi-collector inductively coupled plasma mass spectrometry

We have developed techniques for measuring ²³⁴U and ²³⁰Th on Faraday cups with precisions of 1-3 epsilon units (1 ε-unit=1 part in 10⁴) using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Using a Thermo-Scientific Neptune with desolvation nebulization, we obtained ionization/transmission efficiencies of 1-2% for both U and Th. We set up protocols to correct for tailing, prepared U and Th gravimetric standards, tested a Th mass fractionation correction procedure based on U isotopes, and identified natural calcite samples likely to be in U-Th isotopic secular equilibrium. The measured atomic ratios, ²³⁴U/²³⁸U=54.970 (±0.019)×10^-6 and ²³⁰Th/²³⁸U=16.916 (±0.018)×10^-6, for these calcite samples were identical within errors (quoted 2σ uncertainties calculated combining all sources of error). Half-life values calculated from these ratios are consistent with previous values, but have much smaller errors: 245,620±260a for ²³⁴U and 75,584±110a for ²³⁰Th (quoted 2σ uncertainties calculated using all sources of error). In calculating a ²³⁰Th age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the ²³⁸U half-life value, uncertainty in our gravimetric uranium and thorium standards, and uncertainty in the absolute isotopic composition of the uranium standard), yields effective uncertainties for the purposes of ²³⁰Th dating of ±70a for the ²³⁴U half-life value and ±30a for the ²³⁰Th half-life value. Under ideal circumstances, with our methods, the 2σ uncertainty in age, including uncertainty in half-life values is ±10a at 10ka, ±100a at 130ka, ±300a at 200ka, ±1ka at 300ka, ±2ka at 400ka, ±6ka at 500ka, and ±12ka at 600ka. The isotopic composition of a sample with an age <800ka can clearly be resolved from the isotopic composition of a sample in secular equilibrium, assuming closed system behavior. Using these techniques, we analyzed a Sanbao Cave (Hubei, China) stalagmite that formed between 510 and 640ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite δ¹⁸O and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values.