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 U-234 and Th-230 on Faraday cups with precisions of 1-3 epsilon units (1 epsilon-unit=1 part in 10(4)) 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, U-234/U-238=54.970 (+/- 0.019) x 10(-6) and Th-230/U-238=16.916 (+/- 0.018) x 10(-6), for these calcite samples were identical within errors (quoted 2 sigma 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 +/- 260 a for U-234 and 75,584 +/- 110 a for Th-230 (quoted 2 sigma uncertainties calculated using all sources of error). In calculating a Th-230 age, some of the systematic errors included in estimating the full error in the half-lives effectively cancel. Removing these uncertainties (uncertainty in the U-238 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 Th-230 dating of +/- 70 a for the U-234 half-life value and +/- 30 a for the Th-230 half-life value. Under ideal circumstances, with our methods, the 2 sigma uncertainty in age, including uncertainty in half-life values is +/- 10 a at 10 ka, +/- 100 a at 130 ka, +/- 300 a at 200 ka, +/- 1 ka at 300 ka, +/- 2 ka at 400 ka, +/- 6 ka at 500 ka, and +/- 12 ka at 600 ka. The isotopic composition of a sample with an age < 800 ka 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 640 ka ago. As the half-life values were determined independent of the Sanbao Cave ages, the observed co-variation between stalagmite delta O-18 and Northern Hemisphere summer insolation is consistent with accurate ages and half-life values. (c) 2013 Elsevier B.V. All rights reserved.