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.
Bibliographical noteFunding Information:
We are grateful to the valuable support of Prof. Alexander Klimchouk for sample Kr3, and Mr. Peter Giangrande for all green calcites; Dr. Stephan Richter for providing detailed information about the U standard IRMM-074/10; Prof. Randall R. Parrish for providing detailed instruction for zircon dissolution; and Yu-Te Hsieh for laboratory assistance. This work was supported by US NSF 0502535 , NBRPC 2013CB955902 , NSFC 41230524 , US NSF 0902867 , 0908792 , 1103320 , and 1103403 , Gary Comer Science and Education Foundation Grants CC8 and CP41 , and NSC Grants 97-2745-M-002-009 and 100-2116-M-002-009 .
Copyright 2013 Elsevier B.V., All rights reserved.
- Dating accuracy
- Th dating
- Th half-life
- U half-live