The half-lives of uranium-234 and thorium-230

We have re-determined the ²³⁴U and ²³⁰Th half-lives to be 245,250 ± 490 years (2σ) and 75,690 ± 230 years (2σ), respectively. Using high precision thermal ionization mass spectrometric (TIMS) methods, we measured ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U atomic ratios in 4 different materials that were likely to have behaved as closed systems for 10⁶ years or more: zircons with concordant ²³⁸U-²⁰⁶Pb, ²³⁵U-²⁰⁷Pb, and ²³²Th-²⁰⁸Pb ages, Iceland Spar, Table Mountain Latite, and aliquots of a solution of Harwell uraninite (HU-1). We calibrated the TIMS multipliers using U-500, U and Th gravimetric standards, and U double spike. Consistent ²³⁴U/²³⁸U values for all measured materials and consistent ²³⁰Th/²³⁸U values for all materials with the exception of our HU-1 solution support the secular equilibrium status. The new half-lives agree within error with previously determined values; however, errors in our values are generally smaller than those in the earlier determinations. Our ²³⁴U half-life is about 3‰ higher than that commonly used in ²³⁰Th dating laboratories and our ²³⁰Th half-life is about 4‰ higher. ²³⁰Th ages calculated with the new half-lives are generally older than those calculated with the previously used half-lives. The difference in age, though, is small throughout the ²³⁰Th age range because our revised ²³⁴U and ²³⁰Th half-lives are offset from earlier values in the same sense (both to higher values). In the case of dating materials older than 350 ka in laboratories that rely solely on gravimetric standardization procedures, use of our decay constants and their associated errors will considerably reduce the errors in age arising from uncertainty in the decay constants. (C) 2000 Elsevier Science B.V. All rights reserved.