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

Abstract

We have re-determined the 234 U and 230 Th half-lives to be 245,250±490 years (2 σ) and 75,690±230 years (2 σ), respectively. Using high precision thermal ionization mass spectrometric (TIMS) methods, we measured 234 U/ 238 U and 230 Th/ 238 U atomic ratios in 4 different materials that were likely to have behaved as closed systems for 10 6 years or more: zircons with concordant 238 U– 206 Pb , 235 U– 207 Pb , and 232 Th– 208 Pb 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 234 U/ 238 U values for all measured materials and consistent 230 Th/ 238 U 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 234 U half-life is about 3‰ higher than that commonly used in 230 Th dating laboratories and our 230 Th half-life is about 4‰ higher. 230 Th 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 230 Th age range because our revised 234 U and 230 Th 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.