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

We have re-determined the U-234 and Th-230 half-lives to be 245,250 +/- 490 years (2 sigma) and 75,690 +/- 230 years (2 sigma), respectively. Using high precision thermal ionization mass spectrometric (TIMS) methods, we measured U-234/U-238 and Th-230/U-238 atomic ratios in 4 different materials that were likely to have behaved as closed systems for 10(6) years or more: zircons with concordant U-238-Pb-206, U-235-Pb-207, and Th-232-Pb-208 ages, Iceland Spar, Table Mountain Latite, and aliquots of a solution of Harwell uraninite (HU-I), We calibrated the TIMS multipliers using U-500, U and Th gravimetric standards, and U double spike. Consistent U-234/U-238 values for all measured materials and consistent Th-230/U-238 values for all materials with the exception of our HU-I 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 U-234 half-life is about 3 parts per thousand higher than that commonly used in Th-230 dating laboratories and our Th-230 half-life is about 4 parts per thousand higher. Th-230 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 Th-230 age range because our revised U-234 and Tn 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.