Analysis of the relative decay constants of 235U and 238U by multi-step CA-TIMS measurements of closed-system natural zircon samples

Abstract

The U decay constants (λ238U and λ235U) of Jaffey et al., 1971, Phys. Rev C 4, 1889–1906, are widely regarded as the “gold standard” for U–Pb geochronology and geochronology in general, given their exceptional precision and accuracy compared with all earlier U decay constant measurements and with the decay constants for all other isotopic systems used in geochronology.However, several years ago it became clear that U–Pb analytical methods had improved to the point where even these small uncertainties in the decay constants had become the limiting factor in the accuracy and precision of measured U–Pb ages of the mineral zircon. Earlier work indicates that the accepted value of λ235U is slightly low relative to the accepted value for λ238U, and the accepted value of 137.88 for the 238U/235U isotopic ratio of natural U. The present study applies detailed multi-step CA-TIMS measurements to a suite of zircon samples selected for apparent perfect concordance except for small deviations that could be explained by errors in the constants used in age calculations. Most samples were selected from a 400Ma–560Ma age range, a range where errors from tracer calibration uncertainties and corrections for intermediate daughter isotope disequilibrium are both low. The new measurements yield a value for λ235U=0.98574±0.00021Ga−1. The weighted mean of this new result and two other recent results is 0.98571±0.00012Ga−1. Remarkably, this just overlaps the almost four-decade-old Jaffey et al. (1971) result of 0.98485±0.00135Ga−1 within the stated errors of that study, but the new result is a full order of magnitude more precise. Thus, adoption of the new value for U–Pb geochronology is strongly recommended. Recent research also suggests the need for revision of the 238U/235U isotopic composition of natural U. If a new value for the 238U/235U isotopic ratio is adopted, the λ235U value can be adjusted accordingly. Errors in the 238U/235U propagate over a range of geologic ages quite differently from errors in the decay constants, so it is possible to differentiate between these two effects.