Multi-Step CA-TIMS U-Pb Zircon Analysis of the Relative Decay Constants of 235U and 238U

The U decay constants of Jaffey et al. (1971) are widely regarded as the "gold standard" for geochronology, given their exceptional precision and accuracy compared with earlier U decay constant measurements and with the decay constants for other isotopic systems used in geochronology. Several years ago, it became clear that analytical methods had improved to the point where measurements of carefully selected natural zircon samples could provide a useful independent check of the accuracy of the 235U and 238U decay constants relative to each other. Using this approach, Mattinson (2000) and Schoene et al. (2006) both determined that the accepted decay constant for 235U is too low by ca. 0.09%, resulting in 207Pb/206Pb ages that are systematically too high, relative to 206Pb/238U ages. The relative accuracy of the two decay constants is crucial for the interpretation of "concordance", i.e., agreement of ages calculated from the two independent decay systems. The present study applies detailed multi-step CA-TIMS measurements to a suite of zircon samples selected for apparent perfect concordance; no Pb loss after removal of radiation- damaged zircon by early CA-TIMS steps, and no evidence of any older inherited component. Most samples were selected from a 400Ma to 600Ma age range, a range where errors from tracer calibration uncertainties and corrections for intermediate daughter product disequilibrium are both low. The new measurements strongly confirm the ca. 0.09% error in the 235U decay constant, relative to the Jaffey et al. (1971) 238U decay constant. Use of this recommended correction of the 235U decay constant will allow more accurate assessment of concordant to near-concordant zircon data, and also more accurate concordia intercept ages for more discordant data.