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As the internal precision of radiometric dates approaches the 0.1% level, systematic biases between different methods have become apparent. Many workers have suggested that calibrating other decay constants against the U–Pb system is a viable solution to this problem. We test this assertion empirically and quantitatively by analyzing U–Pb systematics of zircon and xenotime on the single- to sub-grain scale by high-precision ID-TIMS geochronology on 11 rock samples ranging from 0.1 to 3.3Ga. Large statistically equivalent datasets give ²⁰⁷Pb/²⁰⁶Pb dates that are systematically older than ²⁰⁶Pb/²³⁸U dates by ∼0.15% in Precambrian samples to as much as ∼3.3% in Mesozoic samples, suggesting inaccuracies in the mean values of one or both of the U decay constants. These data are used to calculate a ratio of the U decay constants that is lower than the accepted ratio by 0.09% and is a factor of 5 more …