Origin of uranium isotope variations in early solar nebula condensates

High-temperature condensates found in meteorites display uranium isotopic variations (235U/238U) that complicate dating the solar system's formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide 247Cm (t1/2 = 15.6 My) into 235U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of 235U reaching ∼+6% relative to average solar system composition, which can only be due to the decay of 247Cm. This allows us to constrain the 247Cm/235U ratio at solar system formation to (7.0 ± 1.6) × 10-5. This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture.