238U/235U Variations in Meteorites: Extant 247Cm and Implications for Pb-Pb Dating

The U-238/U-235 isotope ratio has long been considered invariant in meteoritic materials (equal to 137.88). This assumption is a cornerstone of the high-precision lead-lead dates that define the absolute age of the solar system. Calcium-aluminum-rich inclusions (CAIs) of the Allende meteorite display variable U-238/U-235 ratios, ranging between 137.409 +/- 0.039 and 137.885 +/- 0.009. This range implies substantial uncertainties in the ages that were previously determined by lead-lead dating of CAIs, which may be overestimated by several million years. The correlation of uranium isotope ratios with proxies for curium/uranium (that is, thorium/uranium and neodymium/uranium) provides strong evidence that the observed variations of U-238/U-235 in CAIs were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial Cm-247/U-235 ratio of approximately 1.1 x 10(-4) to 2.4 x 10(-4).