²³⁸U/²³⁵U variations in meteorites: Extant ²⁴⁷Cm and implications for Pb-Pb dating

The ²³⁸U/²³⁵U 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 (CAls) of the Allende meteorite display variable ²³⁸U/²³⁵U 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 CAls, 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 ²³⁸U/²³⁵U in CAls were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial ²⁴⁷Cm/²³⁵U ratio of approximately 1.1 × 10^-4 to 2.4 × 10^-4.