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

How to Get a Date Radiometric dating relies on measuring the abundance of a radioactive isotope and/or its decay products. By knowing a decay rate and an isotopic starting abundance—both assumed to be constant—an age is determined. Using high-resolution mass spectrometry, Brennecka et al. (p. 449, published online 31 December; see the Perspective by Connelly) show that the known starting abundance of 238U and 235U isotopes in meteorites, which decay into 206Pb and 207Pb, respectively, is actually quite variable. Trace amounts of 247Cm in the early solar system may have unexpectedly contributed additional 235U, skewing the ratio. Pb-Pb dating, the method commonly used to date early solar system materials, may thus need a correction of up to 5 million years. Variable abundances of meteorite isotopes may require correcting the lead-based age of the solar system by 5 million years. 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 (CAIs) 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 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 238U/235U in CAIs 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.