<sup>238</sup> U/ <sup>235</sup> U Variations in Meteorites: Extant <sup>247</sup> Cm 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 238 U and 235 U isotopes in meteorites, which decay into 206 Pb and 207 Pb, respectively, is actually quite variable. Trace amounts of 247 Cm in the early solar system may have unexpectedly contributed additional 235 U, 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.