Philip E. Janney

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.

Tungsten Nuclear Anomalies in Planetesimal Cores

Use of the extinct 182 Hf- 182 W chronometer to constrain the timing of planetary accretion and differentiation rests ontheassumptionthatthesolarnebulahadhomogeneoustungstenisotopiccomposition.Here,wereportdeficiencies of � 0.1 part in 10,000 in the abundance of 184 Win group IVB iron meteorites relative to the silicate Earth. These are mostlikelyduetoincompletemixingattheplanetesimalscale(2Y4kmradiusbodies)oftheproductsof slow(s-)and rapid (r-) neutron-capture nucleosynthesis in the solar nebula. The correction that must be applied to the 182 Hf- 182 W model age of core formation in IVB irons due to the presence of these nuclear anomalies is � 0.5 Myr. Subject headingg minor planets, asteroids — nuclear reactions, nucleosynthesis, abundances — solar system: formation — stars: abundances

Early Solar Nebula Condensates with Canonical, Not Supracanonical, Initial 26Al/27Al Ratios

The short-lived radionuclide 26 Al existed throughout the solar nebula 4.57 Ga ago, and the initial abundance ratio ( 26 Al/ 27 Al)0, as inferred from magnesium isotopic compositions of calcium–aluminum-rich inclusions (CAIs) in chondritic meteorites, has become a benchmark for understanding early solar system chronology. Internal mineral isochrons in most CAIs measured by secondary ion mass spectrometry (SIMS) give ( 26 Al/ 27 Al)0 ∼ (4–5) × 10 −5 , called “canonical.” Some recent high-precision analyses of (1) bulk CAIs measured by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), (2) individual CAI minerals and their mixtures measured by laserablation MC-ICPMS, and (3) internal isochrons measured by multicollector (MC)-SIMS indicated a somewhat higher “supracanonical” ( 26 Al/ 27 Al)0 ranging from (5.85 ± 0.05) × 10 −5 to >7 × 10 −5 . These measurements were done on coarse-grained Type B and Type A CAIs that probably formed by recrystallization and/or melting of fine-grained condensate precursors. Thus the supracanonical ratios might record an earlier event, the actual nebular condensation of the CAI precursors. We tested this idea by performing in situ high-precision magnesium isotope measurements of individual minerals in a fine-grained CAI whose structures and volatility-fractionated trace element abundances mark it as a primary solar nebula condensate. Such CAIs are ideal candidates for the fine-grained precursors to the coarse-grained CAIs, and thus should best preserve a supracanonical ratio. Yet, our measured internal isochron yields ( 26 Al/ 27 Al)0 = (5.27 ± 0.17) × 10 −5 . Thus our data do not support the existence of supracanonical ( 26 Al/ 27 Al)0 = (5.85–7) × 10 −5 . There may not have been a significant time interval between condensation of the CAI precursors and their subsequent melting into coarse-grained CAIs.