A low initial abundance of 247CM in the early solar system and implications for r-process nucleosynthesis

The short-lived nuclide Cm-247 is produced by reprocess nucleosynthesis. When the presolar nebula formed, 247 Cm became isolated from r-process production and its abundance diminished as a result of radioactive decay. Given its short half-life of only similar to 16 million years, Cm-247 is presently extinct, but its former presence should be detectable as small variations in U-235/U-238 in primitive meteoritic material, provided Cm was chemically fractionated from U at the time these solid objects formed. The magnitude of U isotopic anomalies in meteorites can thus be used to elucidate the timing and character of the last r-process nucleosynthetic event for input into models describing the formation and evolution of the early solar system. Here we report coupled U isotopic determinations and Nd/U proxy measurements for Cm[U in a series of acid-etched leachates and mineral assemblages extracted from meteorites containing primitive phases expected to show strong Cm-U fractionations. Using multiple-collector ICPMS, we are able to determine U-235/U-238 with 2 sigma analytical uncertainties of 1 epsilon (1 epsilon = 1 part in 10,000) on sample sizes consisting of < 3 ng of U-238 and < 20 pg of U-235. A double-spiking procedure using a mixed U-236-U-233 spike was employed to allow instrumental mass fractionation to be reliably corrected intemally and at high precision. Uranium isotopic results for almost 40 different phases show no resolvable deviations in U-235/U-238 from the chondritic value, at the similar to 1-2 epsilon level. These data supplement our previous observations for a suite of bulk meteorite samples [C.H. Stirling, A.N. Halliday, D. Porcelli, In search of live Cm-247 in the early solar system, Geochim. Cosmochim. Acta 69 (2005) 1059-1071] and provide evidence for a solar system initial Cm-247/U-235 of < 8 X 10(-5). Such a low value is difficult to explain without a long time-scale of similar to 2.3 X 10(8) years between the last actinide producing r-process event and the formation of the solar system. As such it is difficult to reconcile with a model of actinide production in the same r-process forming event as Hf-182 with a half-life of 8.9 My [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109-L113]. The alternative models of Hf-182 production via a neutron-rich fast s-process, occurring, for example, in the helium burning shell in a 25 solar mass star during explosive nucleosynthesis [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actmides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109-L113], or via a distinct r-process event that is separate from actinide production [G.J. Wasserburg, M. Busso, R. Gallino, K.M. Nollet, Short-lived nuclei in the early solar system: Possible AGB sources, Nucl. Phys. A (in press)], may provide a viable explanation. However, further studies are also required to assess the veracity of Cm-U systematics, which are critically dependent on the suitability of using Nd and the light rare earth elements (LREEs) as a chemical proxy for Cm. (c) 2006 Elsevier B.V. All rights reserved.