In search of live 247Cm in the early solar system

Abstract

The r-process only nuclide 247Cm decays to 235U with a characteristic half-life of ∼16 million years. 247Cm is presently extinct, but offers considerable potential as a short-lived r-process chronometer, providing constraints on the time interval between the last r-process nucleo-synthetic event and the formation of the solar system. The existence of “live” 247Cm in the early solar system should be manifested today as variations in 235U/ 238U, provided Cm was chemically fractionated from U when solids formed in the early solar system. Using multiple-collector ICPMS and a high-purity mixed 233U- 236U spike to monitor instrumental mass fractionation, we are able to resolve variations in 235U/ 238U at the 1–2 epsilon level (2σ M; 1 epsilon = 1 part in 10,000) on sample sizes consisting of 20 ng of uranium. Data can be acquired on smaller (<10 ng) samples with ±2–3 epsilon 2σ uncertainties. Uranium isotopic measurements and U, Nd and Sm concentrations were acquired on bulk samples of a suite of carbonaceous chondrites, unequilibrated and equilibrated ordinary chondrites and eucrites, for which conflicting results had previously been obtained. Our results show no well-resolved excursions in 235U/ 238U away from the terrestrial value at the ∼2 epsilon level, and constrain the amount of 247Cm-produced excess 235U atoms to less than ∼1 × 10 8 atoms per gram of chondritic meteorite, with respect to terrestrial 235U/ 238U. Large (permil- level) anomalies in 235U/ 238U could, however, be artificially generated in the ordinary chondrites during laboratory processing. Therefore, U may be more susceptible to isotopic fractionation during chemical processing than previously recognized, and may reconcile some of the highly conflicting ε 235U results reported by previous workers for chondritic meteorites. Our results indicate that a timescale of ∼1–2 × 10 8 years between the last actinide producing r-process event and the formation of the solar system may not be unreasonable based on the 247Cm- 235U system. However, this conclusion is far from robust at this stage because the only bulk meteorites analysed that display strong Nd/U fractionation are highly metamorphosed chondrites that may have experienced a protracted history of redistribution and re-equilibration. The search for “live” 247Cm in the early solar system can now be extended to early-formed condensates and mineral phases displaying strong Cm-U fractionations.