In search of live Cm in the early solar system |
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| Authors: | CH Stirling AN Halliday |
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| Institution: | 1 Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH, 8092 Zürich, Switzerland
2 Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK |
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| 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 (2σM; 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 × 108 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 ε235U results reported by previous workers for chondritic meteorites. Our results indicate that a timescale of ∼1-2 × 108 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. |
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