HR: 1340h
AN: P33A-1001    [Abstracts]
TI: In Search of {\it r}-Process $^{247}$Cm in the Early Solar System
AU: * Stirling, C H
EM: stirling@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Potter, E
EM: potter@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Andersen, M B
EM: andersen@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AB: The {\it r}-process only nuclide $^{247}$Cm decays to $^{235}$U with a characteristic half-life of $\sim$16 million years. $^{247}$Cm is presently extinct, but offers considerable potential as a short-lived {\it r}-process chronometer, providing constraints on the time interval between the last {\it r}-process nucleosynthetic event and the formation of the solar system. The existence of "live" $^{247}$Cm in the early solar system should be manifested today as variations in $^{235}$U/$^{238}$U, provided Cm was chemically fractionated from U when solids formed in the early solar system. The Cm-U system also has a direct bearing on the fundamental U-Pb cosmochronometer, which currently assumes no Cm effects in early solar system material. Using a Nu Instruments NuPlasma and new techniques in multiple-collector ICPMS, we are able to resolve variations in $^{235}$U/$^{238}$U at the two epsilon level (2$\sigma$; 1 $\epsilon$ = 1 part in 10,000) on sample sizes consisting of $<$20 pg of $^{235}$U. The high precision of our measurements offers the potential to resolve $^{235}$U anomalies, including samples where Cm-U effects had previously been unobserved. Our first uranium isotopic measurements were acquired on bulk samples of a suite of carbonaceous chondrite, ordinary chondrite and eucrite meteorites, for which conflicting results had previously been obtained. These data show no well-resolved excursions in $^{235}$U/$^{238}$U from the terrestrial value at the $\sim$2 epsilon level, and constrain the amount of $^{247}$Cm-produced excess $^{235}$U atoms to less than $\sim$1 x 10$^{8}$ atoms per gram of chondritic meteorite, with respect to terrestrial $^{235}$U/$^{238}$U (Stirling et al., in press, Geochim. Cosmochim. Acta). We have extended the search for "live" $^{247}$Cm in the early solar system to small samples from mineral phases in primitive objects that are likely to display strong Cm-U fractionations. In particular, uranium isotopic measurements have been acquired on acid-etched leachates for a suite of chondritic meteorites, and for a suite of minerals separated from chondrites and angrites. Some of these data show resolvable excursions away from the composition of our terrestrial standard, and as such, have important implications for the $^{247}$Cm-$^{235}$U cosmochronometer and the timing of {\it r}-process nucleosynthesis relative to the formation of the first solar system materials.
DE: 8125 Evolution of the Earth
DE: 5455 Origin and evolution
DE: 5749 Origin and evolution
DE: 6040 Origin and evolution
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting