HR: 1340h
AN: P33A-1002    [Abstracts]
TI: An Internal $^{205}$Pb-$^{205}$Tl Isochron for the Iron Meteorite Toluca and the Initial Solar System Abundance of $^{205}$Pb
AU: * Nielsen, S G
EM: sune@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Rehk\"{a}mper, M
EM: markr@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Halliday, A
EM: halliday@erdw.ethz.ch
AF: ETH Zurich, Sonneggstrasse 5, Zurich, 8092 Switzerland
AB: The radionuclide $^{205}$Pb has long been of particular interest to astrophysicists and cosmochemists because it is the only short-lived isotope that may have been present in the early solar system, which is produced solely by the {\it s}-process. The initial solar system abundance of $^{205}$Pb could therefore provide unique constraints on the sites and the timing of {\it s}-process nucleosynthesis. As $^{205}$Pb decays to $^{205}$Tl with a half-life of about 15 Myr, the former presence of $^{205}$Pb can be inferred, if variations in the $^{205}$Tl /$^{203}$Tl isotope ratio can be identified for meteorites. The numerous analytical studies that were conducted during the last 45 years were unable to determine the initial abundance of $^{205}$Pb, however, and only an upper limit of about 9 x 10$^{-5}$ was established for the initial $^{205}$Pb /$^{204}$Pb ratio of the solar system (Huey & Kohman, 1972, EPSL 16, 401). In a previous abstract (Nielsen, Rehk„mper & Halliday, 2004, GCA 68, A727) we reported Tl isotope variations for the metal phases of various iron meteorites that were interpreted to reflect the former presence of $^{205}$Pb at the time of parent body solidification. Thallium, however, has only two isotopes, such that anomalies that are produced by the decay of $^{205}$Pb cannot be readily distinguished from isotope effects that reflect mass dependent fractionation in the samples. Additional Tl isotope data that were collected for the coexisting metal and sulfide phases of the iron meteorites Canyon Diablo (IAB) and Grant (IIIAB) were not in accord with our previous interpretations. The former existence of an extinct radionuclide in the solar system is best demonstrated if an internal isochron can be obtained for co-genetic samples of an individual meteorite. This approach was taken in the present study, where we analyzed six bulk metal samples and one troilite nodule from the iron meteorite Toluca (IAB). The Tl isotope compositions of the metal samples vary by about $2.5\permil$ and they display an excellent correlation with the $^{204}$Pb/$^{203}$Tl ratios, which range between about 0.1 and 75. The slope of this correlation implies an initial $^{205}$Pb/$^{204}$Pb ratio at the time of parent body solidification of (7.6$\pm$1.5) x 10$^{-5}$. This result is consistent with data obtained for the metal phases of the iron meteorites Canyon Diablo (IAB), Murphy (IIAB), and Navajo (IIAB). The Toluca sulfide, however, does not fall on the metal isochron but is characterized by a $^{205}$Tl/$^{203}$Tl ratio that is too low by about $1.5\permil$. This offset is consistent with the Tl isotope compositions of all other analyzed sulfide-metal pairs of iron meteorites. The Toluca sulfide displays a higher Tl concentration than the metal. It is therefore possible that the Tl isotope compositions of the metal samples may have been altered by sulfide re-equilibration, isotope fractionation during diffusion and sulfide micro-inclusions. It is unlikely, however, that the observed internal metal isochron of Toluca would have been preserved and be consistent with the data for many other metals if such secondary disturbances are associated with major shifts in the Tl and Pb budgets of coexisting metal and sulfide phases.
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting