HR: 08:45h
AN: P31C-04    [Abstracts]
TI: A new chronology for asteroid formation in the early solar system based on $^{182}$W systematics
AU: * Kleine, T
EM: kleine@erdw.ethz.ch
AF: Zentrallabor fuer Geochronologie, Institut fuer Mineralogie, Corrensstr. 24, Muenster, 48155 Germany
AU: * Kleine, T
EM: kleine@erdw.ethz.ch
AF: Institut fuer Isotopengeologie und Mineralische Rohstoffe, ETH Zentrum NO Sonneggstrasse 5, Zurich, 8092 Switzerland
AU: Klaus, M
EM: klaush@nwz.uni-muenster.de
AF: Institut fuer Geochemie und Mineralogie, Zuelpicherstr. 49b, Koeln, 50674 Germany
AU: Herbert, P
EM: palme@gwp-min.min.uni-koeln.de
AF: Institut fuer Geochemie und Mineralogie, Zuelpicherstr. 49b, Koeln, 50674 Germany
AU: Scherer, E
EM: scherer@nwz.uni-muenster.de
AF: Institut fuer Geochemie und Mineralogie, Zuelpicherstr. 49b, Koeln, 50674 Germany
AU: Muenker, C
EM: muenker@nwz.uni-muenster.de
AF: Institut fuer Geochemie und Mineralogie, Zuelpicherstr. 49b, Koeln, 50674 Germany
AB: Chondrites are generally considered to represent the chemically least processed material of the solar system. They contain Ca-Al-rich inclusions (CAIs), which probably condensed at high temperatures from a gas of approximately solar composition. CAIs are widely considered to be the first solids formed in the solar nebula, such that their most precise U-Pb age of $4567.2 \pm 0.6$ Ma is commonly taken as the age of the solar system. Differentiated meteorites such as iron meteorites derive from asteroids that underwent large-scale chemical differentiation, most notably core formation. Chondrites are widely considered to represent the precursor materials from which asteroids accreted and then differentiated. If this succession of events is correct, the accretion of chondrite parent bodies predates core formation in asteroids. The relative chronology of these processes, however, has not yet been determined. We obtained precise Hf-W ages for CAIs, pristine chondrites, and iron meteorites with the aim of understanding the genetic and temporal relationship between chondrites and iron meteorites. The W isotope data reveal that type IIIAB, IVA, IVB, and IC iron meteorites predate the last major thermal overprint of CAIs and the formation of chondrite parent bodies. These irons are remnants of first-generation planetesimals and represent the oldest yet dated material formed in the solar system. They constrain the minimum age of the solar system to $4570.5 \pm 2.0$ Ma. Assuming a homogeneous distribution of $^{26}Al$ the initial $^{26}Al/^{27}Al$ of the solar system must have been higher than $\sim 8 \times 10^{-5}$. These high amounts of $^{26}Al$ can have provided an efficient means to promote rapid differentiation of early-formed asteroids. We suggest that chondrite parent bodies formed by re-accretion of debris produced during collisional disruption of first-generation planetesimals, and that chondrules formed in the vicinity of first-generation planetesimals.
DE: 5455 Origin and evolution
DE: 3662 Meteorites
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting