HR: 08:30h
AN: P31C-03    [Abstracts]
TI: Timescales for Accretion of the First Planetesimals
AU: Markowski, A
EM: markowski@erdw.ethz.ch
AF: ETH Zurich, Department of Earth Sciences ETH Zentrum, NO, Sonneggstrase 5, Zurich, CH 8092 Switzerland
AU: Quitte, G
EM: quitte@erdw.ethz.ch
AF: ETH Zurich, Department of Earth Sciences ETH Zentrum, NO, Sonneggstrase 5, Zurich, CH 8092 Switzerland
AU: * Halliday, A
EM: halliday@erdw.ethz.ch
AF: ETH Zurich, Department of Earth Sciences ETH Zentrum, NO, Sonneggstrase 5, Zurich, CH 8092 Switzerland
AU: Kleine, T
EM: kleine@erdw.ethz.ch
AF: ETH Zurich, Department of Earth Sciences ETH Zentrum, NO, Sonneggstrase 5, Zurich, CH 8092 Switzerland
AB: With new precise chronology becoming available for CAIs and chondrules our understanding of early disc evolution is improving dramatically. However, the timescales for first planetesimal accretion are more problematic. It long has been theorized that the first 1000 km sized planetesimals or planetary embryos accreted by runaway growth over timescales of roughly 10${^5}$ years. However, isotopic evidence for the existence of such early objects has been lacking. Chondrites usually contain CAIs as well as chondrules and the isotopic ages for these objects provide evidence that the parent bodies formed $>$ 2 million years after the start of the solar system. Similarly, the strontium isotope data for basaltic achondrites as well as other lines of isotopic evidence for eucrites and angrites provide evidence that some asteroidal sized differentiated objects did not form until similar, relatively late times. The most likely candidates for samples of very early-formed differentiated planetesimals are certain iron meteorites. Tungsten isotopes provide the opportunity to define the timescales for accretion and metal segregation recorded in these objects. New high precision W isotope data for $>$30 iron meteorites show that the timescales for accretion, metal segregation and re-equilibration are variable for all magmatic iron groups but extend over periods of 10${^6}$ years. Some groups include iron meteorites with very unradiogenic W and these could be vestiges of the earliest primary planetesimals. These objects not only provide constraints on early accretion, they also help define the initial ${^1^8^2}$Hf/${^1^8^0}$Hf of the solar system and the accretion rates and age of later objects, including the Moon.
DE: 5410 Composition
DE: 5455 Origin and evolution
DE: 6205 Asteroids and meteoroids
DE: 1035 Geochronology
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting