HR: 10:35h
AN: P42A-02    [Abstracts]
TI: Resolution of Small Differences in Li and Mg Isotopic Composition Between the Terrestrial Planets and Chondrites. Is the Earth "Chondrulitic"?
AU: Wiechert, U
EM: wiechert@erdw.ethz.ch
AF: ETH, Earth Sciences, Zurich, CH8092 Switzerland
AU: * Halliday, A N
EM: alexh@earth.ox.ac.uk
AF: University of Oxford, Earth Sciences, Oxford, OX1 3PR United Kingdom
AU: Magna, T
EM: magna@erdw.ethz.ch
AF: ETH, Earth Sciences, Zurich, CH8092 Switzerland
AB: Precise Li and Mg isotope data obtained with high resolution MC-ICPMS for samples from the Earth, Moon, as well as meteorites that are considered to be derived from Mars and Vesta, are fractionated to slightly heavy values relative to those of chondrites. The compositions of these major silicate reservoirs of differentiated objects are broadly similar and display no relationship with indices of volatile element depletion or differences in proposed accretion mechanisms. Chondrules also yield heavy Mg isotopic compositions, so the compositions of the differentiated planets may reflect sorting of material in the proto-planetary disk. Such an explanation would be consistent with a class of models previously proposed to explain the onset of accretion and the low Si/Mg of the Earth. In this model chondrules separate from volatile-rich planetary dust by accumulation in stagnant regions between eddies in the solar nebula. Small (~1 km) planetesimals formed by turbulent concentration of millimeter-sized chondrules then develop into planetary embryos and thence to larger terrestrial planets by combinations of runaway growth and collisions. As such, chondrule accumulation provides an explanation that obviates some of the dynamic difficulties associated with the onset of accretion. The isotopically heavy Li and Mg isotopic compositions of the Earth and its possible "chondrulitic" origin have important implications for Earth's bulk composition and putative hidden reservoirs.
DE: 1027 Composition of the planets
DE: 1028 Composition of meteorites (3662, 6240)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Major and trace element geochemistry
SC: Planetary Sciences [P]
MN: Fall Meeting 2005