HR: 13:40h
AN: V22E-01 INVITED     [PDF]
TI: Earth accretion dynamics and time-scales
AU: * Halliday, A N
EM: halliday@erdw.ethz.ch
AF: Earth Sciences, ETH, Sonneggstr 5, Zurich, 8092 Switzerland
AB: The degree to which efficient mixing of new material, losses of volatiles to space and changes in oxidation characterize the impact-driven growth of Earth-like planets in unclear. These processes affect calculated time-scales and can be studied by parallel modeling of data from different radiogenic isotope systems. The W isotope composition of the silicate Earth yields a model time-scale for accretion that is faster than estimates based on terrestrial Pb and Xe isotope data and on Sr, W and Pb data for lunar samples. This is hard to explain unless refractory metals in impacting core material did not always mix efficiently with the silicate portions of the Earth before being added to the Earth's core. Agreement is obtained with a Moon-forming Giant Impact 50 Myrs after the start of the solar system if only a quarter of the W from Theia's core equilibrated with the silicate Earth assuming the Hf/W in silicate reservoirs remained constant. Both W and Sr isotope compositions of the Moon provide evidence that the average composition of proto-planets was in fact more like Mars, with a low Hf/W, volatile-rich, oxidized mantle. Growth from such protoplanets decreases to a few percent the calculated amount of equilibration between Theia's core and the silicate Earth during the Giant Impact.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting