Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v23h..03k&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V23H-03
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1212 Earth'S Interior: Composition And State (7207, 7208, 8105, 8124), 1213 Earth'S Interior: Dynamics (1507, 7207, 7208, 8115, 8120), 3630 Experimental Mineralogy And Petrology, 8125 Evolution Of The Earth (0325)
Scientific paper
The 182Hf-182W system should, in principle, be well suited to date core formation in Earth but the calculated ages strongly depend on the core formation/accretion model applied [e.g.,1-5]. An essential parameter for calculating Hf-W ages of core formation is the degree of metal-silicate equilibration during core formation. For instance, early core formation with complete metal-silicate equilibration and late core formation with only limited equilibration can result in the same W isotope composition of Earth's mantle [3-5]. Although this limits the application of Hf-W as a chronometer for core formation, W isotopes can be used to constrain the conditions of metal-silicate separation in Earth's mantle. This requires the timing of core formation to be determined independently. The age of the Moon provides the timing of the last major accretion stage in Earth's history and as such likely provides the time at which Earth's core had entirely formed. Here we use the ~40 Myr age of the lunar magma ocean [6] and the W isotope compositions of the lunar [6] and terrestrial mantles to infer the conditions prevailing during core formation in Earth. The W isotope composition of Earth's mantle after the giant impact is only slightly more radiogenic than chondrites at that time. This could indicate that the giant impact led to large-scale metal-silicate equilibration erasing a previously radiogenic W isotope composition. Alternatively, core formation prior to the giant impact involved extensive equilibration of metal and silicate, such that a more radiogenic W isotope composition would have never evolved in Earth's mantle. Mass balance considerations that are consistent with the initial W isotope composition of the Moon indicate that the Earth's mantle prior to the giant impact cannot have been radiogenic. Even with full equilibration, the Moon-forming impact could not have sufficiently reduced an originally radiogenic W isotope signal in the Earth's mantle. Core formation in Earth's mantle prior to the giant impact thus must have involved large-scale metal-silicate equilibration. Mass balance calculations suggest that prior to the Moon-forming impact on average more than 80% of the incoming W equilibrated with the mantle before entering Earth's core. This suggests that core formation occurred by the physical separation of small metal droplets from molten silicate in a magma ocean. However, W isotopes cannot constrain the degree of equilibration during the giant impact and would be consistent with both complete and absent re-equilibration of the impactor's core with Earth's mantle. [1] Kleine et al. 2002, Nature 418; [2] Yin et al. (2002), Nature 418; [3] Halliday 2004, Nature 427; [4] Kleine et al. (2004), EPSL 228; [5] Nimmo and Agnor (2006), EPSL 243; [6] Kleine et al. (2005), Science 310.
Bourdon Bernard
Kleine Thorsten
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