Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u42a..01w&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #U42A-01
Other
1015 Composition Of The Core, 1025 Composition Of The Mantle, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
Scientific paper
The chemical similarities between the Earth and chondritic meteorites enable us to use the concentrations of refractory elements in the mantle to estimate their concentrations in the core. Experimental data on metal- silicate partitioning then constrain the conditions under which the core segregated as the earth accreted. Current results on a range of refractory elements of different degrees of siderophile character (Ni, Co, V, W, Nb, Cr ) are consistent with segregation of the core at the bottom of a deep magma ocean. Assuming single- stage core-mantle equilibration, pressure-temperature conditions around 40GPa and 3800K (ie >1000 km depth in earth of present size) would be required to match the composition of the mantle. Such a deep magma ocean would be short-lived and it is more reasonable to consider core-segregation as a continuous process occurring under progressively higher pressure conditions as the earth accreted. 182Hf-182W chronometry implies a mean-life of accretion of 12 Myr for such a process. Assuming that core segregation occurred progressively during accretion, metal-silicate partitioning data imply that the average magma-ocean depth was about 35% of mantle depth. Furthermore, the data are best matched if the Earth became more oxidized as accretion progressed. Recent experiments demonstrate that Earth-sized planets are capable of self-oxidising during core segregation because of the disproportionation of Fe(II) into Fe(III) plus Fe(0) as perovskite crystallizes. In the final stages of this process iron metal segregation would have ceased and small amounts of iron-nickel sulfide liquid would have been the only additions to the core. Recently, it was proposed by Wood and Halliday (Nature v.437 p.1345) that late addition of sulfide to the core could have caused substantial fractionation of Pb from U and been responsible for the apparently young model lead age of the silicate earth. New experiments on metal-silicate partitioning of Pb and other `chalcophile' elements indicate however that Pb is not chalcophile enough for a small amount of sulfide addition to generate the observed effect. Furthermore, consideration of the extinct isotopic system 205Pb-205Tl (half life 15 Myr) enables us to use the observations that the Pb/Tl ratio of the silicate Earth is 2.5 times chondrite while the 205Tl/203Tl ratio is chondritic. Core formation appears to be incapable of generating these observations (Tl is neither siderophile nor chalcophile enough), lending support to the hypothesis that the earth lost heavy volatile elements during and perhaps after formation of the moon.
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