Other
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmmr23a0036d&link_type=abstract
American Geophysical Union, Fall Meeting 2005, abstract #MR23A-0036
Other
1645 Solid Earth (1225), 3904 Defects
Scientific paper
It is thought that the silicate mantles of smaller terrestrial bodies, such as Mars and the Earth's moon, contain higher iron contents than the larger terrestrial planets, such as Earth and Venus. If material within the perovskite region of planetary mantles were to preferentially loose iron to the core, this discrepancy of iron content with planetary size might be explained. At the same time, Galimov (2005) proposed late-stage core accretion on Earth from geothermal arguments. Lauterbach et al. (2000) and Frost et al. (2004) provide a mechanism for producing lower-mantle iron depletion by disproportionation of iron as a necessary requirement of the Fe3+ content of lower mantle magnesium silicate perovskite. Recent measurements of dihedral angle of Fe-O liquids in perovskite (Shannon and Agee 1998, Takafuji et al., 2004) suggest that, at least within the deep lower mantle, metallic melts will form an interconnected network and iron can be extracted into the core. We present mass balance calculations for iron disproportionation in perovskite and fractional extraction from the lower mantle. For reasonable core-mantle interaction layer thicknesses the calculated depletion rates require 10-50 convective overturns of the mantle to reach a terrestrial composition from a Mars-like starting composition. Several tens of convective overturns is reasonable during the last 4.5 billion years of Earth's history. In addition, the model also makes several predictions which can be tested: First, Fe depletion should be much more rapid during the earliest Archaean when mantle convection was most vigorous and the mantle iron content was highest. Secondly, the composition of mantle-derived melts should become increasingly iron-poor with time. This appears to be true of Archaean continental crust and MORB. Third, the upper mantle should become increasingly oxidized with time, until a gas-phase oxygen buffer is reached (C-CO). Excess oxygen could then be removed from the mantle by oxidation of mantle carbon and CH4 and degassing of CO/CO2 and water. The lack of young mantle diamond might be significant in this respect.
Dobson David P.
Frost Daniel
Price Geoffrey D.
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