Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.u43b..03d&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #U43B-03
Physics
1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
Scientific paper
Elemental and isotopic differences between the Earth, Mars, and meteorites indicate that the bulk of each terrestrial planet formed from very narrow (<0.5 AU) feeding zones. The formation of Earth's core, the crystallization of its magma ocean, the formation of early crust, and the outgassing of Earth's oceans and atmosphere are natural consequences of planetary accretion. The environment within which primordial differentiation of the Earth occurred appears to have been a deep magma ocean. This conclusion derives from the observation that the abundances of moderately siderophile elements in Earth's mantle appear to be set by equilibrium with metal at the base of a deep magma ocean. Highly siderophile elements are in chondritic relative abundances and may point to their delivery in an oxidized "late veneer" followed by very efficient mixing into a by now metal-free (the metal had segregated into Earth's core) magma ocean. It has generally been thought that the accretion disk was too hot at 1 AU for hydrous minerals to be stable. Paradoxically, the Earth appears to have accreted "wet", a conclusion that derives from modeling, D/H observations of comets, and Os isotope measurements of meteorites. However, dust in the accretion disk was bathed for some time in a sea of H and O, and some amount of water must have formed. Stimpfl et al. (2004) MAPS 39, A99 show that several Earth oceans of water could be adsorbed onto grains in the inner solar system and subsequently accreted into the terrestrial planets. This conclusion suggests that H is dissolved in Earth's core, as water dissociates in the presence of Fe metal at even modest pressures and temperatures. The implication is the progressive oxidation of the magma ocean as Earth grows, H is segregated into the metal phase, and OH is liberated, thus accounting for the observation that the upper mantle of Earth is about 3 log units more oxidizing than required for equilibrium with metal. Thus the "late veneer" might have contained Fe metal, which would have oxidized because of the overwhelming excess of oxygen in the magma ocean. Different Xe isotopic reservoirs in the Earth can most readily be explained by separation of I from Xe through the medium of liquid water, implying the presence of seas or oceans within 100 Ma of nucleosynthesis of 129I.
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