Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v42a..01h&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V42A-01
Physics
1025 Composition Of The Mantle, 1027 Composition Of The Planets, 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1155 Extinct Radionuclide Geochronology
Scientific paper
It is well-established that the silicate Earth is non-chondritic in its Si/Mg ratio. Some of this may reflect core formation, but the total Earth is also non-chondritic in Mg/Fe. Late stage erosion of the outer portions of the Earth or its constituent proto-planets, as is thought to have affected Mercury, could, in principle, produce some such effects, as well as explain differences in volatile budgets. However, mantle-derived samples from the Earth, Moon, Mars and Vesta all appear to have similar stable isotopic compositions of the slightly volatile elements Li and Mg regardless of accretion history. These isotopic compositions are heavy relative to chondrites and may be better explained in terms of sorting of dust in the disk, resulting in basic differences in the earliest material that formed the planets compared with chondrites. Chondrules have both low Si/Mg and isotopically heavy Mg and may be analogous to the size-sorted particles from which the terrestrial planets first developed into objects that were large enough for gravity to play a major role. This must be a very early feature. The parent bodies of irons, martian meteorites, angrites and eucrites appear to have undergone very rapid (<1Myr) accretion and core formation but were variably oxidized and formed proportionally smaller cores compared with Earth's. This contrasts with the reduced early Earth with a nebular atmosphere sometimes envisaged for the main stages of accretion and core formation. In fact the Earth is unlikely to have been large enough to have trapped a significant hydrogen-rich nebular atmosphere until a few Myrs after the start of the solar system by which time much of the gas may have dispersed or been swept into the Sun. It has been argued that some of the non-chondritic features of the silicate Earth as we sample it (such as its high ^{142}Nd atomic abundance) might reflect early-formed complementary hidden silicate reservoirs. However, the formation of the Moon and its early proximity to the Earth would have led to large tidal heating effects in the lower mantle and it is difficult to envisage how any such discrete primordial silicate reservoirs could have survived.
Halliday Alex N.
Zahnle Kevin J.
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