Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v11e..01a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V11E-01
Physics
1027 Composition Of The Planets, 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1115 Radioisotope Geochronology
Scientific paper
Since the discovery of a difference in 142Nd/144Nd between the Earth and chondrites (Boyet and Carlson, 2005), a puzzle has been created for understanding Earth evolution and composition: do we have a chondritic bulk Earth composition, necessitating a hidden enriched reservoir in its interior, or is the Earth non-chondritic? Are we losing the historical reference frame for the composition of the Earth? We propose a hypothesis that explains the observed difference in 142Nd/144Nd as a result of different initial 142Nd/144Nd ratios for the Earth and chondrites, with the Earth sampling the solar nebula later than chondrites, thus giving additional time for 146Sm decay. This hypothesis implies fractionation between Sm and Nd during condensation of solid materials from a nebula with higher Sm/Nd ratio. Several lines of evidence in meteorites as well as in experiments have indeed shown that Sm is more volatile under reducing conditions; this results in significant reduction in Sm/Nd ratio in condensates relative to the nebula. Furthermore, observations of nebulae around young stars have shown that they indeed possess reducing conditions (high C/O ratios). Observations also indicate that the process of formation of solid materials is not a simple one-way condensation, but a complex sequence of several steps of evaporation-condensation processes that could last for several million years. In addition, 129Xe has shown that the Earth accreted its final materials 120 m.y. later than meteorites, providing support for our hypothesis. We interpret the observation of 142Nd/144Nd ratios in Archean (3.8 Ga) rocks, showing positive 142ɛ regardless of acidic or basaltic composition, as witnesses of late accreting materials with high 142Nd/144Nd from the nebula. These anomalies vanish with time, because they are destroyed by the geodynamic cycle.
Allègre Claude J.
Hart Stan R.
Shimizu Nobumichi
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