HR: 08:45h
AN: V21F-04    [Abstracts]
TI: Compositional Structure of Earth's Interior: The Importance of Early Terrestrial Differentiation
AU: * Boyet, M
EM: boyet@dtm.ciw.edu
AF: Carnegie Institution of Washington, 5241 Broad Branch Rd, NW, Washington, DC 20015 United States
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Carnegie Institution of Washington, 5241 Broad Branch Rd, NW, Washington, DC 20015 United States
AB: The Sm/Nd ratio needed to evolve the 20-ppm excess in ¹⁴²Nd/¹⁴⁴Nd (produced by the decay of 103 Myr half-life ¹⁴⁶Sm) found in terrestrial rocks compared to chondrites (Boyet and Carlson, Science, 2005) would lead to a ¹⁴³Nd/¹⁴⁴Nd similar to that of modern mid-ocean ridge basalts (MORB). Thus, the positive ε¹⁴³Nd of MORB is not the result of mantle depletion caused by continent formation, but may be due to fractionation occurring during an early (> 4.53 Ga) terrestrial global differentiation event. To minimize the additional increase in the Sm/Nd ratio of the depleted mantle caused by continent formation over Earth history, and hence avoid producing a reservoir with ¹⁴³Nd/¹⁴⁴Nd higher than observed for terrestrial rocks, the depleted MORB mantle (DMM) must occupy a much larger mass fraction of the mantle (70-95%) than previously modeled (30-50%). Adding the continental crust back into DMM, for DMM masses corresponding to 70-95% of the mantle, eliminates the negative Pb concentration anomaly typical of DMM, creating a smooth BSE normalized incompatible element pattern, with a small negative Nb anomaly, with the most highly incompatible elements depleted to about half BSE abundances. Unless the BSE has non-chondritic abundances of the refractory lithophile elements, the superchondritic ¹⁴²Nd/¹⁴⁴Nd of terrestrial rocks requires the presence of an enriched component in Earth's mantle formed during the early differentiation event. By mass balance, and if restricted in volume to the D" layer, this enriched component has a relatively smooth BSE-normalized incompatible element pattern rising from Lu= 2 x BSE to Rb = 11 x BSE with a small positive Nb and negative Hf anomaly. The pattern is not consistent with deep fractionation of a magma ocean involving high-pressure phases such as perovskite or majorite, but instead suggests that this ancient enriched reservoir may be a subducted terrestrial proto-crust formed near Earth's surface. In this case, the ability of the post-perovskite phase to solvate substantial amounts of Fe and incompatible elements may have allowed this subducted proto-crust to form a buoyantly stable layer at the base of the mantle where it has remained throughout Earth history.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005