HR: 09:45h
AN: U21D-08    [Abstracts]
TI: Varying rates of mantle convection in the early Earth
AU: * Carlson, R W
EM: rcarlson@ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015, United States
AU: Boyet, M
EM: maud.boyet@univ-st-etienne.fr
AF: 2 Laboratoire Magmas et Volcans, UMR CNRS 6524, Observatoire de Physique du Globe, Universite Blaise Pascal, 5 rue Kessler, Clermont-Ferrand, 63038, France
AB: The transition from ¹⁴²Nd/¹⁴⁴Nd approximately 35 ppm higher than chondritic in early Archean rocks from Isua (Caro et al., GCA, 2006; Boyet and Carlson, EPSL, 2006; Bennett et al., 38th LPSC, 2007) to the 20 ppm excess measured in rocks younger than 3.5 Ga (Boyet and Carlson, Science 2005) suggests a period of mixing between LREE enriched and depleted reservoirs probably formed within 30-50 Ma of Earth formation. In order to reach ¹⁴²Nd/¹⁴⁴Nd as high as 35 ppm above chondritic requires a quite high Sm/Nd ratio (¹⁴⁷Sm/¹⁴⁴Nd = 0.22). This value is similar to the Sm/Nd ratio needed to explain the elevated initial ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd of ancient lunar crustal rocks (Boyet and Carlson, EPSL, 2007). This observation suggests that extensive differentiation of the Earth through a magma ocean event was completed before the Moon formed from a LREE-depleted terrestrial mantle. Between 4.5 and 3.8 Ga, mixing in the mantle must have been relatively minor in order to preserve the very high ¹⁴²Nd/¹⁴⁴Nd seen in the ancient rocks from Isua. Between 3.8 and 3.5 Ga, relatively rapid mixing between high- and low-Sm/Nd reservoirs produced during the magma ocean event must be invoked to lower the ¹⁴²Nd/¹⁴⁴Nd to the value measured in all rocks younger than 3.5 Ga. A possible explanation for this evolution in mantle composition notes geodynamic models that rapidly crystallize (within millions of years) a magma ocean to leave a buoyantly unstable cumulate pile that then overturns rapidly (e.g. Elkins-Tanton et al., EPSL, 2005). After the overturn, the mantle has cold dense material underlying buoyant hot material, which inhibits mantle convection until internal heating overwhelms the chemical buoyancy. The evidence for time varying ¹⁴²Nd/¹⁴⁴Nd in the early Archean suggests that by 3.8 Ga, this inhibition had been removed and that convection throughout a large portion of the mantle had begun. This model provides the unexpected prediction that the Hadean may have been a relatively quiescent time on Earth between the violence of Earth formation, global melting, magma ocean crystallization and overturn, and the resumption of wholesale mantle convection in the early-Archean that continues to the present day.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1155 Extinct radionuclide geochronology
SC: Union [U]
MN: 2007 Fall Meeting