HR: 10:55h
AN: U12A-03    [Abstracts]
TI: Plate tectonics, ancient crust, and the geochemical evolution of the mantle
AU: * Brandenburg, J
EM: jpbrande@umich.edu
AF: Dept. of Geological Sciences, Univ. of Michigan, 2534 C.C. Little Building 425 East University Avenue, Ann Arbor, MI 48109-1063, United States
AU: Hauri, E
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution of Washington, Dept. of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015, United States
AU: van Keken, P
EM: keken@umich.edu
AF: Dept. of Geological Sciences, Univ. of Michigan, 2534 C.C. Little Building 425 East University Avenue, Ann Arbor, MI 48109-1063, United States
AU: Ballentine, C
EM: chris.ballentine@man.ac.uk
AF: Dept. of Earth Sciences, Univ. of Manchester, M13 9PL, Manchester, M13 9PL, United Kingdom
AB: Isotopic ratios measured in oceanic basalts indicate the presence of variably mixed depleted, enriched and pristine components in the convecting mantle. A persistent dense layer at the base of the mantle may modulate the isotopic composition of enriched compositional end members, particularly if this layer is at least partially composed of ancient oceanic crust. We model the generation of a dense layer from subducted crust in models with stiff mobile plates approximated by the force-balance method [1][2]. The inclusion of plates allows us to self- consistently model crust petrogenesis at divergent plate boundaries. An embedded geochemical model tracks the isotopic evolution of Pb,U,Th,Sm,Nd,Rb,Sr,Re,Os, and He within this model framework. By varying thermal convective vigor and chemical density within reasonable parameter values, we investigate the admissible range of model behavior. In all cases we find that the extraction of continental crust is essential to reproduce the spread of observed isotope ratios. The size and longevity of pools of ancient oceanic crust at the base of the mantle further modulate isotope systematics by delaying the remixing of ancient oceanic crust. With no chemical buoyancy the average age of melting forms a gradient between lower and upper mantle values of 1.5 and 2.75 Byr respectively. With dense oceanic crust the gradient becomes discontinuous, producing an older upper mantle age in excess of 3.0 Byr. This indicates an average residence time of pooled crust on the order of 250 to 500 Myr, and leads to more distinct separation between isotopic end-member compositions. Using these relationships, we construct a series of viable continental crust extraction scenarios and compare these with published models. Mass balance calculations are performed and the role of the pooled oceanic crust as a complimentary reservoir to the continental crust is evaluated. [1] Gable, C.W., R.J. O'Connell, B.J. Travis (1991) "Convection in 3 dimensions with surface plates; generation of a toroidal flow," J. Geophys. Res., 89, 8391--8405 [2] Brandenburg, J.P., P.E. van Keken (2007) "Methods for thermochemical convection in Earth's mantle with force-balanced plates," Revised version submitted to Geochem. Geophys. Geosyst.
DE: 0545 Modeling (4255)
DE: 1009 Geochemical modeling (3610, 8410)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8125 Evolution of the Earth (0325)
SC: Union [U]
MN: 2007 Fall Meeting