HR: 0800h
AN: U21B-0426    [Abstracts]
TI: Subadiabatic geotherm and compositional gradient in the mantle
AU: * Xu, W
EM: xuwenbo@umich.edu
AF: University of Michigan, Department of Geological Sciences, 2534 C. C. Little Building, 1100 North University Ave, Ann Arbor, MI 48109-1005, United States
AU: Lithgow-Bertelloni, C
EM: crlb@umich.edu
AF: University College London, Dept. of Earth Sciences, University College London, Gower St, London, WC1E 6BT, United Kingdom
AU: Stixrude, L
EM: stixrude@umich.edu
AF: University College London, Dept. of Earth Sciences, University College London, Gower St, London, WC1E 6BT, United Kingdom
AU: Ritsema, J
EM: jritsema@umich.edu
AF: University of Michigan, Department of Geological Sciences, 2534 C. C. Little Building, 1100 North University Ave, Ann Arbor, MI 48109-1005, United States
AB: We further explore the properties of a mantle model consisting of a mechanical mixture of basalt and harzburgite with varying basalt fraction. We have found that such a mechanical mixture is faster and better explains 1-D seismic profiles than an equilibrated pyrolite of the same bulk composition, particularly in the transition zone [Xu et al., 2007]. We now invert for the geotherm that would best fit seismic profiles, given our bulk compositional models. We find that we are not able simultaneously to fit a range of seismological 1D VS and VP models by varying the temperature alone. Hence we explore the simultaneous optimal solution for temperature and basalt fraction throughout the whole mantle. Our results indicate a radial gradient in bulk composition throughout the mantle: basalt depletion in the upper mantle and enrichment in the lower mantle. The transition zone is insensitive to basalt fraction for the mechanical mixture, which gives us an independent thermometer for this region. The comparison between model and observed seismic structure suggests significant subadiabaticity. For the lower mantle, the basalt fraction monotonically increases with depth: from 20% below the 660 km discontinuity and 80% near the core-mantle boundary. Remarkably, this is in agreement with the basalt enrichment expected in the dynamical calculations of Nakagawa and Buffett [2005]. A remaining uncertainty is the role on the phase equilibria and physical properties of high-pressure basaltic phases stabilized by Na, such as calcium ferrite. We test the dynamical consequences of our results by constructing a velocity-density scaling, which we use to predict the Earth's geoid and plate motions.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3621 Mantle processes (1038)
DE: 3939 Physical thermodynamics
DE: 7208 Mantle (1212, 1213, 8124)
SC: Union [U]
MN: 2007 Fall Meeting