HR: 08:30h
AN: U41A-02    [Abstracts]
TI: Compositional and Thermal Effects on Transition Zone Structure
AU: * Xu, W
EM: xuwenbo@umich.edu
AF: University of Michigan, Dept of Geological Sciences, Ann Arbor, MI 48109, United States
AU: Lithgow-Bertelloni, C
EM: crlb@umich.edu
AF: University of Michigan, Dept of Geological Sciences, Ann Arbor, MI 48109, United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: University of Michigan, Dept of Geological Sciences, Ann Arbor, MI 48109, United States
AB: Our present picture of Earth's chemical and dynamical evolution points to an uncertain knowledge of the composition of the mantle, which is important for us to understand the thermal and dynamical evolution of Earth. Dynamical models suggest a mantle made of a mechanical mixture of basalt and harzburgite, especially perhaps a basaltic gradient and a concentration of basalt in the lower mantle. We use a self-consistent thermodynamic calculation of phase equilibria and physical properties to explore the Vs structure of the mechanical mixture in the transition zone based on the mantle composition of Workman and Hart [2004]. We performed calculations for a mechanical mixture model with varying fractions of basalt (0% to 40%), along adiabats with potential temperature ranging from 1400K to 1800K. In general, seismic structure is insensitive to basalt fraction in the transition zone, except the location and thickness of the 410 and 660 km discontinuities, which are linearly dependent on the basalt fraction. Another important characteristic is that the 660 km discontinuity shows a double-step discontinuity, in which the second step is caused by the sudden transition of garnet- perovskite+Ilmenite. Compared with the influence of basalt fraction, the influence of the geotherm is very significant for the shear-wave velocity, including the location and thickness of 410 and 660 km discontinuities, which are linearly related to temperature, in the transition zone. The non-global 520 km discontinuity structure could be explained by the variation of temperature. In fact it is not caused by the global transformation of wadsleyite to ringwoodite. Instead, the existence of the 520 km discontinuity corresponds to low temperature, which causes large amount of garnet to transform to cpv in the transition zone of wadsleyite to ringwoodite, and the reverse is true at high temperature. When comparing our results with PREM, we find a velocity deficit that increases in magnitude from 400 to 740 km depth, which indicates a ~100 K colder subadiabatic geotherm. Besides the deficit in shear wave velocity, the gradient from 410 to 660 km would also indicate a basaltic gradient in the mantle.
DE: 1025 Composition of the mantle
DE: 3900 MINERAL PHYSICS
DE: 3939 Physical thermodynamics
DE: 5460 Physical properties of materials
DE: 7208 Mantle (1212, 1213, 8124)
SC: Union [U]
MN: 2007 Joint Assembly