HR: 0800h
AN: T21C-0495    [Abstracts]
TI: Density Variations in Subducting Slabs and Surrounding Mantle at the 670 km Discontinuity: Are the Slabs Lighter or Heavier?
AU: * Ganguly, J
EM: ganguly@geo.arizona.edu
AF: Jibamitra Ganguly, Department of Geosciences, University of Arizona, Tucson, AZ 85721 United States
AU: Saxena, S
EM: saxenas@fiu.edu
AF: Surendra K. Saxena, CESMEC, Center for the Study of Matter under Extreme Conditions, Florida International University, Miami, FL 33199 United States
AU: Freed, A M
EM: freed@purdue.edu
AF: Andrew M. Freed, Department of Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN 47907 United States
AB: It is commonly accepted that the 670 km seismic discontinuity in the Earth's interior is primarily due to the transformation of gamma-spinel or Ringwoodite to perovskite, and that this transition boundary has a slightly negative P-T slope, which makes the slab somewhat lighter than the surrounding mantle, thereby resisting further subduction. Seismic tomographic images, however, show both piling-up of the slab at this depth and penetration to greater depths. The latter phenomenon implies that at least under some conditions the slab is heavier than the surrounding mantle. Therefore, in order to understand the fate of the subducting slab at the 670 km depth, it is essential to have estimates of the density of the slab and the surrounding mantle taking into consideration all major mineralogical transformations at this depth, instead of just Ringwoodite to perovskite. We have, thus, calculated the equilibrium mineral assemblages and thereby the densities of the mantle and the subducting oceanic slab as function of temperature (1600 to 700 C) at 24.6 GPa (corresponding to 670 km depth) by minimizing the Gibbs free energy for specific bulk compositions within the system MgSiO3-FeSiO3-CaSiO3-Al2O3. Some of the thermodynamic mixing properties are not well constrained, and have thus been estimated by empirical methods. The mantle was assumed to have a pyrolite composition, whereas the slab was considered to be compositionally stratified with a layer of lherzolite composition at the bottom that is followed upward by layers of harzburgite and basalt (10%) compositions. The results suggest that all slabs that have attained interior temperature of ~ 750 C or less (e.g. Mariana: 155 Myr old, 4.4 cm/yr velocity, 64 degrees dip angle) are heavier than the surrounding mantle at 670 km depth, and should penetrate into the lower mantle, though the process of penetration may be greatly slowed by an inferred sharp increase of viscosity with depth across the boundary between the upper and the lower mantles. At 670 km depth, the lateral transition from pyrolite mantle to lherzolite bottom layer of a slab causes a density drop, but that from lherzolite to harzburgite at T < 900 C causes a density jump within the slab. The harzburgite to lherzolite ratio and the temperature of a slab play critical roles in determining the density of the slab relative to the surrounding mantle at 670 km depth.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
SC: Tectonophysics [T]
MN: Fall Meeting 2005