HR: 11:05h
AN: U42A-04    [Abstracts]
TI: Density Structures of Oceanic Slabs and Surrounding Mantle Around the 660 km Discontinuity: Implications for the Fate of Old and Young Slabs
AU: * Ganguly, J
EM: ganguly@email.arizona.edu
AF: University of Arizona, Department of Geosciences, Tucson, AZ 85721, United States
AU: Saxena, S K
EM: saxenas@fiu.edu
AF: Florida International University, CeSMEC, Center for the Study of Matter Under Extreme Conditions, Miami, Fl 33199, United States
AU: Freed, A M
EM: freed@purdue.edu
AF: Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47906, United States
AB: We calculated density variations as a function of temperature around the 660 km deep seismic discontinuity in the Earth's mantle in different types of compositional units associated with a subducting slab and the ambient mantle. The calculations are based on computational thermodynamic approach of minimization of Gibbs free energy at a specified P-T condition, subject to the bulk compositional constraints of the system, that simultaneously yield stable mineral assemblage, mineral compositions and modal abundances. These results are converted to density profiles using appropriate data for physical properties and equations of state that are applicable to high P-T conditions. In addition, we also calculated thermal structures of several slabs, with the extremes being given by Tonga (140 Myr at trench, vertical velocity: 14 cm/yr; average dip: 60 degrees) and Peru (41 Myr, 4.4 cm/yr, 35 degrees). A slab was assumed to be lithologically stratified with a top basaltic crust, followed downwards by residual harzburgite and slightly depleted pyrolite. The surrounding mantle is taken to be undepleted pyrolite. Integration of the results of thermal and density calculations show that that all components of the Tonga slab are heavier than both ambient and thermally perturbed adjacent mantle, which has been cooled due to contact with the slab. Thus, old slabs with thermal minimum below 750 C, as in Tonga, should easily penetrate into the lower mantle unless resisted by slab roll back and/or a viscosity jump at the top of the lower mantle. In contrast, the harzburgite layer in warmer slabs, such as Peru, Marianas and Izu-Bonin, is slightly lighter than ambient mantle, causing near neutral or marginally negative net buoyancy of the slab. In this case, other factors, such as rollback and slab dip angle, may explain why some slabs in the northwest Pacific appear to penetrate into the lower mantle while others do not. In Peru-type warm slabs, buoyancy of the harzburgite layer may cause it to stagnate at the top of lower mantle, and thus deform the slabs. Finally, we find that regardless of temperature, the basaltic top part of a slab is always significantly heavier than surrounding mantle. Thus, should it become mechanically decoupled, it should sink into the lower mantle instead of peeling off from the slab to form a "perched eclogite layer", as suggested by some earlier studies.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3611 Thermodynamics (0766, 1011, 8411)
SC: Union [U]
MN: 2007 Joint Assembly