HR: 09:15h
AN: DI41B-06    [Abstracts]
TI: Water Induced Instabilities at the top of Stagnant Slabs in the Transition Zone: Context and Consequences
AU: * Richard, G
EM: richard@geophysik.uni-frankfurt.de
AF: J.W. Goethe University, Earth Sciences Institute, Frankfurt, 60428, Germany
AU: Bercovici, D
EM: david.bercovici@yale.edu
AF: Yale University, Dept Geology and Geophysics, PO Box 208109, New Haven, CT 06520- 8109, United States
AB: Water enters the Earth's mantle at trenches by subduction of oceanic lithosphere. Most of this water immediately returns to the atmosphere through arc volcanism, but a part of it, retained in Dense Hydrous Magnesium Silicates (DHMSs) and Nominally Anhydrous Minerals (NAMs) like olivine, is expected as deep as the mantle transition zone (410-660 km depth). There, slabs can be deflected and linger before sinking into the lower mantle. Because it lowers the density and viscosity of mantle minerals in the transition zone, water is likely to affect the dynamics of the stagnant slab. The consequence of water's presence on the dehydration of a stagnant slab is explored. In particular, we focus on the possible onset of small-scale convective instabilities despite the 'adverse' thermal gradient (i.e., mantle above the floating slab is cooled from below). The competition between the thermal and hydrous effects on the density, and thus on the convective stability of the top part of the slab, is investigated using a numerical model including water dependent density and viscosity and temperature dependent water-solubility. For relatively high initial water content in the floating slab (≥ 1wt%), small-scale convection is likely to occur. Other important controlling parameters are the water dependence of density and viscosity. If small scale convection occurs at the top of a stagnant slab it enhances the rate of slab dehydration (otherwise controlled by the slow diffusion of water) and provides an efficient way to heat up the slab. Model results suggest that young and wet stagnant slabs are unlikely to reach the lower mantle because they would thermally equilibrate relatively quickly with the surrounding transition-zone mantle.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8121 Dynamics: convection currents, and mantle plumes
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting