HR: 1340h
AN: T23A-0522    [Abstracts]
TI: Implications of a plume-fed asthenosphere layer for mantle flow and mid-ocean ridge melting processes
AU: * Hasenclever, J
EM: hasenclever@dkrz.de
AF: Institute of Geophysics, Hamburg University, Bundesstrasse 55, Hamburg, 20146 Germany
AU: Phipps Morgan, J
EM: jp369@cornell.edu
AF: Department of Earth and Atmospheric Science, Cornell University, 4164 Snee Hall, Ithaca, NY 14853 United States
AU: Hort, M
EM: hort@dkrz.de
AF: Institute of Geophysics, Hamburg University, Bundesstrasse 55, Hamburg, 20146 Germany
AB: Based on current evidence for a relative low density and low viscosity layer beneath most oceanic lithosphere, we propose the asthenosphere layer to be formed by upwelling mantle plumes. In our scenario the asthenosphere is a hot, thus buoyant and weak layer that is chemically depleted due to melt extraction at the plume locations (hotspots volcanism) and the subsequent melting processes beneath mid-ocean ridges. At the base of the asthenosphere a transition to cooler, more viscous, not plume-fed mantle material is assumed. We investigate mantle flow in response to forced plate motion and possible asthenosphere entrainment at subduction zones. We use a two-dimensional numerical model that solves for viscous flow (FE-solver) and temperature (Smolarkiewicz FD-solver). Flow and entrainment of a 200~km thick asthenosphere layer are tracked by a tracer particle advection scheme. Speed of the oceanic plate, asthenosphere viscosity, and age of the subducting slab are varied. The angle of subduction is held constant (45°) and both sides of the slab are modeled in separate numerical experiments. We find the lower (hot) side of the slab to entrain a 10-30~km thick downdragged layer of asthenosphere, whose thickness depends upon the subduction rate and the asthenosphere viscosity and density. The upper (cold) side entrains as much by thermal 'freezing' onto the slab's top as by downdragging. Underneath the oceanic plate a relative pressure high at the subduction zone tilts the asthenosphere bottom and drives a return flow within the deeper asthenosphere towards the mid-ocean ridge. In the mantle wedge a recirculation forms whose shape, extension, and circulation speed depend strongly on the model parameters. Limited asthenosphere entrainment at subduction zones implies that a plume-fed asthenosphere is relatively easy to establish during mantle evolution. The implications of a plume-fed asthenosphere for mid-ocean ridge melting processes (in comparison to other models for the Earth's mantle) are investigated using a modified tracer advection scheme. Each tracer is able to track multiple pieces of geophysical and geochemical information, for example its degree of melting. This method can simultaneously track more than one mantle component, for example a depleted and an enriched component melting at different depths and rates, which allows to model mantle heterogeneities of various sizes. Representative trace element contents and isotope ratios for each mantle component are also stored with the moving tracers particles so that the geochemical evolution of a small mantle volume is recorded in each tracer. Data from tracers leaving the melting zone are used to calculate the residual melting column. Initial results allow us to put constraints on plausible mantle source compositions and let one further explore properties and possible mechanisms for the origin of the asthenosphere.
DE: 0545 Modeling (4255)
DE: 0560 Numerical solutions (4255)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
SC: Tectonophysics [T]
MN: Fall Meeting 2005