HR: 15:25h
AN: DI43A-08    [Abstracts]
TI: Isotopic Modeling of Dynamic Geochemical Layering in the Mantle
AU: * Lee, V E
EM: vlee@eps.berkeley.edu
AF: University of California, Berkeley, Dept. of Earth & Planetary Science 307 McCone Hall, MC 4767, Berkeley, CA 94720-4767 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Berkeley, Dept. of Earth & Planetary Science 307 McCone Hall, MC 4767, Berkeley, CA 94720-4767 United States
AB: In order to understand the dynamics of the Earth's mantle, it is vitally important to reconcile geophysical and geochemical lines of evidence that suggest discrepant models for mantle convection. For example, seismic tomography provides evidence that subducted slabs of oceanic lithosphere can penetrate through the transition zone into the lower mantle, implying whole-mantle convection, whereas the distinct ranges of isotopic values for mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) has been invoked as evidence for layered mantle convection. We present here a model that results in dynamic geochemical layering in the mantle (allowing for whole-mantle convection while creating distinct isotopic reservoirs in the shallow and deep mantle). A key aspect of this model is that the process of wet partial melting beneath mid-ocean ridges generates three chemically distinct regions: enriched basaltic crust (BC), a depleted harzburgite/lherzolite layer (DHL), and an incipiently melted lherzholite layer (IML) that is only slightly depleted. These three layers can also have different, distinct physical properties as well (e.g., Braun et al. 2000). During subduction and penetration of the subducted slab lithologic column through the transition zone, any preferential separation of the slab material based on physical properties (e.g., retention of low-viscosity DHL material in the upper mantle or penetration of the cold, high-viscosity BC layer to the lower mantle) will result in distinct εNd and εHf values for the upper and lower mantle. Box models are used to investigate the evolution of εNd and εHf during Earth's history, and the resulting values are compared to measured values for MORBs and OIBs. These results may have implications for constraining the slab flux from the upper to lower mantle.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005