HR: 1330h
AN: T32B-0924    [PDF]
TI: Seismic Heterogeneity Caused by Oceanic Crust Differentiation and Segregation in the Convecting Mantle
AU: Xie, S
EM: sxie@ess.ucla.edu
AF: Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567 United States
AU: * Tackley, P J
EM: ptackley@ucla.edu
AF: Department of Earth and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567 United States
AU: * Tackley, P J
EM: ptackley@ucla.edu
AF: Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567 United States
AB: This presentation focuses on the seismic signature of mantle heterogeneity associated with crustal differentiation and segregation in the lower mantle. Segregation of subducted oceanic crust above the CMB has often been invoked as a way of explaining the isotopic signature of OIB geochemical endmembers such as HIMU. Here a mantle convection model that includes melting-induced differentiation and plate tectonics is run for billions of years and the resulting thermo-chemical heterogeneity is studied. Statistical diagnostics such as radial correlation functions (Jordan et al., 1993) and spectral heterogeneity maps (Tackley et al., 1994) are used to characterize the observational signature of the thermo-chemical structures and compare them to global seismic tomographic models. In the reference case, crust is denser than the background mantle at the CMB. Due to this density contrast, the crustal material forms a thick and dense layer at the bottom of the mantle, although the layer interface is not sharp as is commonly obtained in models where a layer is inserted a priori. An enormous amount of long-wavelength volumetric heterogeneity is found in the lower mantle. The presence of oceanic crust near the surface also contributes to heterogeneity at the top of the mantle. In cases where the subducted crust is neutrally buoyant or buoyant in the deepest mantle, a large amount of heterogeneity instead exists in the the mid-mantle region, which is not observed in tomographic models of the real Earth. Unlike the reference case with a thick layer at the bottom of the mantle, these cases have heterogeneity right at the CMB, and this strong heterogeneity exists at both short and long wavelength. When comparing to Earth, it appears that models in which dense subducted crust settles into a layer above the CMB are consistent with constraints from seismic tomography; such a layer is therefore a viable location for the storage of geochemical endmembers.
DE: 1010 Chemical evolution
DE: 7207 Core and mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
SC: Tectonophysics [T]
MN: 2003 Fall Meeting