HR: 17:45h
AN: U34A-08    [Abstracts]
TI: Explaining Topography of the 410-km Discontinuity: Chemical vs Thermal Heterogeneity in the Mantle
AU: * Chambers, K
EM: Kit.Chambers@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford Parks Rd, Oxford, OX1 3PR United Kingdom
AU: Woodhouse, J H
EM: John.Woodhouse@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford Parks Rd, Oxford, OX1 3PR United Kingdom
AU: Deuss, A
EM: deuss@esc.ca.ac.uk
AF: Department of Earth Sciences, University of Cambridge Madingley Rd, Cambridge, CB30EZ United Kingdom
AB: Discussion of $SS$ and $PP$ precursors from the 410-km discontinuity (termed $P410P$ and $S410S$) has largely focused on the long-wavelength topography of this discontinuity. Recent models suggest precursor derived topography of the 410-km discontinuity is dominated by low degree spherical harmonics. However, there remain discrepancies as to the details of the topography and its relation to mantle processes. In particular, global models of discontinuity topography do not show significant correlations with expected regions of thermal anomaly in the mantle. This could partly be explained by the influence of water and other chemical heterogeneities on the olivine phase change which produces the 410-km discontinuity. We present a new global model of topography for the 410-km discontinuity which is constrained using both $P410P-PP$ and $S410S-SS$ observations. We examine the relationship between our observations of 410-km discontinuity topography and seismic velocity anomalies in the transition zone. We find a moderate negative correlation between discontinuity topography and seismic velocity anomalies at low spherical harmonic degrees. This is consistent with the expected relationships between the $\alpha -\beta$ olivine phase boundary position, seismic velocity anomalies, and lateral variations in mantle temperature. However, when shorter wavelengths are included the relationship becomes more complex. In smaller regions beneath Northeast Asia and the Western Pacific we observe positive correlations between seismic velocities and discontinuity topography, which we interpret as evidence for the effects of chemical heterogeneities on seismic velocities and the position of 410-km discontinuity. Our results suggest that long wavelength discontinuity topography is primarily controlled by temperature variation in the mantle, but at shorter wavelengths the influence of chemical heterogeneities becomes important. This separation of the spectra for thermal and chemical heterogeneities suggests that chemical heterogeneities can survive in a convecting mantle.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7207 Core and mantle
DE: 1734 Seismology
DE: 1212 Earth's interior--composition and state (8105)
SC: Union [U]
MN: 2004 AGU Fall Meeting