HR: 08:24h
AN: DI51B-03 INVITED    [Abstracts]
TI: Upper Mantle Discontinuity Topography from Thermal and Chemical Heterogeneity
AU: * Schmerr, N
EM: nschmer@asu.edu
AF: Arizona State University, School of Earth and Space Exploration PO Box 871404, Tempe, AZ 85287-1404, United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Arizona State University, School of Earth and Space Exploration PO Box 871404, Tempe, AZ 85287-1404, United States
AB: We investigate the depth of upper mantle seismic discontinuities utilizing high-resolution stacks of underside- reflected, horizontally-polarized precursors to the seismic phase SS. Bootstrap-derived histograms of discontinuity depths are used to retrieve detailed topographic information from stacks of a high-quality broadband dataset. This dataset consists of over 20,000 seismograms that sample beneath South America, the Hawaiian hotspot, and the surrounding Pacific and Atlantic Oceans. Beneath South America, our dataset reveals the 660-km discontinuity is 20 km deeper on the down-dip side of subduction, in agreement with cold material descending to this boundary. However, there is also a 10-15 km depression in the overlying 410-km discontinuity that is difficult to explain by thermal effects alone. In the same region, multiple discontinuities are detected on the 410-km boundary where it intersects the slab. We also find the 660-km boundary to be unexpectedly shallow beneath the mid-Atlantic and eastern Pacific ridges, consistent with warmer material rising through these depths. The 660-km discontinuity is shallow beneath the location of the Hawaiian hotspot, supporting a hot mantle plume passing through this boundary. We explore different mechanisms connecting chemical heterogeneity to our observed topography, including the transport and isolation of H₂O at the 410-km discontinuity and/or the extraction of Fe from wedge material later entrained into the mantle transition zone. An overlying partial melt layer can explain multiple reflectors near 410-km depth, and phase changes in the ilmenite system can explain our observations of multiple reflectors below 660-km depth. These findings strongly support the idea that the origin of upper mantle heterogeneity has both chemical and thermal contributions, and is associated with deeply rooted tectonic processes.
DE: 3621 Mantle processes (1038)
DE: 7200 SEISMOLOGY
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7245 Mid-ocean ridges
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting