Upper Mantle Discontinuity Topography from Thermal and Chemical Heterogeneity

Upper Mantle Discontinuity Topography from Thermal and Chemical Heterogeneity A cross section explaining our results, whereby iron is removed by melting in the mantle wedge and the Mg-enriched residue is swept into the transition zone along with the subducting lithosphere. This increases the pressure at which olivine transforms into wadsleyite, deepening the 410 km boundary.
Utilizing high resolution stacks of precursors to the seismic phase SS, we investigated seismic discontinuities associated with mineralogical phase changes at approximately 410 and 660 kilometers deep within the Earth beneath South America and the surrounding oceans. We utilized a dataset of over 17,000 broadband seismograms collected from the IRIS DMC and EarthScope. Our maps of phase boundary topography revealed deep 410- and 660-km discontinuities in the down-dip direction of subduction, inconsistent with cold material at 410-km depth. We explored several mechanisms invoking chemical heterogeneity within the mantle transition zone to explain this feature. In some regions, we detected multiple reflections from the discontinuities, consistent with partial melt near 410-km depth and/or additional phase changes near 660-km depth. Thus, the origin of upper mantle heterogeneity has both chemical and thermal contributions, and is associated with deeply rooted tectonic processes.
</p><p>Schmerr, N., and E. Garnero (2007), Upper mantle discontinuity topography from thermal and chemical heterogeneity, Science, 318(5850), 623-626.
</p><p>Acknowledgements: NSF Awards EAR-0453944 and EAR-0711401</p>


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