Seismic Discontinuities and Compositional Heterogeneities in the Mid-Mantle

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes
Type: Oral
Date: 4/29/2020
Time: 11:00 AM
Room: 120 + 130
Description: 

Mid-mantle discontinuities are usually attributed to isochemical phase transformations of olivine and its high-pressure polymorphs. However, recent seismic observations have shown complexities in these discontinuities that cannot be explained by conventional models of thermal variations. Here we analyse SS precursor stacking results to investigate mid-mantle discontinuities on a global scale. The 520-km discontinuity, with varying amplitude and depth, is consistently observed in all resolved regions. We also observe a sporadic 560-km discontinuity, indicative of the phase transition from garnet to Ca-perovskite. A megameter-scale reflector at ~800 km depth, as strong as the 660-km discontinuity, is observed near the Kamchatka slab. Its strong amplitude can only be explained by an abundance of majoritic garnet that transforms to bridgmanite at this depth. We speculate that the 800-km reflector indicates an oceanic plateau that was subducted at the Kamchatka Trench. Such a structure may be related to a large igneous province created by the initiation of the Hawaii hotspot.

Presenting Author: Songqiao Wei

Authors