"How Heterogeneous is the Earth's Mantle? Combining New Observations from Seismology with Geochemistry and Geodynamics" by Dr. Nicholas Schmerr (University of Maryland)

Abstract:

Improved constraints on mantle composition are fundamental to understanding the accretion, differentiation, and thermochemical evolution of our planet. A key component for unraveling the complexity of the mantle is the imaging of seismic discontinuities; these interfaces are the consequence of solid-to-solid phase changes in mantle mineralogy, changes in chemical composition, the presence of melt, and/or changes in mantle anisotropy. To investigate the lateral continuity and depth of these boundaries, I use underside reflections of S-wave seismic energy arriving as precursors to the seismic phase SS to image the depth and impedance contrast present across mantle discontinuities. Comparing the resulting observations to mineral physics predictions for seismic structure from a mineral physics reveals the average mantle composition of the Pacific is fit by a mixture of ~20% basalt and 80% harzburgite, roughly consistent with the bulk chemistry of mantle peridotite (18% basalt, 82% harzburgite). In some regions beneath hotspots and subduction, the presence of discontinuities in the depth range of 220-350 km, as well as at 1000 km depth suggests a bulk chemistry enriched in basalt (basalt fraction ~30-35%), lending evidence to the hypothesis that the mantle is laterally heterogeneous, and that dynamics are stirring an enriched component, perhaps from the deep or shallow earth, into the upper mantle.