EAR-PF: Global Investigation of the Mantle from Shear Velocity Profiles with a Focus on the Water Content and Temperature of the Mantle Transition Zone

Mann, Michael Everett, Ithaca NY

Investigators

Abstract

Dr. Michael Mann has been granted an EAR Postdoctoral Fellowship to carry out research and education plans at Brown University. This project focuses on the mantle transition zone (MTZ), a layer found approximately 410-km and 660-km below the Earth’s surface. The MTZ is poorly understood but is thought to incorporate high concentrations of water. Determining the depth to the MTZ boundaries has been difficult because of its deep depth. The fact that the MTZ is so deep has prevented high-resolution analyses of seismic properties within and around it, and observations at such depths have yielded a variety of competing compositional hypotheses. Dr. Mann will produce accurate, high-resolution models of Earth’s layers from the surface through the MTZ, which will help constrain depth (and therefore temperature), composition and state of hydration (water content). These properties are important for models of Earth formation, the solid Earth water cycle, and mantle composition and convection. The proposed project will use complementary datasets to generate profiles of the Earth (from surface to MTZ) to determine temperature more accurately, as well as reconcile competing hypotheses for the composition and state of hydration within and around the MTZ beneath different tectonic regions. Dr. Mann will also participate in mentoring undergraduate researchers through the Brown Research Experience for Undergraduates (REU): Dynamic Earth in the 21st Century and participate in teaching and professional development workshops offered by the Brown University Sheridan Center for Teaching and Learning. This project will determine shear velocity profiles from the surface through the MTZ using a joint inversion of receiver functions, which are sensitive to sharp changes in velocity, and surface wave phase velocities, which are sensitive to velocities over a broad range of depths. Standard analyses using these complementary observables incorporate only fundamental-mode Rayleigh wave phase velocities, which are sensitive to velocities in the uppermost 300-350 km of the Earth. The method used here will also incorporate surface wave overtones, which have sensitivities that extend beyond MTZ depths (>700 km). These velocity profiles will constrain temperature, water content, and composition beneath long-lived seismometer stations worldwide. This project aims to better resolve the seismic properties of the MTZ, including negative velocity gradients near the base of the MTZ, and to understand their implications for the present-day water content, temperature, and composition of the mantle. Recent research has revealed additional seismic discontinuities that represent negative velocity gradients with depth both within and surrounding the MTZ. The negative velocity gradient near the bottom of the MTZ is of particular interest, as it has been interpreted as the result of low temperatures in a pyrolytic mantle, metastable phases in subducting slabs, and ponded subducted oceanic crust at the base of the MTZ. These conflicting hypotheses cannot all be true simultaneously. Hence, a deeper investigation into the extent and characteristics of MTZ seismic velocities and boundaries is necessary to constrain MTZ structure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.