[S14-1] Upper mantle and transition zone dynamics and structure I

[S14-1-01] Observations of Upper Mantle Discontinuity Structure

^○Nicholas Schmerr (University of Maryland, College Park, USA)

invited

There exist a number of seismic discontinuities throughout Earth's mantle that arise from a wide variety of mechanisms ranging from changes in mineral structure, anisotropy, the introduction of new mineral phases, the presence or production of melts, to more subtle changes in mantle composition. Of the upper-mantle boundaries, the 410 km (410) and 660 km (660) discontinuities resulting from phase changes in the mineral olivine are the most prominent, and globally define the upper mantle transition zone. The topography, sharpness, and impedance contrasts present 410 and 660 are of great value owing to their potential for revealing lateral variations in mantle temperature and composition as well as mantle dynamics. Imaging of body waves scattered from the boundaries are providing new insights on the heterogeneity present on the 410 and 660 throughout the mantle.

In addition to the 410 and 660, the ever-increasing number of seismic stations deployed around the globe is enabling study of the geographical characteristics of weaker, regional seismic discontinuities. These weaker and regionalized discontinuities are proposed to arise from a variety of mechanisms, ranging from stalled or trapped melt in the upper mantle, either at the base of the lithosphere, or above the 410 km discontinuity, transitions in the anisotropy of the olivine phase, the presence of phase changes in the pyroxene and garnet of the mantle, to the formation of seismic discontinuities from the presence of the coesite to stishovite phase transition in the vicinity of subducted and/or plume entrained eclogitic materials. These new results are adding to the diversity of seismic discontinuities found in the upper mantle and significantly improving our understanding of mantle chemical and thermal heterogeneity.