Collaborative Research: CSEDI--Multi-scale Analysis of Mantle Discontinuities Using Inverse Scattering of SS Waves and Experimental Mineral Physics

点击翻译按钮获取中文摘要

Through a concerted effort of seismic imaging and mineral physics we aim to improve our knowledge of the transition zone between the upper and lower parts of Earth?s mantle, which plays a central role in our understanding of mantle evolution, composition, and dynamics. The transition is marked by phase transformations in the dominant mantle silicates (e.g., olivine), and the associated jumps in elastic parameters can be detected and imaged with seismological techniques. The existence and interpretation (as isochemical phase transitions) of global interfaces near 410 and 660 km depth are no longer disputed. But even for the 410 and 660 many issues are unresolved. Laboratory experiments show that the depth to and the magnitude and transition profile of radial changes across discontinuities depends on various physico-chemical factors, such as temperature, pressure, major element composition and partitioning, and presence of water. In situ estimation of these parameters from seismic data is often complicated by contamination of signal due to shallower mantle heterogeneity and the need to make prior assumptions about the location and character of interfaces. For the 220 and 520 even the lateral extent and cause are debated, and it is unclear if other interfaces exist in the depth range of interest. We aim to investigate the transition zone using a generalized Radon transform of very large numbers (100,000) of broad-band SS waveforms that contain reflections at the underside of these interfaces. Specifically, we wish to (i) detect and locate elasticity contrasts, (ii) characterize the (radial) changes across them, and (iii) determine the lateral extent, variations along, and correlations between different interfaces. The seismological estimates of (local) discontinuity properties and (regional) topographies will be compared with predictions from different (e.g., olivine, pyroxene, garnet) multi-component systems in order to identify and understand compounded transitions and to produce in situ estimates of temperature, composition, and water content. Data coverage is insufficient for a global study: our initial geographical focus is a 2-D transect across import mantle dynamic regimes: from Hawaii (mantle upwelling), across NW Pacific (normal ocean), Kuriles (subduction zone), to Siberia (stable continent). This research is likely to improve our general understanding of the composition and phase chemistry of the upper mantle transition zone and as such provide constraints on the interplay between thermo-chemical mantle convection and upper mantle stratification. Furthermore, the proposed collaboration and integration of inverse scattering, observational seismology, and mineral physics offers a unique educational experience for students, post-doctoral scholars, and senior staff involved.