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.

Project Report

Deep Earth processes have been believed to be connected and sometimes play critical roles for the geological phenomena on the surface. However, because the depths greater than a few km are inaccessible, it is critical to combine imaging techniques and laboratory measurements to understand the interior. One of the powerful ways to image the interior structures is to use seismic waves. While seismic wave speeds increase gradually with depths, they change abruptly at certain depths in the Earth’s mantle (seismic discontinuity). From numerous laboratory measurements, we now know that some of the seismic discontinuities are related to atomic-scale changes in the structures of mantle minerals (phase transition). From the thermal and compositional responses of the phase transitions measured in laboratory, the imaged depth variations of the mantle seismic discontinuities can be used to infer thermal and compositional structures in the mantle. Through close collaboration between seismologists and mineral physicists in this research project, we have developed a robust technique to image 3D structures of mantle discontinuities and combined the imaging with laboratory experiment results. We have applied this method to understand the thermal and compositional structures beneath Hawaii. The origin of Hawaiian volcanism and the seamount chains extended for thousands of km from the island has been under intense debate since the birth of the plate tectonics theory. The most popular theory has been that the source of the volcano may reside in the boundary between the core and the mantle at 3000-km depth and the distinct chemical signatures of the volcanic rocks at Hawaii may be related to the core-mantle boundary region. However, our results show that the warmest structure may exists 1000-2000 km west of Hawaii at 400-800 km depths. At the same depth range right beneath Hawaii, we found that the mantle is only slightly warmer than normal mantle. This indicates that high-temperature structure which may supply heat from the deep mantle to Hawaii may not be vertical but may develop significant lateral components. This also implies that much of the distinct chemical signatures of the volcanic rocks at Hawaii may be related to processes at shallower depths.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Robin Reichlin
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Massachusetts Institute of Technology
United States
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