Compositional heterogeneity and seismic anisotropy near the 410 km discontinuity
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The mantle is the largest compositional layer within Earth and it is continually mixed by thermal convection associated with the movement, creation, and destruction of tectonic plates. It is clear from volcanic rocks that this mixing has not resulted in a homogeneous mantle composition, however, there are few observational constraints that allow mapping of the compositional variations within Earth's mantle. This project seeks to measure the range of compositional variations in the mantle and map their spatial distribution across the globe. The focus will be variations in the fraction of the mineral olivine because it is the most abundant mantle mineral and variations in its local abundance change the properties of a seismically reflective boundary at about 400 km depth. The interface represents a change in the crystal structure of olivine, which transforms to wadsleyite as pressure and temperature increase with depth. Laboratory mineral physics experiments to determine the seismic properties of olivine and wadsleyite will be combined with analyses of seismic data from dense regional arrays to infer compositional variations. In addition to estimating the fraction of olivine, the project will also estimate the abundance of hydrogen defects in olivine, which represent a potential geochemical reservoir of water in the deep Earth. The combined studies will provide novel insights into global mantle mixing and spatial variations in the ability of the mantle to generate volcanic activity. Grant funding will primarily be used to support research training and education of student scientists using modern mineral physics and seismic methods.
This project will take advantage of the petrological simplicity of the 410 km discontinuity, which is the boundary between the upper mantle and the transition zone, to investigate spatial variations in the bulk and volatile composition of the mantle. The olivine-wadsleyite transition is the only phase transition that can cause the globally observed 410. Wadsleyite has about five times greater water storage capacity than olivine and the existence of structural water can profoundly affect sound velocities of minerals. Therefore the magnitude of the 410 provides a good estimation of the olivine content and water concentration near 410 km depth in the Earth's interior. High olivine content in the source rock results in less capability of producing magma, thus olivine content is a good indicator of mantle petrology. In addition, olivine is also strongly anisotropic relative to wadsleyite, so depending on the olivine content and strain-induced lattice-preferred orientation of olivine polymorphs, the 410 can be a sharp anisotropic boundary as well. The investigators will also use broadband P to SV/SH scattering signals collected by a global compilation of dense regional seismic arrays to better assess the isotropic/ anisotropic seismic structures near the 410. The results will be compared with new laboratory single-crystal high pressure-temperature sound velocity measurements. The ultimate goal of the proposed study is to estimate the lateral compositional variations in terms of olivine content and water concentration near 410 km depth, and the anisotropic velocity jumps across the 410 discontinuity. The majority of project funding will support an interdisciplinary team of students supervised by two early career PIs. This project will create unique opportunities for graduate and undergraduate students, especially underrepresented minority students, to participate in scientific research both on campus and at the state-of-the-art synchrotron facilities.
