Grain and Phase Boundaries in Mantle Assemblages: Atom Probe and Electron Microscopy/Diffraction Approaches

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The research pursued under this grant seeks to enhance understanding of the chemistry and physics of grain boundaries in crystalline solids, with particular emphasis on boundaries in silicate materials representative of the upper mantle of the Earth. Typical crystalline solids consist of myriad crystals (grains), frequently having random spatial orientation, bonded together across ribbon-like, two-dimensional boundaries. These boundaries have specific structure and chemistry, different in fundamental ways from the grains on either side; the boundaries frequently control the physical properties of the solid overall, e.g., strength, viscosity, electrical and ionic conductivity, optical transmittance, etc. In Earth and planetary science, the community’s interests reflect the breadth of these properties, although particularly the mechanical and electrochemical ones: boundaries between grains of the same mineral, boundaries between grains of different minerals (phase boundaries) and boundaries between minerals and melts affect directly phenomena such as (i) the rate of mantle deformation (which effects plate tectonics), (ii) the attenuation of seismic wave energy, and (iii) the chemical composition of magmas (particularly of trace elements, which facilitates understanding of melting and magma-migration processes in the Earth). Prominent in this specific research is the application of a new and exciting analytical technique to the study of grain- and phase-boundary structure and chemistry: atom probe tomography (APT). With appropriate care in specimen preparation and data analysis, APT can characterize boundary chemistry at the very atomic scale, allowing precise questions concerning materials dynamics to be posed and scrutinized. A beauty of the approach is that one can so learn the physics of scaling of mechanical and chemical responses from the sub-nanometer to the kilometer-plus dimensions. The research has implications both economic and in workforce development. Grain- and phase-boundary structure and chemistry in ionic and covalent-bonded solids (as are minerals) is a primary concern in the development of advanced ceramics for battery/fuel cell, photovoltaic, and structural applications. Employing APT in an effective way for the chemical design of grain/phase boundaries is a direct extension of the research supported here. The “effective way” caveat has everything to do with advances in APT approach (specimen preparation, imaging conditions, data analysis): these necessary advances will constitute no small part of the education accumulated—and promulgated—by the program participants. The research specifically focuses on grain and phase boundary structure and chemistry in (a) deformed rock of upper-mantle chemistry/mineralogy (olivine plus pyroxenes) and (b) olivine grain boundary interface(s) with a host magma. The former focus examines mantle aggregates deformed (experimentally) in diffusion creep–grain/phase boundary sliding. Characterizing the impact of the spatial orientation of deviatoric stress on boundary structure and chemistry can elucidate aspects of the physics of plastic instability in the mantle—a crucial issue in creating and sustaining plate tectonics. The latter focus addresses issues in the crystallization of basaltic magma and the role grain boundaries might play in (i) storing incompatible trace elements as well as (ii) affecting/effecting the mobilization of large magma bodies that are partially crystallized.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.