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The Study of Intergranular Segregation and Elemental Partitioning in Partially Molten Olivine-bearing Geological Composites by STEM-EDX

Understanding the deformation behavior of composite ceramic materials at high temperature and applied stress is a common goal of geologists and the developers of ceramic materials for hightemperature structural applications. For example, the nature of grain boundaries at the “operating” temperature of the composite has significance to the creep behavior of both silicon nitride ceramics used in advanced automotive engines and for olivine-rich partially molten rocks that comprise the Earth’s upper mantle. The presence of nanometer-scale glassy intergranular phases at the grain boundaries can provide fast diffusion paths and promote grain boundary sliding as a significant creep mechanism [1]. Scanning transmission electron microscopy (STEM) in conjunction with energydispersive X-ray (EDX) or electron energy-loss (EELS) spectroscopies is a powerful method for characterizing the grain boundary phases and segregants at nanometer resolution. However, the STEM-EDX analysis of such composites can be compromised by changes in chemistry, including sublimation, elemental redistribution, and bond-breaking, that may occur under the intense irradiation of the focused electron beam. The goal of the present study is to characterize by STEM-EDX techniques the elemental distributions and intergranular segregation in partially molten olivine-bearing geological composites, to better understand the rheological behavior of these materials in the Earth’s upper mantle. The technical challenges encountered in the characterization of these materials suggest strategies for such characterization of beam-sensitive materials in general.