The resolving power of the APT makes it a promising tool for investigating the microphysics of rock deformation, bridging the atomic scale all the way to the plate-tectonic scale. The near-boundary gradients of olivine-insoluble ions are presented as evidence of the phase transformation which either dissolves olivine into clinopyroxene or vice versa. In this model, grain-scale transport of the shared (slowly-diffusing) mineralogical component Si 4+ is not required. The hypothesis presented here is that the ‘bulk rock’ – a wehrlite – deforms rapidly because conversion of one phase to the other occurs at phase boundaries (e.g., Sundberg & Cooper, 2008). Lower effective viscosities of phase mixtures may be critical to the initiation of plate tectonics and the formation of mantle shear zones. This enhanced deformation in the two-phase material is due to stress-driven reactions at the phase boundaries. The experiments show that the mixtures deform much more rapidly than either mineral endmember. Here we report Atom Probe Tomography (APT) analyses of grain and phase boundaries of laboratory-deformed, fine-grained mixtures of clinopyroxene and olivine (Zhao, et al., 2019). Marquardt, K., Dobson, D., Hunt, S., and Faul, U.: Grain boundary character information via individual imaging or statistical analyses: complexion transitions and grain boundary segregation, EGU General Assembly 2021, online, 19–, EGU21-14614,, 2021. The thus obtained grain boundary character distribution (GBCD) is discussed in relation to bulk viscosity. Using stereology, we extract the geometry of the interfacial network. To investigate if the distribution of grain boundaries is affected by grain boundary chemistry, we analysed grain orientation data from over 4x10 4 grains, corresponding to more than 6000 mm grain boundary length per sample. Polycrystalline olivine samples show different viscosity related to grain boundary segregation of impurities. We conclude that sintering pressure affects grain-boundary strength and we will evaluate how this may influence anelastic energy loss of seismic waves through elastic or diffusional accommodation of grain-boundary sliding. Our data indicate a grain boundary structural change occurs from “low-pressure” to “high pressure” grain boundaries, where the grain boundary facets change from >100 nm – 20 nm to 3-7 nm length scale, respectively. The HRTEM data were acquired using an image-corrected JEOL ARM 300 to achieve the highest resolution at low electron doses using a OneView camera. Here we used high resolution transmission electron microscopy to study the structure of grain boundaries from polycrystals synthesized at low (4-8 GPa) and high (8-15) GPa sintering pressure. Past elasticity measurements showed that the Youngs modulus of garnet polycrystals changes as a function of sintering pressure (Hunt et al. Data from these complementary methods will be discussed on two systems garnet and olivine polycrystals. The focus is on the combination of structural, chemical and statistical analysis as obtainable using transmission electron microscopy and electron backscatter diffraction. Here we introduce the principles of grain boundary structure to property relations and potent methods to study these. This led to the term “grain boundary complexions” to mark the phase-like behaviour of grain boundaries while differing from phases in the sense of Gibbs (Cantwell 2014). In the past two decades, observations of marked bulk material property changes have been associated with changes in the structure and composition of grain boundaries. Grain boundaries affect bulk properties of polycrystalline materials, such as electrical conductivity, melting or bulk viscosity.
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