Determining the orientations of ice crystals using electron backscatter patterns.

Knowing the orientations of the ice crystals in a polycrystalline aggregate is essential for understanding and modeling the flow of naturally occurring ice. Here we show, for the first time, that the orientation of crystals in polycrystalline ice can be determined with a higher angular and spatial resolution and more rapidly than any currently used method by using electron backscatter patterns (EBSPs) in a cold-stage equipped scanning electron microscope. We also present an orientation image map constructed from EBSPs, and discuss possible applications of the technique for ice. The results indicate that obtaining EBSPs and orientation images from other frozen water-containing materials, such as clathrate hydrates, may also be possible.

SEM/EDS comparison of polar and seasonal temperate ice.

We have developed a method employing a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy for examining uncoated ice specimens. By permitting the ice to sublimate in the SEM at temperatures between -115 degrees and -60 degrees C, enough ions are produced to prevent specimen charging. The absence of a conductive coating permits both the acquisition of uncompromised X-ray spectra, and a dynamic view of impurities as the ice sublimates. The method has enabled us to examine the microstructure and impurities in ice in ways not possible through standard melt chemistry measurements or even through using a SEM to study coated samples. Soluble impurities appeared either as white spots in grain interiors, grain boundaries, and triple junctions, or as filaments in grain boundaries. Inclusions of insoluble impurities have also been observed in natural ice. Thus, we have been able to compare the microstructural location and concentration of impurities in ice from different terrestrial locations. Even when ion chromatography of the melt from two core sections reveals similar levels of impurities, the morphology and location of the impurity aggregates can be quite different. Analysis of impurity type and location can provide clues to the depositional environment and history of the ice.