63 rd EASTERN SNOW CONFERENCE Newark, Delaware USA 2006 211 Microstructural Characterization of Firn I. BAKER 1 , R. OBBARD 1 , D. ILIESCU 1 , AND D. MEESE 1,2 ABSTRACT In this paper, we use a scanning electron microscope (SEM) coupled with x-ray spectroscopy and electron back-scattered diffraction patterns to examine firn in cores retrieved by the United States International Trans-Antarctic Scientific Expedition. From grain boundary grooves we were able to see where the previously-existing snow crystals were joined, and can determine grain sizes. From the SEM images, the porosity and the surface area per unit volume of the pores were measured. Finally, we have shown that we can determine the microchemistry of impurities in firn and demonstrated that we can determine the orientations of the firn crystals. Keywords: Firn, snow, porosity, surface area, scanning electron microscopy, x-ray microanalysis INTRODUCTION In a number of recent papers, we have used a scanning electron microscope coupled with energy-dispersive x-ray microanalysis (EDS) to determine the microstructural location of impurities in ice cores (Cullen and Baker 2000; 2001; 2002a; 2002b; Cullen et al. 2002; Iliescu et al. 2002; Baker et al. 2003; Baker and Cullen 2003a; 2003b; Obbard et al. 2003a; 2003b; Iliescu and Baker 2004; Baker et al. 2006). A key feature of this work is that the ice specimens were uncoated, and examined at -80 ± 10 °C so that slight sublimation from the ice prevented charge build-up. This is a technique that we have also used to examine snow (Iliescu and Baker 2002) and that Barnes and co-workers used to examine both ice cores and snow from Antarctica (Barnes et al. 2002a; 2002b; 2003). This approach has an advantage over the use of aluminum or gold coating on ice to prevent charging in the SEM (Mulvaney, Wolff and Oates 1988; Wolff, Mulvaney and Oates 1988; Barnes et al. 2002b) since the metal coatings can obscure weaker X- ray fluorescence signals from the sample. In other work, we have demonstrated that Raman spectroscopy coupled with a confocal scanning optical microscope can be used to analyze impurities in grain boundaries and triple junctions that cannot be detected by EDS (Iliescu et al. 2004). MicroRaman spectroscopy was first used to look at impurities in a triple junction in ice by Fukazawa and co-workers (1998). More recently, we have demonstrated that we can obtain the complete orientations of crystals in polycrystalline ice (not simply the c-axis direction) with high angular (0.1 o ) and spatial resolution (50 nm) by using electron back-scattered patterns (EBSPs) obtained in a SEM (Iliescu et al. 2004; 2005; Obbard et al. 2006). In this paper, we show, for the first time, that we can obtain SEM images, EDS and EBSPs from uncoated firn using the same approach. Ultimately, such information can provide an efficient and accurate method of measuring porosity, internal surface area and grain size and will allow us an 1 Thayer School Of Engineering, Dartmouth College, Hanover, NH 03755, Email: Ian Baker (Ian.Baker@Dartmouth.Edu ) 2 Climate Change Institute, University Of Maine, Orono, ME 04469