Charge contrast imaging of geological materials in the environmental scanning electron microscope

The environmental scanning electron microscope (ESEM) allows high-resolutio n high-magnification imaging of conductivity differences in uncoated geolog ical samples. Under normal ESEM operating conditions, negative charge build up at the sample surface (from bombardment by the electron beam) is prevent ed by the presence of a gas (usually water vapor) in the sample chamber. Ba ckscattered and secondary electrons from the sample ionize this chamber gas , and the resultant positively charged gaseous ions migrate toward the nega tively charged sample. When chamber gas pressures lower than approximately 250 Pa are used, however, charging of the sample can occur because insuffic ient charge balancing positively charged gaseous ions are produced. Charge implantation in the sample alters secondary electron emission, and, because intracrystalline conductivity contrasts occur in response to variations in defect density, secondary electron images reflect compositional variations and/or microstructural features. These secondary electron images are refer red to as charge contrast images (CCT). To demonstrate potential geological applications of CCI, we present images of growth zones, microfractures, di fferential diffusion domains, pleochroic haloes, and relict fluid pathways from zircon (strongly luminescent), quartz (weakly luminescent), and biotit e and cordierite (non-luminescent). CCT detect defects in a similar way to cathodoluminescence (CL), but have a higher resolution because the CCI sign al is composed of secondary electrons that are generated from a much smalle r interaction Volume than photons utilized in CL. CCI imaging also can be a pplied to a wider variety of geological samples than CL, because electronic charge trapping is not restricted to wide-band gap electronic configuratio ns. One of the most important potential applications of the CCI technique m ay Lie in the direct imaging of relict fluid pathways in rocks that have ex perienced metasomatism or alteration.