THE VARIABLE-PRESSURE SCANNING ELECTRON MICROSCOPE: AN ALTERNATIVE TECHNIQUE FOR CATHODOLUMINESCENCE IMAGING OF ZIRCONS AND OTHER GEOLOGICAL MATERIALS

Zircons are rarely homogeneous and often display a complex internal texture generated by oscillatory growth and sector zoning, recrystallization, and strain-induced crystal defects. The advent of high-precision U-Pb-zircon dating techniques (SHRIMP and laser-ablation ICP-MS) has led to the routine application of cathodoluminescence imaging (CL) of zircons (and other minerals such as monazite). The most common CL technique in the geosciences is the monochromatic (gray-scale) “hot cathode” method that utilizes a SEM/EPMA equipped with a dedicated photo-multiplier tube (PMT) CL detector. When a dedicated CL detector system is not available, a properly-equipped variable-pressure SEM (VPSEM) may offer a workable substitute. The VPSEM is designed to generate secondary electron images at relatively high pressures (10-2500 Pa) compared to a standard SEM/EPMA which requires a high operating vacuum (10^-3 Pa or greater). VPSEMs utilize a variety of detector technologies to generate a high pressure secondary electron image. The Zeiss Supra 55 VPFESEM at the University of Idaho is equipped with a variable pressure secondary electron detector (VP SE) that collects photons generated by cascade collisions between secondary electrons and gas molecules in the sample chamber. When a luminescent sample such as a zircon is imaged under high-vacuum and high voltage conditions (i.e. standard SEM/EPMA operating conditions) the VP SE detector can be used to collect a monochromatic CL image. For comparison purposes, a series of CL images of zircon separates from the Precambrian Belt Super Group (N Idaho/Montana) were collected on an AMRAY 1830 SEM equipped with a dedicated PMT-based CL detector, and on the Supra 55 VPSFESEM utilizing a VP SE photon detector. Digital images were collected under similar beam (15 kV), vacuum (10^-3 Pa) and frame-buffering conditions. Gray-scale sensitivity was lower in the VPFESEM images but otherwise images were of comparable resolution and quality. For reasons of cost and ease of use, a dedicated CL detector system on a SEM or EPMA is still the preferred method for CL imaging of most geological materials. However, when these tools are unavailable, a VPSEM with a photon-based SE detector may offer a practical alternative.