UTILIZATION OF CCSEM METHODS TO DETERMINE MINOR/ACCESSORY MINERAL ABUNDANCES FROM INLAND DESERT DUNE SANDS OF THE GRAN DESIERTO

Recent studies of sand samples from the Gran Desierto using thermal infrared (TIR) emission spectroscopy in the laboratory and remote-sensing data acquired from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument revealed the bulk mineral composition of the inland dunes. The dunes are dominated by mainly quartz-rich sands of the ancestral Colorado River that mix with locally-sourced, feldspar-rich sands from the surrounding alluvial fans of the Basin and Range Province and the Sonoyta River. However, more detailed analysis is required to understand sediment sources and transport pathways.

The scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), especially under the control of a computer (CCSEM), can be deployed in a particle-by-particle or point count mode to characterize thousands of particles per sample. Because a particle observed in the backscattered electron imaging mode has a brightness proportional to its average atomic number, the analysis can be set up to characterize all particles, or only the bright accessory minerals. In the point count mode, the electron beam dwells at point locations on a grid and characteristic x-rays are emitted. The EDS spectrum is acquired and the mineral phase is interpreted. An SEM image and EDS spectrum are saved for possible further review. In the particle-by-particle mode, the particle periphery is also detected and physical measures such as size and morphology can be derived.

The analytical methods of CCSEM, including advantages and disadvantages, will be described and illustrated using examples of inland dune sands from the Gran Desierto. The combination of the TIR remote sensing and the SEM intimate sensing, each yielding complementary information, can be used to provide a more complete understanding of sediment sources and processes in this environment.