ON THE USE OF ELECTRON MICROPROBE FOR ACCURATE AND PRECISE ANALYSIS OF REE-MINERALS

For 65 years, Electron Microprobe (EMP) has provided extremely valuable microanalyses to assess and locate precious or critical elements. Several recent hardware improvements have triggered new interest in EMP data, notably with the development of large-area crystals, Si-drift EDS detectors, and field-emission guns. Additional software advances allow faster data acquisition at sub-micron spatial resolution, and with greater accuracy, precision, and sensitivity.

Identification and analysis of REE-minerals can be challenging. Small grain size, complex texture, and rarity of some mineral species preclude a complete mineral identification by optical means, hence the need for a microanalytical tool. Analytical challenges reside in the complex chemistry (with >20 elements commonly present), multiple peak and background interferences, and the beam sensitive nature of carbonate and hydrous minerals. Software improvements allow for better peak interference and matrix corrections, and the multipoint background acquisition allows for more precise background correction. The mean atomic number background correction (empirical background correction) limits the risk for background interference and cuts the analysis time in half. For beam sensitive material, time-dependent intensity correction is necessary to yield accurate data, without lowering the beam current or increasing the beam size. Element mapping (by WDS for high sensitivity or hyperspectral EDS mapping) renders mineral identification easy. For instance, element maps in Allaz et al. (2015, AmMin 100, 2123-2140) reveal complex mineral zoning from an allanite-rich rim (± monazite), transitioning to a fine-grained fluorbritholite-rich core (+ monazite, quartz, fluorite; ± bastnäsite, törnebohmite, uraninite), and locally an intermediate zone of törnebohmite, cerite and REE-carbonate. EMP further provides a complete quantitative analysis that can be used to determine REE-fractionation among the different REE-phases and to obtain U-Th-Pb EMP ages of monazite (and uraninite). EMP data alone are insufficient for a complete petrological history, but they often remain necessary, especially in the field of ore mineralogy where textural and quantitative information are essential.