GSA Connects 2021 in Portland, Oregon

Paper No. 33-7
Presentation Time: 3:05 PM

QUANTITATIVE WDS COMPOSITIONAL MAPPING USING THE ELECTRON MICROPROBE


DONOVAN, John1, ALLAZ, Julien2, VON DER HANDT, Anette3, SEWARD, Gareth G.E.4, NEILL, Owen5, GOEMANN, Karsten6, CHOUINARD, Julie1 and CARPENTER, Paul7, (1)CAMCOR, University of Oregon, 1443 E. 13th St, Eugene, OR 97403-1241, (2)Institute of Geochemistry and Petrology, ETH Zurich, 8092 Z├╝rich, Zurich, Switzerland; Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, (3)Earth Sciences, University of Minnesota, Minneapolis, MN 55455, (4)Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106, (5)Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48013, (6)Central Science Laboratory, University of Tasmania, Tasmania 7001, Hobart, Australia, (7)Department of Earth and Planetary Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130

Quantification of pixels in X-ray intensity maps requires the same level of rigor applied in traditional point analysis. Accordingly, X-ray map quantification should include pixel-level corrections for WDS detector deadtime, corrections for changes in beam current (beam drift), changes in standard intensities (standard drift), high accuracy removal of background intensities (off-peak or mean atomic number), quantitative matrix corrections, quantitative correction of spectral interferences, and, if required, time dependent intensity corrections (for beam and/or contamination sensitive materials). The purpose of quantification at the pixel level is to eliminate misinterpretation of intensity artifacts, inherent in raw X-ray intensity signals, which distort the apparent abundance of an element. Major and minor element X-ray signals can contain significant artifacts due to absorption and fluorescence effects. Trace element X-ray signals can contain significant artifacts where phases with different average atomic number produce different X-ray continuum (bremsstrahlung) intensities, or where a spectral interference, even an apparently minor one, can produce a false-positive intensity signal. The methods we propose for rigorous pixel quantification requires calibration of X-ray intensities on the instrument using standard reference materials, as we already do for point analysis, which are then used to quantify multiple X-ray maps, and thus the relative time overhead associated with such pixel by pixel quantification is small.

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Donovan, J.J., Singer, J.W., and Armstrong, J.T. (2016) A new EPMA method for fast trace element analysis in simple matrices. American Mineralogist, 101(8), 1839-1853.