CHARACTERIZING MINERALOGICAL CONTRIBUTIONS TO DUST FROM AEOLIAN SAND ABRASION: METHODS DEVELOPMENT FOR PXRF ANALYSIS OF DUST AND SAND FROM PI-SWERL EXPERIMENTS

The Portable In-Situ Wind ERosion Lab (PI-SWERL) is a field instrument used to assess wind erosion and dust emissions. A sample of dust generated during a PI-SWERL sand abrasion experiment is captured on a filter, representing the PM10 fraction (<10 μm). Geochemical analysis offers the potential to identify which sand components (mineralogical and/or lithological) preferentially contribute to the dust fraction. Comparative quantitative analysis of dust and initial sand compositions could be used to evaluate this, but is not practical due to vast differences in the character of the available samples: a sand aliquot vs. small sample on dust filter (several mg dispersed across several cm² of filter). We developed a semiquantitative approach utilizing a portable X-ray fluorescence spectrometer (pXRF). The relationship between grain size and signal response for different elements (due to differential X-ray attenuation) was evaluated by analyzing crushed, sieved basaltic glass sand. For a limited set of elements (Al, Si, K, Ca, Fe, S, Ti, Mn) peak intensities were ratioed to one another (to remove effect of sample mass), ratios for dust and sand of the same sample were ratioed to each other (the target ratio), and these ratios were ratioed to the same ratios in dust vs sand from glass (to remove effect of grain size). The resulting values were evaluated to assess what elements had been enriched or depleted in dust. The sample suite, collected in a planetary geology analog context, is from sites with predominantly volcanic (mostly basaltic) geology in the Western U.S., Hawaii, and Iceland, and varies in mineralogical/lithological complexity. Dust was generated from these sands by PI-SWERL experiments in a laboratory setting. Components expected to be enriched in dust include: softer minerals, constituents of mechanically weak lithic grains, coatings on grains, and already-fine-grained constituents. Preliminary evaluation suggests that in some compositionally simple sands predictable enrichments are observed. For example, a basaltic Hawaiian olivine-rich beach sand with shell grains produced dust enriched in Ca from the shells. Complex sands are more challenging to interpret. Many basaltic sands yield Ti-enriched dust, consistent with fine Fe-Ti oxides from groundmass preferentially contributing to dust.