Micro-X-ray Fluorescence Mapping Data Analysis and Interpretation

Abstract

The Planetary instrument for X-ray Lithochemistry (PIXL) is a micro-XRF instrument on NASA’s Perseverance rover. XRF has long been used for analyzing geologic materials, but instrumentation is constantly evolving and capabilities have improved (Anzelmo and Lindsay, 1987a). This work outlines several methods and considerations for analysis and interpretation to make the most of available XRF data. We describe an optimized analytical method for quantifying light elements often omitted from XRF analysis based on Compton and Rayleigh scattered peaks the spectrum in Chapter 2, and in Chapter 3 we outline practical guidelines and limitations of the analysis. This method requires that the scattered peaks have a few hundred counts, which is easily attainable on a standard XRF instrument, and slightly more difficult on a micro-XRF instrument, which has a diminished count-rate. If used, however, it can help differentiate between different classes of minerals that are otherwise indistinguishable by XRF, such as carbonates and oxides. Micro-XRF mapping uses an x-ray beam focused to ~100 microns to collect spectra across a surface to provide spatially resolved element abundances. This information is often used qualitatively in biology and forensics, but in geology there is potential to lose information about the bulk rock when focusing on such a small area (Tian et al., 2014; Zheng et al., 2019). Chapter 4 addresses how to combine groups of small measurements to extract bulk data, particularly when a surface is heterogeneous on a 100-micron scale. We found that with a 100-micron beam, 25 spots were sufficient, though characterization of mineral components is limited by the size of the grains relative to the beam. Chapter 5 examines the effect of smaller scale structures of only a few microns and how they can alter measurements of sulfur. Coatings of only 0.2 nm around sulfate altered measured sulfur up to 13%, and thicker coatings of 2 microns diminished measured sulfur by 85%.