Particle Induced X-ray Emission spectrometry (PIXE) of Hawaiian volcanics: An analogue study to evaluate the APXS field analysis of geologic materials on Mars

Abstract

The Alpha Particle X-ray Spectrometer (APXS), a field instrument onboard four martian rovers, measures largely unprepared, in situ samples on Mars. The APXS has high precision that enables the determination of elemental concentrations in a wide range of geologic materials. However, lack of sample preparation can lead to heterogeneous matrix effects, and understanding the associated uncertainty is essential for interpreting APXS data. Here we use Particle Induced X-ray Emission spectrometry (PIXE) to analyze a suite of geologic samples from Hawai'i as an analogue study to better understand APXS analyses of martian samples. Wavelength-Dispersive X-ray Fluorescence (WDXRF) analyses of fused glass beads establish higher-accuracy standards for the Hawaiian samples. Sulfate-silicate mixtures were made to evaluate sulfur analysis by PIXE. Results show that the PIXE concentrations for most major elements have 2–6% accuracy, which is comparable to the APXS. However, the PIXE concentrations are systematically high in Al and low in Mg, resulting in lower accuracy (13% and 20%, respectively). Olivine-phyric lavas and most of their altered products have the largest discrepancies with Al concentrations up to 25% high and Mg up to 35% low. Sulfur is systematically high (up to 30% in a basalt matrix) compared to gravimetric S concentrations in the sulfate-silicate mixtures. These systematic deviations in Mg, Al, and S are linked to heterogeneous matrix effects, because PIXE and APXS analyses assume all atoms in a sample to be homogeneously mixed on the sub-micrometer scale, which is not the case. Two key implications for APXS results are: (1) Olivine-bearing samples likely have reported concentrations of Mg that is too low and Al that is too high. Thus, olivine-phyric basalts in Gusev crater and the basaltic sand and soil at three landing sites may have Mg and Al concentrations closer to those of the olivine-phyric shergottites and modelled martian crust than previously thought. (2) Sulfate-silicate mixtures may have overestimated S concentrations reported, resulting in greater uncertainty in the stoichiometry of Ca-sulfates, which is used to deduce the geochemical associations of sulfur in samples.