Constraining the Chemistry of Sub-cm Diagenetic Features with Curiosity's Alpha Particle X-ray Spectrometer

The Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Science Laboratory (MSL) rover Curiosity has acquired approximately 1,300 geochemical analyses since landing in 2012. The APXS utilizes a combination of X-ray fluorescence and particle-induced X-ray emission to determine the chemical composition of materials within its 15+ mm diameter field of view (FOV) [1, 2]. Diagenetic features provide a means to further understand and constrain the habitability of Curiosity's landing site, Gale crater. These features often present as veins or nodules with an areal extent on the sub-cm scale. APXS analyses of these features therefore contain a mixture of signals from the feature and host substrate.

To probe the composition of these sub-FOV features, Curiosity has developed a technique whereby multiple APXS measurements are conducted in close proximity to the primary target (referred to as a raster). The data are then analyzed to not only localize APXS FOVs, mitigating arm placement uncertainty which is on the order of 1-2 cm [2], but also infer the composition of the various endmembers within the workspace. The original raster analysis method (e.g., [2, 3]) has proven useful at deconvolving the chemistry of diagenetic features from the surrounding substrate. However, this method utilizes APXS oxide data as the primary input. These data are derived assuming a homogeneous sample for the purposes of calculating and correcting for matrix effects (the attenuation of induced X-rays by other elements in the sample). In instances of clear chemical heterogeneities, these matrix corrections can result in skewed compositions of the derived endmembers, such as a vein or nodule.

Here we present an improvement to this method whereby we utilize low-level data products and isolate matrix effect calculations for each individual endmember (e.g., [4]). The derived results show significant improvements (10-30%) compared to the oxide method in stoichiometric ratios when applied to calcium sulfate veins, an ideal proof-of-concept sample. Subsequent analyses of magnesium-sulfate dominated nodules hint at other potential mobile elements within the fluids present during diagenesis, such as P, Mn, Ni, and/or Zn. Similar elements were enriched in nodules at the Ayton/Groken field site, where P₂O₅ and MnO concentrations in the nodular material totaled over 25 wt% at a ~2:1 P:Mn molar ratio [4]. The improved analytical method will be particularly useful as Curiosity continues to explore the Marker Band and sulfate unit.

[1] Gellert & Clark (2015), Elements, 11.
[2] VanBommel et al. (2016), XRS, 45.
[3] VanBommel et al. (2017), XRS, 46.
[4] VanBommel et al. (2023), Icarus, 392.